Imaging modalities for non-acute pathologies of the foot and ankle

Abstract

Chronic foot and ankle pain, in contrast to acute traumatic injuries, presents a diagnostic challenge due to its diverse underlying causes. Accurate diagnosis often necessitates the utilization of various imaging modalities, emphasizing the importance of selecting the most appropriate one. The intricate structure of the foot, composed of multiple bones and supported by soft tissues like ligaments and plantar fascia, gives rise to a spectrum of mechanical disorders, including stress fractures, plantar fasciitis, Morton’s neuroma, and more. In addition to mechanical issues, non-acute abnormalities encompass inflammatory diseases affecting tendons and joints, benign tumors, tumor-like lesions, vascular abnormalities, and others. This article reviews the indispensable role of imaging in the assessment of these conditions, with a focus on plain radiography, computed tomography (CT), magnetic resonance imaging (MRI), and nuclear medicine studies, tailored to the specific clinical presentation. By providing insights into the selection and interpretation of imaging modalities, this article aims to assist clinicians in achieving accurate diagnoses and optimizing patient care for nonacute foot and ankle pathologies.

1. Introduction

Non acute or chronic foot and ankle pain is a common ailment and unlike acute ankle and foot pain, which are mostly traumatic injuries, the underlying diseases of chronic foot and ankle pain are diverse and often difficult to diagnose clinically. Hence, various imaging modalities are often needed and it is necessary to select the appropriate imaging modality. The foot is a symphony of several bones supported by soft tissue structures such as ligaments and plantar fascia to maintain its arch. Several mechanical ailments develop depending on the foot morphology such as stress fractures, plantar fasciitis, Morton’s neuroma etc. Non acute abnormalities also include inflammatory disease involving tendons and joints, benign tumour and tumour like lesions, vascular lesions, etc.

Imaging of foot and ankle pathology depends on the exact site, nature of symptoms and chronicity. The imaging modalities include Plain radiograph, CT, MRI and nuclear medicine studies, tailored to the clinical condition. Often multiple imaging modalities may be necessary for evaluation and to narrow the differential diagnosis.

2. Discussion

2.1. Imaging modalities

There are various imaging modalities to evaluate the foot and ankle, with plain radiographs being the fundamental imaging modality for primary assessment (Table 1). The other imaging modalities include Ultrasound, Computed Tomography (CT), Magnetic Resonance Imaging (MRI), and Nuclear Medicine studies. The modalities are chosen carefully based on the clinically suspected pathology, for. eg US examination if Achilles tendon pathology is suspected.

Table 1.

Commonly used imaging modalities with their advantages and disadvantages.

Modality	Advantage	Disadvantage
Plain Radiograph	•Easy access •Cost effective •Bone assessment •Matrix evaluation •Metalwork evaluation •Weight bearing	•Limited soft tissue evaluation •Not cross sectional •No bone marrow oedema evaluation •Radiation
Ultrasound	•Easy access •Cost effective •Dynamic/real time •Excellent soft tissue evaluation •No radiation	•User dependent •Questionable evaluation of deep-seated pathology •Soft tissue resolution inferior compared to MRI
CT	•Widely available •Excellent bone resolution •Excellent matrix evaluation •Cross sectional •Option of weight bearing CT •Alternate option is MRI is contraindicated	•Radiation •Limited evaluation of bone marrow oedema •Inferior soft tissue resolution •Majority non weight bearing
MRI	•Excellent soft tissue resolution •Cross sectional •Excellent bone marrow evaluation	•Limited availability •High cost •Time consuming •Evaluation of metalwork limited
Nuclear Medicine	•Functional study •Cross sectional •Trouble shooting	•Low resolution •Wide differential diagnosis •Radiation

2.1.1. Plain radiograph

The bone anatomy, alignment as well as joint congruency can be assessed through a routine set of three radiographic views: the anteroposterior (AP), also sometimes referred to as dorsoplantar or (DP) view, the internal oblique (or mortise) view, and the direct lateral view for the ankle. For the assessment of the foot, DP view, lateral view and oblique views are routinely performed. Weight bearing DP and lateral views are performed for specific indications. Special views such as sesamoid bone view of the great toe is performed where necessary.

The AP view of the ankle is used to evaluate the ankle mortise. The mortise view is essential to evaluate the talus and all margins of the joint space accurately. In adults, all portions of this clear space should be symmetric and should measure no greater than 4 mm in width.

The lateral view is essential for the evaluation of ankle effusions.

The lateral view should also include the proximal portion of the fifth metatarsal, as fractures at the base can present with ankle pain.¹

Weight-bearing radiographs of the ankle must be obtained whenever possible. Apart from acute ankle injuries, destruction of the hindfoot caused by long standing polyarthritis, including seronegative arthritis or diabetes, hindfoot impingement are some examples, that would benefit from weight-bearing radiographs.² Other conditions include Charcot foot, tarsal coalitions, and hindfoot deformities including pes planus (flat foot)³

Plain radiograph is the foremost imaging technique for the assessment of arthritis with the detection of subchondral sclerosis, osteophyte formation, and joint space narrowing and it is a routine clinical practice to obtain appropriate radiograph to evaluate and monitor arthritis and related ailments.

Although globally accepted initial modality of imaging, a study showed that X-ray was more cost-effective for the examination of bone tissue but was unable to identify early chronic inflammatory lesions in articular cartilage and synovium, which is better seen on MRI.⁴ The soft tissues have low x-ray absorptive ability and therefore soft tissue lesion detection and characterization is significantly inferior to that of MRI.

Soft tissue shadowing on plain radiograph can often be helpful. Swelling over the medial or lateral malleolus is an important clue for underlying fractures or ligamentous injury, which can be detected on plain radiographs and when correlated with clinical information, the image findings can be helpful. The presence of small lucencies within soft tissue swellings could represent gas pockets may represent gas forming organism abscess formation. Tendon and ligament disorders may be suggested as well on plain radiographs, such as for ligamentous ossifications, xanthomatosis of Achilles tendon, and insertional Achilles tendinosis.

Apart from acute foot and ankle injuries, destruction of the hindfoot caused by long standing polyarthritis, including seronegative arthritis or diabetes, hindfoot impingement are some examples, that would benefit from weight-bearing radiographs.¹ Other conditions include Charcot foot, tarsal coalitions, and hindfoot deformities including pes planus and Haglunds deformity (Fig. 1).^2,3

Fig. 1.

