Clinical Applications for Radiotracer Imaging of Lower Extremity Peripheral Arterial Disease and Critical Limb Ischemia

Abstract

Peripheral arterial disease (PAD) is an atherosclerotic occlusive disease of the non-coronary vessels that is characterized by lower extremity tissue ischemia, claudication, increased prevalence of lower extremity wounds and amputations, and impaired quality of life. Critical limb ischemia (CLI) represents the severe stage of PAD and is associated with additional risk for wound formation, amputation, and premature death. Standard clinical tools utilized for assessing PAD and CLI primarily focus on anatomical evaluation of peripheral vascular lesions or hemodynamic assessment of the peripheral circulation. Evaluation of underlying pathophysiology has traditionally been achieved by radiotracer-based imaging, with many clinical investigations focusing on imaging of skeletal muscle perfusion and cases of foot infection/inflammation such as osteomyelitis and Charcot neuropathic osteoarthropathy. As advancements in hybrid imaging systems and radiotracers continue to evolve, opportunities for molecular imaging of PAD and CLI are also emerging that may offer novel insight into associated complications such as peripheral atherosclerosis, alterations in skeletal muscle metabolism, and peripheral neuropathy. This review summarizes the pros and cons of radiotracer-based techniques that have been utilized in the clinical environment for evaluating lower extremity ischemia and common pathologies associated with PAD and CLI.

INTRODUCTION

Peripheral arterial disease (PAD) is an occlusive artery disease most commonly caused by progressive atherosclerosis that can result in significant health-related problems for the lower extremities [1, 2]. A resting ankle-brachial index (ABI) of ≤ 0.9 is used as a hemodynamic definition of PAD, and approximately one-third of PAD patients suffer from claudication while the other two-thirds remain asymptomatic or have atypical symptoms. The prevalence of PAD increases with increasing age and the number of cardiovascular risk factors/comorbidities (i.e. diabetes mellitus (DM), smoking, chronic kidney disease, hyperlipidemia, hypertension, etc.) [3], and more than 200 million individuals worldwide suffer from PAD [4, 5]. Critical limb ischemia (CLI) is considered the severe end of the PAD spectrum and is characterized by chronic ischemic rest pain, ulcers, or gangrene attributable to arterial occlusive disease [1, 2]. Of patients with PAD, 1–3 percent initially present with CLI and 5–10 percent progress to CLI within 5 years [2]. Among CLI patients who are not candidates for revascularization, 25 percent will have died and another 25 percent will have had a major amputation within 1 year following diagnosis [2]. In addition to lower extremity ischemia, other complications often present in the setting of PAD and CLI such as infection, osteomyelitis, and Charcot neuropathic osteoarthropathy (CN). Due to the complex interplay of multiple factors contributing to the progression of PAD and CLI, quantitative non-invasive imaging techniques capable of detecting underlying pathophysiology as well as responses to medical treatment could greatly improve the evaluation of high-risk PAD and CLI patients and potentially guide therapeutic interventions.

The radiotracer-based imaging approaches single photon emission computed tomography (SPECT) and positron emission tomography (PET) have been utilized in the clinical setting for decades for various cardiovascular applications such as myocardial perfusion assessment, but more recently have emerged as approaches for assessing PAD pathophysiology [6]. SPECT and PET allow for three-dimensional (3D) functional imaging of targeted radiolabeled probes via non-invasive detection of emitted gamma rays. While radiotracer-based approaches possess high sensitivity for molecular imaging of PAD pathophysiology, both SPECT and PET have relatively low spatial resolution when compared to magnetic resonance (MR) and computed tomography (CT) imaging modalities. This issue has been addressed in recent decades by combining high-sensitivity functional imaging (SPECT, PET) with high-resolution anatomical imaging (CT, MR) to produce hybrid scanners (e.g., SPECT/CT, PET/CT, and PET/MR)[7, 8]. Hybrid scanners offer improved quantitative assessment of radiotracer retention within specific anatomical sites, as well as decreased image artifacts from surrounding non-target tissues, and corrections for scatter, attenuation, and partial volume effects. While standard imaging modalities such as Duplex ultrasound, digital subtraction angiography (DSA), CT angiography, and MR imaging and angiography exist for assessing PAD, these modalities primarily assess vascular anatomy and/or large vessel (arterial) flow, whereas SPECT and PET imaging offer complementary assessment of various molecular processes that may facilitate identification of underlying pathophysiology within lower extremity vasculature and skeletal muscle. Additionally, patients with PAD often present with comorbidities such as DM and chronic kidney disease, which may limit the application of CT and MR imaging techniques that require iodinated- or gadolinium-based contrast agents, whereas radiotracer imaging techniques do not necessitate intravascular contrast agents for functional imaging. This review summarizes the historical, current, and emerging radiotracer-based imaging techniques that have been utilized for evaluating various pathologies associated with PAD and CLI, with particular emphasis on non-invasive imaging of skeletal muscle perfusion, osteomyelitis, infection, and CN.

RADIOTRACER IMAGING OF LOWER EXTREMITY PERFUSION

A variety of radiotracer-based approaches have been developed and utilized since the 1940s for evaluation of lower extremity perfusion, which have included a mixture of intramuscular, intravenous, and intra-arterial administration methods [9–14]. However, by the 1980s, the majority of lower extremity perfusion imaging investigations were performed using intravenous administration of thallium-201 [²⁰¹Tl] as scintigraphy became increasingly utilized for evaluation of myocardial perfusion. [²⁰¹Tl] specifically became a widely utilized radiotracer due to its redistribution characteristics that allow for evaluation of skeletal muscle perfusion under stress and rest conditions with only a single intravenous injection. Initial scintigraphy studies demonstrated that exercise stress distribution and rest redistribution of [²⁰¹Tl] allowed for the diagnosis of PAD [15, 16] while Hamanaka et al. [17] later validated the findings of [²⁰¹Tl] scintigraphy in