Distal arch tendinopathy, retro calcaneal bursitis and haglund deformity a) Plain film of ankle showing bony enlargement (long arrow) at the posterosuperior aspect of calcaneum consistent with Haglund deformity. Soft-tissue thickening corresponding to insertional Achilles tendinosis (short arrow) and retrocalcaneal bursitis noted. b) Sagittal PDFS MRI shows insertional Achilles tendinosis (red arrow) and fluid distension of retro calcaneal bursa (blue arrow) consistent with Haglund’s syndrome. Note the subtle bone marrow oedema of the posterosuperior calcaneum (white arrow). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

The plain radiographs are the primary imaging tool to evaluate the complications of instrumentation of the foot and ankle, such as fractures, displacement, loosening, or infection. A plain radiograph can fairly assess the density or matrix of a bone lesion, including the margins, and to look for any periosteal reaction; in order to characterize it as either aggressive or non-aggressive (Fig. 2). However, it lacks in-depth tumour characterization as compared to MRI which has superior soft tissue detail.⁴

Fig. 2.

a) Plain radiograph of ankle showing marked sclerosis of the body of the talus. b) MRI of ankle T1 and PDFS sequences showing bone marrow infiltration with loss of normal fatty bone marrow signal on T1 and diffuse heterogeneous high T2 signal. Small ankle joint effusion. Open biopsy confirmed lymphoma of bone.

2.1.2. Ultrasound

Ultrasound (US) is a widely available modality that is commonly utilized for the assessment of joint diseases. It is inexpensive, dynamic and accessible. The US offers excellent soft tissue resolution and is dynamic and allows functional assessment to certain extent. US is regularly used in one-stop rheumatology clinics, foot and ankle clinics etc.⁵ One of the most pertinent advantages of US is real-time imaging, making it an excellent tool for guiding minimally invasive procedures like fluid aspiration, biopsies, dry needing procedures and injections. Its lack of ionizing radiation is significant in the management of paediatric and pregnant patients. Another vital benefit of the US unit is that it is mobile, and hence easily performed at the patient’s bedside. US is regularly used in one-stop rheumatology clinics, foot, and ankle clinics etc.

US is highly operator dependent, hence adequate training is of vital importance, especially when it comes to the evaluation of multiple structures at the foot and ankle area. US must be applied along with clinical history and physical examination in the hands of an experienced operator. It is known to visualize the periosteum and cortex, for periostitis or cortical erosions, such as for inflammatory arthritides for example rheumatoid arthritis at the ankle,⁶ periostitis of paediatric osteomyelitis, etc. However the ultrasound waves are unable to penetrate the bone, hence it is not utilized to identify lesions within the bone with intact overlying cortex such as tumors and osteomyelitis. Where there is breach of cortex and extraosseous soft tissue, US is useful in identifying the lesion and helpful for image guided biopsy.

There are several nonacute foot and ankle disorders that can be evaluated on US, which include chronic injuries, inflammatory, infectious, and degenerative conditions. It is the first-line imaging technique to assess the tendons (Fig. 3, Fig. 4) and ligaments of the ankle.

Fig. 3.

a) Axial and sagittal PDFS MRI of ankle joint showing fluid distension of retro malleolar peroneal tendon sheath (blue arrow), longitudinal splitting of the peroneus brevis tendon (red arrows) and thickened peroneus longus tendon (yellow arrow). Also note the peroneus quartus tendon (green arrow) which is from an accessary muscle. b) Dynamic ultrasound scan shows subluxation of peroneal tendons anteriorly from the retro malleolar grove. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4.

a) Long axis panoramic ultrasound, short and long axis power doppler of Achilles tendon (blue arrow) showing abnormal thickening, loss of normal echotexture and neovascularization. b) Sagittal and axial PDFS MRI showing diffuse fusiform thickening of the Achilles tendon (blue arrow) with intrasubstance high T2 signal changes and oedema of the paratenon (yellow arrow). Appearances in keeping with severe Achilles tendinopathy. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Nerve entrapment around the foot and ankle by primary or secondary causes can be evaluated by ultrasound. The ultrasound is the first line of investigation in several known foot and ankle entrapment neuropathy such as sural nerve entrapment, Baxters neuropathy, tarsal tunnel syndrome, anterior tibial nerve entrapment, digital nerve entrapment due to Morton’s neuroma (Fig. 5) etc.^6,7

Fig. 5.

a) Long axis and b) short axis ultrasound scan of the 2nd inter metatarsal space foot showing a hypoechoic nodule (arrow) in keeping with Mortons neuroma.

US has several benefits for the evaluation of the tendons and ligaments of the ankle, especially during active and passive ankle movements and upon stress manoeuvre, and offers excellent resolution and functional details. In addition, dynamic US with a dorsiflexion-eversion manoeuvre of the ankle is helpful for the detection of intermittent dislocation and intra sheath subluxation of the peroneal tendons.⁶ Dynamic assessment of forefoot is possible with image guided replication of Moulders click to detect Morton’s neuroma. A stress test during US can provide diagnostic confidence regarding the differentiation between partial and complete tears of the ligaments.⁶ Similar principle can be applied to the tendon tears to differentiate between complete and incomplete tears by assessing dynamic paradoxical movements as expected in complete tears.

The tendinitis and tenosynovitis affecting different tendons of foot and ankle are likely a result of a spectrum of abnormalities such as repetitive local stress, trauma, systemic joint disorders, such as rheumatoid arthritis, ankylosing spondylitis, or infection In tendinitis or tendinosis, the tendon is oedematous and hypoechoic, while the thickened tendon sheath with presence of fluid in the tendon sheath is called tenosynovitis. Colour doppler is vital to detect the presence of neovascularity to assess the extent of inflammation of the tendon or tendon sheath and can be used to evaluate the degree of disease and monitor healing as well.⁶

2.1.3. Computed tomograph

Computed Tomography (CT) scan is obtained in axial projections, and standard coronal and sagittal reconstructions are performed in soft tissue and bone algorithms. It provides three-dimensional imaging of the foot and ankle with increased spatial resolution compared to plain radiograph, which is essential for the foot and ankle surgeons for preoperative planning. CT is particularly useful in evaluating the bony cortex and identifying pathological fractures, periosteal reactions, or matrix mineralization. In nonacute conditions, CT examination is of significant importance in the evaluation of fracture healing to assess for evidence of delayed union or non union and to evaluate the potential complications of fracture fixation instrumentation such as infection, fracture, osteolysis or loosening. Another advantage of CT imaging is for suspected osseous or fibrous midfoot and hindfoot coalition (Fig. 6). It is of importance in evaluating instability of the ankle joint, particularly the subtalar joint. It is the predominant imaging modality prior to reconstructive surgeries such as corrective osteotomy for fracture malunion of the foot and ankle.⁸

Fig. 6.

a) CT axial and sagittal reformats showing bilateral calcaneo navicular tarsal coalition (blue). b) Sagittal T1, PDFS and axial PDFS MRI showing calcaneo-navicular fibrous coalition (blue arrow) with associated bone marrow odema on ether side of the abnormal articulation. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Weight bearing cone beam CT scan is a recent innovation. An important indication in a nonacute setting is to accurately measure hindfoot alignment, which is fundamental prior to operative procedure. The weight bearing CT also allows dynamic assessment of pes planus and worsening of various hindfoot impingements.⁹

CT has been shown to be extremely useful to diagnose and assess the severity of osteoarthritis and inflammatory arthritis, such as rheumatoid arthritis, reactive arthritis, and crystal deposition disease, most notably gouty arthritis. CT shows bony anatomy better prior to considering surgical intervention. CT is also the imaging modality of choice if MRI is not conclusive in concluding a fracture, or to identify underlying bone lesion in case the MRI shows nonspecific oedema. Dual energy CT (DECT) is a relatively recent technique for the visualization, characterization, and quantification of monosodium urate crystal deposits which allows early diagnosis, treatment, and follow‐up of gout(Fig. 7).¹⁰

Fig. 7.

A case of Gout. a) plain x-ray ankle showing extensive dense soft tissue swelling (yellow arrows) around the ankle joint. Plain x-ray of the big toe shows a punched out erosion (blue arrow) with over hanging edges involving the distal phalanx. b) Dual energy CT of the both feet and ankles show large deposits of monosodium urate tophi (red arrows) around both ankles and left big toe. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

CT has a role to play in osteomyelitis, particularly when MRI is contraindicated or presence of metal work degrading the MRI image quality. It is superior to both MRI and plain film in delineating the bony margins and identifying a sequestrum or involucrum. The main disadvantage is the inability to detect marrow oedema, hence early infection can easily be missed. Additionally, a normal CT does not exclude osteomyelitis. CT has been shown to clearly delineate soft tissue calcifications and ossifications including enthesophytes, which may not be well visualised on plain radiograph and MRI. Abnormalities such as myositis ossificans can look heterogenous on MRI which can potentially be misled to be reported as a neoplastic lesion and CT scan can differentiate between myositis ossificans and sinister abnormalities.¹¹

The main pitfall of CT includes poor depiction of the tendons, ligaments, surrounding soft tissues, and tumors; where MRI is the first choice of imaging. Nonetheless the soft tissue structures are still visualised on CT with less resolution compared to the MRI and every effort should be made to characterize the soft tissue anatomy and identify abnormalities. It is not unusual to find subluxed or dislocated tendon around the ankle joint on a CT scan performed to evaluate bony anatomy and the reporting radiologist should evaluate the soft tissue anatomy by switching the bone window to the soft tissue window. A further drawback includes relatively high dose of ionizing radiation compared to plain film and MRI, and a decision to consider CT scan in young individuals and pregnant women should be exercised with caution. In case of a CT scan performed when there are metalworks present, there could be significant image degradation from beam artifacts, these can be overcome to certain extent by using metal artefact reduction software (MARS) in the newer scanners.

2.1.4. Magnetic resonance imaging (MRI)

Magnetic resonance imaging (MRI) provides the greatest soft tissue contrast resolution of any imaging modality and is most useful for the evaluation of soft tissue abnormalities as well as osseous lesions. Routine MRI evaluation of the ankle is performed in the standard axial, coronal, and sagittal planes with possible 3D reconstruction of the images. The hind foot evaluation is possible in ankle MRI and no separate MRI examination is necessary. In case of evaluation of midfoot and forefoot abnormalities, a separate foot MRI should be considered which is tailored and performed along the sagittal, long axis and short axis of the foot. The principal utility of MRI is in the evaluation of chronic foot and ankle pain, nonacute lesions, and it can confidently differentiate between fat, water, muscle, and other soft tissue components which are not easily evaluated on plain radiograph or CT images. There is a reduced role of MRI in the immediate emergency setting, as the high signal characteristics of oedema and haemorrhage are nonspecific and may obscure other findings and in acute setting, CT is the imaging modality of choice.¹²

The excellent soft-tissue contrast resolution of MRI allows the clear visualization of the tendons and ligaments which is able to evaluate sprains, tendinosis, tenosynovitis, entrapment, rupture, and dislocation. The MRI can also quantify the severity of disease based on the signal characteristics and also capable to predict the course of disease when serial imaging is considered to monitor the disease process. In addition, other soft tissue abnormalities such as impingement syndromes, sinus tarsi syndrome, compressive neuropathies, and synovial disorders are better visualised on MRI. Neoplastic lesions of bone and soft tissue are better characterized by an MRI and often helpful in differentiating benign and malignant lesions with a fair level of confidence. Added diffusion weighted imaging sequences aid visualization of the ankles tendon injuries, and provide more beneficial and diagnostic information than routine MR imaging and help detect tendinosis, tenosynovitis, partial and full thickness tears of the tendons.

It has been also utilized in the assessment of inflammatory arthritides, particularly in the early phase of disease. MRI can identify synovitis, tenosynovitis (Fig. 8), disease distribution and subchondral osteitis is well visualised on MRI.¹³ Erosions related to inflammatory arthropathy can be visualised on plain films, CT as well as MRI, but MRI has an edge to differentiate acute erosive lesions as opposed to chronic inactive lesions (Fig. 9).¹⁴

Fig. 8.

Coronal, sagittal and axial PDFS MRI of ankle joint showing fluid distension of the Flexor hallucis longus tendon sheath (blue arrow) in keeping with tenosynovitis. Note the absence of ankle joint effusion (white arrow). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 9.

A case of psoriasis. a) Plain x-ray foot showing pencil in cup appearance of the 4th and 5th MTP joints (blue arrows) which is a typical feature of psoriatic arthritis. Note the partial resorption of 3rd MT head (green arrow) b) Coronal T1 and PDFS MRI foot showing destructive changes of the 3rd, 4th and 5th MTP joints (yellow arrows) with pencil in cup appearance. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

MRI is invaluable in the evaluation of soft tissue and osseous neoplasms and other non-neoplastic lesions. The MRI is almost synonymous with local staging in a suspected neoplastic lesion. The MRI signal characteristics such as chondroid (Fig. 10), osseoid (Fig. 11), fibrotic, cystic etc are well identified on MRI and able to predict the nature of the lesion with a reasonable confidence to consider if soft tissue biopsy is necessary.

Fig. 10.

a & b. Plain radiograph of forefoot showing well defined punched out lucent lesions involving lateral aspect of the middle, proximal phalanges and proximal metatarsal giving scalloped appearance of the adjacent bony cortex with a sclerotic periosteal reaction. c, d & e. Coronal T1, PDFS and STIR MRI of 2nd toe and metatarsal showing lobulated high T2 and isointense T1 signal lesion abutting the adjacent cortex. No associated medullary bone or soft tissue oedema or matrix calcification. Appearances in keeping with periosteal chondromas.

Fig. 11.

a) CT foot with multiplanar reformats showing small focal lucent lesion involving the cuboid bone in the subarticular location with a tiny focal dense nidus (blue arrow) b) MRI of ankle sagittal and coronal PDFS and axial T1 sequences showing significant bone marrow oedema with in the cuboid bone which is high on T2 and low on T1 sequences (yellow arrow). Appearances in keeping with Osteoid osteoma. C) Planning CT of foot showing the osteoid osteoma (green arrow) which was treated with RF ablation. Note the RF ablation probe in situ (red arrow). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

MRI is considered relatively safe in pregnancy compared to CT scan because of lack of radiation, MRI is also considered ahead of CT in children for the same reason.¹⁵

There are several contraindications for MRI which includes patients with cochlear implants, aneurysm clips, pacemaker, defibrillators, and other implanted electronic devices which are considered unsafe in the magnetic field.¹⁶ Some of the other drawbacks of MRI include inferior spatial resolution compared to CT scan especially in wide field of view, high cost, and long scan times, patient required to remain still for a longer duration, although rare, often a need of general anaesthesia can be quite demanding patients who cannot lay still, especially children.¹⁷

2.1.5. Nuclear medicine

The use of nuclear medicine techniques lies primarily in the evaluation of subacute or chronic foot and ankle pain, for which the radiographic examination has either been negative or inconclusive.¹⁸ The most common examination is a bone scan or bone scintigraphy and may be performed with 1, 3, or 4 phases. Single photon emission computed tomography (SPECT) and Positron emission tomography (PET) are the other two imaging modalities that are often utilized, and these are combined with CT scans.19, 20, 21

The most common indications for SPECT are degenerative disease, osteochondral lesions, stress fracture, postoperative evaluation of joint fusion of foot, tarsal coalition, Achilles tendonitis, stress fractures and diabetic foot.²² When multiple lesions of the foot and ankle are detected on MRI, it may be difficult to identify the major culprit lesions associated with the patient’s symptoms, and hence SPECT (Fig. 12) may be used as complementary imaging to MRI to enhance the diagnostic specificity.²⁰

Fig. 12.

a. CT image, b. SPECT fused image of a patient with previous ankle joint fusion and on going pain. The study shows increased uptake at the talonavicular joint (yellow arrow) with subchondral sclerosis on CT component (blue arrow) in keeping with secondary osteoarthritis of the Talonavicular joint exaggerated by ankle joint fusion. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

The use of nuclear medicine techniques lies primarily in the evaluation of subacute or chronic ankle pain, for which the radiographic examination has either been negative or inconclusive. The most common examination is a bone scan or bone scintigraphy and may be performed with 1, 3, or 4 phases. Single photon emission computed tomography (SPECT) and Positron emission tomography (PET) are the other two imaging modalities that are often utilized and these are combined with CT scans which gives cross sectional details of radionuclide uptake. The SPECT uses gamma emitting radioisotope tracers, such as 99 m Technetium labelled phosphonate agents while PET utilizes F-18 fluorodeoxyglucose (FDG) as the relevant radionuclide agents.

The SPECT and PET imaging can both be used for foot and ankle evaluation, with the former being favoured as it is readily available and is of significantly lower cost. PET, however, demonstrates superior contrast and spatial resolution as a result of a better target-to-background signal ratio, than SPECT. PET however has a higher radiation dose and in practice usually reserved with evaluating disease in the rest of the body and not only periphery.

Although SPECT delivers less radiation to the patient compared to PET, the radiation dose is still high and the radiation dose is a concern. As a result of a radiopharmaceutical intravenous injection, and subsequent CT performed, the total radiation dose is greater with SPECT (±5.5 mSv), when compared with CT scan alone¹⁸ (Fig. 12).

The functional information given by the nuclear medicine studies are invaluable. The bone scan, SPECT and PET can show increased bone metabolism or bone turnover in the affected bones, which when correlated with other imaging modalities with better spatial resolution such as MRI scan, blood markers and clinical history, it is possible to reach an accurate and specific diagnosis.

Although SPECT-CT combines the advantages of two imaging modalities to give functional information from SPECT and morphological information from CT, the low dose CT in the study lacks spatial resolution. The newer developed hybrid SPECT-CT with high-resolution multi slice CT images have increased diagnostic accuracy.¹⁹

Bone scintigraphy or bone scan is highly sensitive for detecting bone lesions, but the poor specificity is a major drawback along with lack of cross sectional component in the study whereas SPECT/CT is able to clearly localize the abnormality as a result of its functional/metabolic activity.²⁰

The most common indications for SPECT are degenerative disease, osteochondral lesions, stress fracture, postoperative evaluation of joint fusion of foot, tarsal coalition, soft tissue pathologies such as Achilles tendonitis, and diabetic foot. When multiple lesions of the foot and ankle are detected on MRI, it may be difficult to identify the major culprit lesions associated with the patient’s symptoms, and hence SPECT may be used as complementary imaging to MRI to enhance the diagnostic specificity.¹⁸

Stress fractures commonly involve the metatarsals however they do occur in the tarsal bones. Radiographs are often inconclusive and these are well visualised on radionuclide bone scintigraphy. Although MRI is known to be highly sensitive for stress injuries demonstrated as bone marrow oedema, bone scintigraphy is also a very sensitive diagnostic modality for early stress fractures.²¹ Nuclear medicine studies can also be used as complimentary imaging modality along with MRI for trouble shooting. Moreover, SPECT can diagnose stress fractures with greater accuracy by the combination of CT findings of fracture lines and SPECT findings of increased metabolic activity and bone turn over.

3. Common and uncommon pathology

3.1. Avascular necrosis

Avascular necrosis is caused by disruption of the bloody supply to the subchondral bone as a result of local trauma or a wide range of systemic and iatrogenic causes.²³ Although the femoral head accounts for the majority of cases around 75%, approximately 4% of cases are seen in the foot and ankle.²⁴ Described sites of avascular necrosis/osteonecrosis include the talus (Fig. 13), navicular referred to as Kohler’s disease in children and Muller-Weiss disease in adults, metatarsal heads which if in the 2nd metatarsal is known as Freiberg’s infraction and Renander’s disease if the sesamoid is involved.²⁴

Fig. 13.

a) Coronal reformats of bilateral ankle CT showing markedly comminuted left talar fractures (red arrow). b) Check x-ray post internal fixation of talar fracture with metallic screws in situ. c) Proton density Fat Sat (PDFS) sagittal and coronal images of ankle showing established avascular necrosis of talus with collapsed talar articular surface with severe subchondral bone marrow oedema (white arrow). Secondary OA changes of ankle joint. Also note severe Achilles tendinopathy with fusiform thickening of the Achilles tendon (blue arrow) approximately 4 cm from its distal attachment with intrasubstance high T2 signal changes. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

The response to ischaemic bone necrosis secondary to insufficient blood supply and oxygen is re-ossification, revascularisation and resorption. These changes can account for the appearances seen on x-ray, CT and MRI. Radiographic appearances include radiolucency of surrounding bone due to resorption and increased density of necrotic bone due to re-ossification which can progress to collapse. This sclerosis and collapse can be referred to as crescent sign on x-ray.²⁴ MRI is more sensitive in early AVN due to the diffuse bone marrow oedema seen in the involved regions, this would represent low T1 and high fluid signal. A pathognomonic sign of osteonecrosis or bone infarct on MRI is a ‘double line’ consisting of serpiginous inner bright and outer dark line on fluid sensitive sequences.²⁵ As the disease progresses and sclerosis is present then some areas will be low signal on all sequences.²⁴

3.2. Bone infarcts

Bone infarcts occur within the medullary cavity of bone with a predilection for long bones. They are often atraumatic with many of the same risk factors as AVN. This includes haematological disorders, malignancy, steroid use, and alcohol.²⁵ X-ray may show areas of serpiginous or ill-defined sclerosis with interposed areas of radiolucency.²⁶ MRI will show geographic medullary areas of central preserved marrow with surrounding serpiginous sclerosis and high fluid signal, with aforementioned ‘double line sign’.²⁵

3.3. Tumour & tumour like conditions

Bone and soft tissue masses of the foot and ankle are not uncommon but true neoplasia has to be differentiated from tumour-like conditions. Majority of the neoplasia in foot and ankle are benign, however rarer malignant lesions must be ruled out. Although the list is not exhaustive, the benign lesions include glomus tumour (Fig. 14), primary synovial osteochondromatosis, tendon sheath giant cell tumour (TSGCT) (Fig. 15), unicameral bone cyst, aneurysmatic bone cyst, enchondroma, juxtacortical chondroma (Fig. 10) intraosseous lipoma (Fig. 16), haemangioma, plantar fibromatosis (Fig. 17), brown tumors (Fig. 18), osteoid osteoma (Fig. 11) etc.^27,28 Malignant lesions include chondrosarcoma, osteosarcoma, lymphoma (Fig. 2), Ewing’s sarcoma, synovial sarcoma etc.²⁹

Fig. 14.

A case of glomus tumour a) Normal appearance of plain film of big toe, b) Ultrasound of the nail bed showing subungual hypoechoic nodular lesion (red arrow) at the region of maximum tenderness. Marked hypervascularity on doppler noted. c) MRI T2 STIR images showing hyperintense nailbed nodule (thin red arrow) causing scalloping of the underlying bone (blue arrow). T1 sequence showing low to intermediate signal subungual nodule. T1 post contrast images showing avid contrast enhancement due to high vascularity (thin red arrow). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 15.

a) US scan of a soft tissue lump (blue arrow) with well defined smooth margins over the plantar aspect of the flexor tendon of 2nd toe at the level of the PIP joint. It is predominantly hypo to isoechoic to the subcutaneous fat with no significant internal doppler signal. b) Sagittal PDFS and Axial T1 sequences show high T2 and isointense T1 soft tissue lump (blue arrow) attached to the flexor tendon sheath in keeping with giant cell tumour of tendon sheath. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig 16.

a) Plain film of the lateral ankle showing a well defined thin walled lucent lesion with no matrix calcification. b) Sagittal T1 image shows fatty lesion with hypointense inclusions. Sagittal and coronal PDFS images show fat suppression with areas of hyper intensity suggesting granulation tissue (stage 2 intra osseous lipoma).

Fig. 17.

Sagittal PD and PDFS, axial T1 and PDFS MRI of the ankle showing diffuse fusiform thickening of the plantar fascia with high T2 signal changes in keeping with plantar fibromatosis. Areas of abnormal high signal intensity on T2 and STIR images reflect the relative proportions of cellular elements within the mass.

Fig. 18.

a) Plain film showing an expansile cystic lesion (blue arrow) involving the 3rd metatarsal shaft with no cortical destruction. No matrix calcification. b) Plain x-ray of the lumbar spine showing rugger jersey appearance and widening of the bilateral SI joints (white arrow) which are sequelae of renal osteodystrophy. c) Early and delayed MIBI scan showing retained hyperactivity of parathyroid glands (red arrow) on the delay images suggesting secondary hyperparathyroidism. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

The initial investigation for bone lesions is usually plain films, whereas for the soft tissue lesions, it can be US. Invariably, the lesions are characterized further by MRI, tailored, and protocolled to the anatomical location. If the lesions need tissue diagnosis, the biopsy can be performed either under US, fluoroscopy or CT guidance³⁰

3.4. Osteomyelitis

Osteomyelitis can occur in similar pathogenesis to septic arthritis via haematogenous spread, direct injury or metalwork related. However, in relation to the foot and ankle, osteomyelitis can occur as a complication of diabetes or any other causes of peripheral neuropathy (Fig. 19). Patient with diabetes may develop peripheral neuropathy and peripheral vascular disease which can give a 25% risk of developing a foot ulcer.³⁰ Of those patients with a foot ulcer, up to 50% may go on to develop secondary infection³¹

Fig. 19.

A case of Charcot arthropathy with midfoot osteomyelitis. a) Radiographs of the foot in AP, oblique and lateral projections. There is involvement of the tarso metatarsal articulations (blue arrows) with lateral subluxation of the metatarsal bones. The lateral radiograph shows the typical inferior subluxation of the talar head (yellow arrow); the cuboid (red arrow), typically becoming the most inferior weight bearing bone of the foot. Note the lager plantar ulcer (arrow head) b) Sagittal MRI of foot and ankle (T1, STIR and post contrast sequences) T1 image shows huge signal drop (arrows) in the fatty bone marrow (white asterisks). STIR image shows massive bone marrow oedema (yellow asterisks) in metatarsal bone, navicular bone, and cuneiform bones. There is soft tissue oedema with a large plantar ulcer (white arrow) reaching up to the bone surfaced of cuboid (blue asterisks). There is a degree of bone destruction and fragmentation in the mid foot joints. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

There are multiple modalities that can be utilized in the evaluation of pedal osteomyelitis including radiography, MRI, and nuclear medicine. Nuclear medicine in the form of PET-CT or WBC scintigraphy has excellent specificity with regards to osteomyelitis and are particularly useful in identifying residual disease after antibiotic treatment.³⁰ The bony changes involved in osteomyelitis however may not be identified on x-ray for 7–15 days after the acute onset of disease.³² On MRI the extent of bone involvement is assessed with the extent of bone marrow changes which is identified by high fluid signal on fat saturated images with concomitant low T1 signal.³¹ MRI is the best modality to assess for associated soft tissue involvement which includes abscess, cellulitis, the ulcer and associated sinuses (Fig. 20).31, 32, 33

Fig. 20.

PD and PDFS and post contrast images of the big toe at the level of 1st MTP joint showing deep plantar ulcer (white arrow) reaching up to the sesamoid bones. Bone marrow oedema noted involving the lateral sesamoid bone (red arrow) with surrounding fluid signal. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

3.5. Mortons neuroma

Morton’ s neuroma is not considered a true neoplasm, it in fact represents an entrapment neuropathy of the common plantar digital and proper plantar digital nerves secondary to neural and perineural fibrosis because of mechanical stress. Mortons neuroma is most seen in the 3rd webspace, and although neuromas in the other web spaces have different names including Joplin and Iselin, they are all generally referred to as Morton’s neuroma.^34,35

Ultrasound in experienced hands is a fantastic tool for assessing for Mortons Neuroma (Fig. 5). Scanning can be performed from the dorsal or plantar side. The digital nerves run plantar to the transverse intermetarsal ligament between the metatarsal heads and is normally 1 mm in thickness. A mortons neuroma is identified as a non-compressible hypoechoic rounded mass centred on the digital nerve.^34,36 Mortons neuroma can often be present with concomitant intermetarsal bursitis which will show a fluid filled hypoechoic compressible bursa. The bursal-neuroma complex may be partially compressible.³⁷ On MRI the mortons neuroma will represent a plantar ovoid mass or if large enough will dumbbell through intermetarsal space. On T1 and T2 weight images the neuroma will be intermediate/low signal, if there is associated bursitis this will be low on T1 sequences and high on T2 or fluid sensitive sequences. On post-contrast imaging the fibrotic neuroma will generally not enhance however any bursal component will enhance on post-contrast sequences.^34,38

3.6. Arthritis

The diagnosis and management of arthropathy depends on imaging heavily. This includes establishing a diagnosis and its confirmation, disease monitoring, treatment response, and identifying complications of the condition. The foot and ankle are affected by a range of mechanical arthritides due to complex biomechanics and trauma and a range of inflammatory arthropathy. Inflammatory arthropathy can be rheumatoid arthritis, seronegative arthropathy (Fig. 9), gout (Fig. 7), calcium pyrophosphate deposition disease, etc. Radiographs remain the initial imaging modality. Where necessary, cross-sectional studies such as CT & MRI are considered to study the complex three-dimensional anatomy, particularly of the midfoot and hindfoot (Fig. 21).

Fig. 21.

a) A case of secondary osteoarthritis due to haemophilia. AP and lateral views of the plain film of the ankle joint showing subchondral sclerosis, severe joint space narrowing, incongruence between joint surfaces and large anterior and posterior osteophytes. b) Sagittal and coronal CT reformats of ankle joint showing subchondral cyst, severe loss of joint space and marginal osteophytosis. Note the involvement of the subtalar joint with severe OA.

The conventional radiographs remain mainstay of the assessment of foot and ankle arthropathy. The radiographs are good in identifying erosions, joint space loss, periarticular osteopenia, marginal osteophyte formation and sub articular cystic change. However, the radiographs lag in accurately assessing the complex three-dimensional anatomy of the midfoot and hindfoot. Particularly in comparison to MRI, the radiographs are unable to visualize cartilage, synovitis, and bone marrow oedema (Fig. 22). US is operator dependant able to visualize synovitis, joint effusion, soft tissue oedema, erosions, and enthesophytes, but limitations include its inability to identify marrow oedema, inaccessibility to deep portions of the joint, and difficulty making temporal comparisons.³⁸

Fig. 22.

a) Plain x-ray of the ankle joint showing a thin walled, non expansile, cystic lesion (blue arrow) with in the distal tibia reaching the joint surface. b) CT also shows a thin walled benign looking, cystic lesion (yellow arrow) which appears to be communicating with the joint surface. c)S agittal, coronal and axial MRI of ankle showing thin walled, non expansile cystic lesion (red arrow) containing thing septations and no fluid levels. Appearances most in keeping with a Geode. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

3.7. Madhura foot

Mycetoma pedis, also termed “Madura foot” arises either from Actinomyces, leading to Actinomycetoma, or from an authentic fungal infection, known as Eumycetoma.³⁹ This persistent granulomatous condition typically begins in the skin layers, primarily affecting the feet, which are susceptible to injury.

Plain X-ray images reveal bone destruction characterized by numerous lytic bone lesions that resemble a moth-eaten pattern (Fig. 23). MRI scans display multiple abnormal regions with heightened T2 and STIR signals, accompanied by a central dark spot and an outer bright ring in both bones and soft tissues, indicative of granuloma development (Fig. 24). This specific characteristic is known as the “dot-in-circle” sign, a critical diagnostic clue.³⁹

Fig. 23.

Plain films of the foot showing moth eaten appearance with multiple well defined punched out lytic defects (yellow arrows), complete destruction of 3rd metatarsal (red arrow), marked joint space reduction, cortical irregularity, periosteal reaction, and patchy areas of increased sclerosis. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 24.

a) Sagittal and axial PDFS MRI images showing multiple rounded abnormal T2 and STIR bright signal bone lesions showing “dot in a circle sign” (yellow arrow) with central hypointense dot representing the fungal granule and surrounding high T2 signal rim representing granulation tissue. b) T1 pre contrast MRI shows moth eaten appearance of the bones (blue arrows). c) Post contrast MRI images showing florid contrast enhancement of the bone (blue arrows) and soft tissues (red arrows). Appearances in keeping with Madura foot. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

3.8. Painful toes (hallux valgus, hammer toe, sesamoiditis)

Deformities of the toes are a frequently-encountered clinical presentations associated with significant pain and morbidity. Each patient, and indeed each toe, is unique and understanding of the anatomy and pathology of the greater and lesser toe deformities is essential.⁴⁰

Hallux valgus is defined as medial deviation of the first metatarsal bone, lateral deviation of the hallux, and a prominent first metatarsal head and is more common in women. The most important static stabilizers on the medial side of the hallux are the articular capsule, collateral ligaments, and medial sesamoid ligament. Insufficiency in any one of those structures leads to the development of hallux valgus. Hallux valgus is associated with disabling sequelae, including nerve entrapment, intermetatarsal bursitis, cartilage damage, and articular degeneration.⁴¹

Hammertoes are among the most common deformities of the forefoot. It is though to be a result of imbalance between the weak intrinsic muscles and the stronger extrinsic muscles surrounding the metatarsophalangeal joints of the lesser digits. Hammertoe is a deformity that involves flexion at the interphalangeal joints (IPJ) and can be distinguished into categories including the classic hammertoe, mallet toe or claw toe. With the lesser digits being an important component in the balance of the foot, as well as in pressure distribution, deformities may lead to compensatory gait changes, distortions in cosmetics, callous formations, and pain.

Sesamoiditis is an inflammatory condition affecting the sesamoid bones, primarily the hallux sesamoids resulting from recurrent strain injury to the plantar aspect of the forefoot. Patients usually experience metatarsal pain during the toe-off phase when walking and might tend to bear weight on the lateral side of the affected foot to alleviate the pain.⁴²

On plain radiograph, sesamoid view offers a detailed profile of both sesamoids. Late stages might display fragmentation and sclerosis of the sesamoids. On CT scan, osteonecrosis is likelier when subtle sclerotic changes are identified. MRI shows bone marrow oedema within the great toe sesamoids. On STIR, there’s a high signal, while T1 may present a normal or reduced signal. Early sesamoiditis can be hard to distinguish from osteonecrosis.⁴²

3.9. Sever disease

Calcaneal apophysitis, commonly referred to as Sever disease, is an inflammation of the calcaneus apophysis leading to pain. This condition often affects active young children and adolescents involved in jumping and running activities.⁴³

Although foot radiographs often appear normal, when signs are evident, they might display an increase in the density of the calcaneal apophysis or its fragmentation. In unilateral cases, there might be an asymmetrical soft tissue thickening over the posterior calcaneum on the ultrasound scan. At times, fragmentation of the apophysis, which can present with a saw-toothed appearance, may be noticeable. MRI is considered the most accurate for assessing apophysis damage (Fig. 25), MRI may reveal oedematous changes within the calcaneal apophysis, possibly extending into the neighbouring calcaneal tuberosity. There might also be evidence of a retrocalcaneal bursal effusion.

Fig. 25.

a) Lateral plain x-ray of a child with posterior heel pain shows increase in the density of the calcaneal apophysis (white arrow). b) Sagittal PDFS and T1 MRI of ankle shows oedematous changes within the calcaneal apophysis (blue arrow) which appears to extend into the adjacent calcaneal tuberosity (yellow arrow). Appearances in keeping with Sever’s disease. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

3.10. Sinus tarsi syndrome

Sinus tarsi syndrome is characterized by pain and tenderness on the lateral side of the hindfoot. Commonly seen in athletes, overweight individuals, and those with foot deformities, it typically arises from trauma, such as ankle sprains, or inflammatory conditions.⁴⁴

On plain radiographs, advanced cases show osteoarthritis of the subtalar joint with intraosseous cystic changes. CT is better at detecting these secondary bone changes earlier than conventional radiography. MRI is likely the most effective method for visualizing soft tissue alterations in the sinus tarsi. Changes can include inflammation, scar tissue formation, or ligament injuries. Normal fat in the sinus tarsi, typically T1-hyperintense, might be replaced by fluid or scar tissue, and disrupted ligaments could be observed (Fig. 26, Fig. 27).

Fig. 26.

Coronal PDFS and T1 MRI of ankle showing high T2 signal (oedema) in the sinus tarsi fat which appears almost completely obliterated on the T1 sequence (blue arrow). Axial PDFS sequence of the same patient shows a ganglion cyst (yellow arrow) over the lateral aspect of the foot over lying cuboid arising from the sinus tarsi. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 27.

Sagittal PDFS and T1 MRI of ankle showing bone marrow oedema with in the Os trigonum (blue arrow) with posterior capsular thickening (yellow arrow) and oedema in keeping with posterior impingment. Note the prominent anterior osteophytosis of tibia (red arrow) with bone morrow oedema of tibial plafond suggesting anterior impingment. There is also obliteration of sinus tarsi fat (green arrow) on T1 sequence in keeping with sinus tarsi syndrome. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

3.11. Posterior ankle impingement syndrome

Posterior-ankle impingement (PAI) syndrome arises from the repetitive compression of soft tissues or bony processes in the foot, leading to bone contusions and synovitis in the posterior regions of the tibiotalar and subtalar joints. This condition is often associated with certain anatomical variations, notably the os trigonum. The syndrome is particularly prevalent in activities that involve a lot of foot plantar flexion, such as ballet dancing, soccer, or basketball.⁴⁵

On plain radiograph there can be os trigonum or a Stieda process. MRI is the preferred method to evaluate the bony and soft tissue structures in detail (Fig. 27). MR images can highlight bone marrow oedema in the os trigonum and its synchondrosis with the posterior talar tubercle.

3.12. Tarsal coalition

Tarsal coalition refers to the full or partial union of two or more midfoot and hindfoot bones, stemming from developmental fusion rather than acquired causes like trauma or rheumatoid arthritis.

The most common tarsal coalitions (about 90% of cases) involve the calcaneonavicular (Fig. 6) or talocalcaneal joints (Fig. 28). The former is best viewed on oblique radiographs and might have an associated hypoplastic talus. Anteater nose sign due to tubular elongation of anterior process of calcaneum over lapping navicular sometimes visible on lateral radiographs in calcaneonavicular coalitions. The latter is best seen laterally and may show signs like the C-sign and talar beak sign (Fig. 26).

Fig. 28.

a) lateral view of the ankle showing a continuous C-shaped arc (blue arrows) which is formed by the medial outline of the talar dome and posteroinferior aspect of the sustentaculum tali due to their bridging. b) PDFS sagittal and coronal MRI of ankle showing an extra-articular, non-osseous coalition located posterior to the middle facets of the talus and calcaneus with an os sustentaculum (white arrow). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

CT provides precise details about the coalition’s extent and is useful for planning resection or deciding on arthrodesis, especially in cases where plain radiographs aren’t definitive, like talocalcaneal coalition. MRI helps to distinguishes between osseous, fibrous, and cartilaginous coalitions. In osseous coalitions, there’s continuity of marrow signal. Fibrous coalitions exhibit close surfaces with fat plane loss and low signals on T1/T2 sequences.

3.13. Tendinosis/tenosynovitis

Tendinopathy and tenosynovitis are common in the foot and ankle region (Fig. 8). They can be associated with mechanical factors, chronic repetitive stress, overuse, mucoid degeneration or can be secondary to inflammatory arthritis. Tendinopathy can also be associated with previous surgeries, especially for ankle fractures, resulting in tendon impingement by metalwork.⁴⁶

US & MRI are the imaging investigations of choice to evaluate tendons in the ankle and foot. MRI is used for diagnostic purposes but is generally less accurate than ultrasound, as ultrasound has better spatial resolution. Ultrasound has an edge over MRI in assessment of chronic tendinopathy, to assess its activity and for image guided intervention.⁴⁷

The Achilles tendon is a large weight-bearing tendon and receives insertion from both gastrocnemius and soleus muscles. Achilles tendinosis is very common and has a multifactorial aetiology including the role of genetics, other disease processes and tendon injury. There are a number of theories for the development of tendinosis, but it is likely to be due to failed healing because of multiple factors.⁴⁸ Non insertional Achilles tendinosis shows fusiform tendon thickening and neovascularization, with vessels usually inserting in the tendon through the ventral side of it from the Kager’s fat (Fig. 4).

3.14. Painful heel (plantar fasciitis, plantar fibromatosis)

Plantar fasciitis, a very common reason for heel pain is a low-grade inflammation involving the plantar aponeurosis and the perifascial structures. It is characterized by chronic deep pain in the subcalcaneal area and along the medial aspect of the plantar surface of the foot. In general, the causative factors can be mechanical, degenerative, and systemic. Mechanical causes of plantar fasciitis include overuse syndromes, various foot deformities, tightness of Achilles tendon, increased body weight, leg length inequality, and externally rotated lower extremity. Degenerative factors include age-related increases in foot protonation and atrophy of the heel fat pad. Systemic factors include various rheumatoid disorders, especially rheumatoid arthritis, seronegative spondyloarthropathies, and gout.⁴⁹

Plantar fibromatosis, also termed Ledderhose disease, is a benign growth of the plantar fascia and falls under the category of musculoskeletal fibromatosis. It can manifest in both children and adults, with males being more commonly affected. It’s related to other fibroproliferative conditions like Dupuytren disease, keloids, and Peyronie disease. Patients often present with nodules or masses in the medial band of the plantar fascia. These lesions may cause discomfort due to their size or by affecting adjacent foot structures (Fig. 17).

On ultrasound, the lesions are generally seen as hypo to mixed echogenicity formations, appearing as distinct, spindle-shaped, and multinodular thickening of the plantar fascia, distinct from the calcaneal insertion. Most don’t display vascularity on Doppler ultrasound. On the MRI scan, these masses present as well defined regions of spindle-shaped or multinodular thickening of the plantar fascia, way from the calcaneal origin. On T1, they exhibit iso to low signal relative to muscle, while on T2, the signal ranges from low to intermediate compared to muscle. Contrast-enhanced T1 scans reveal varying degrees of enhancement.⁴⁹

3.15. Flat foot

Adult acquired flatfoot deformity typically affects middle-aged and elderly women, resulting in foot pain, malalignment, and loss of function. The degeneration of the posterior tibialis tendon (including tenosynovitis, tendinosis, tendon elongation and tearing) is thought to the primary reason, which normally functions to maintain the talonavicular joint at the apex of the three arches of the foot. The tendon dysfunction commonly leads to secondary damage of the spring ligament and talocalcaneal ligaments and may be associated with injury to the deltoid ligament, plantar fascia, and other soft-tissue structures that provide midfoot stabilisation. Failure of multiple stabilizers appears to be necessary for development of the characteristic planovalgus deformity associated in adult acquired flatfoot, with a depressed plantar-flexed talus bone, hindfoot and/or midfoot valgus, and an everted flattened forefoot. The four-tier staging system for the adult acquired flatfoot deformity emphasizes physical examination findings and metrics of foot malalignment.⁵⁰

3.16. Septic arthritis

Septic arthritis can be a destructive process with high morbidity therefore early diagnosis is crucial. There is irreversible joint function in 25–50% with 75% of survivors having significant functional deficit in the affected joint. Septic arthritis can occur due to haematogenous spread secondary to indwelling cather, endocarditis or other system infection, direct inoculation following surgery, injection or open injury and spread from adjacent cellulitis, bursitis, infected ulcer or osteomyelitis.⁵¹ Clinical diagnosis is often the mainstay when considering septic arthritis however imaging can be used for confirmation, identify target for sampling, assess for complications and for follow-up. Radiographic assessment is limited early in the disease barring the identification of effusion on x-ray which can be utilized in the ankle if large enough. If septic arthritis progression the erosions and bone destruction may be identified on x-ray. Ultrasound can be used to identify an effusion, synovitis, collection followed US guided aspiration if appropriate.⁵² Appearances on MRI include joint effusion, synovial thickening, peri-articular bone marrow oedema with high fluid signal and low T1 signal. An associated collection and subcutaneous oedema may be seen. On post-contrast imaging there may be synovial, bone marrow and fluid enhancement. Erosions, chondral loss and destruction can be seen as the disease process progresses.^51,53

4. Conclusion

Imaging plays a key role in evaluating non acute ailments of the foot and ankle region. The modalities used include plain radiographs, ultrasound, MRI, CT and uncommonly, nuclear medicine studies. The imaging should be tailored to the anatomical site and the nature of the suspected abnormality for effective use of imaging resources. Imaging is also necessary for diagnostic and therapeutic intervention. This article elaborates on different causes of non acute foot and ankle pain which include mechanical, inflammatory, infective, neoplastic, infective, and miscellaneous causes.

CRediT authorship contribution statement

Vijay Ram Kumar Papineni: Lead author. Matthew Mariathas: draft & literature search. Sandeep Singh Sidhu: draft and literature search. Basavaraj Chari: review and, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.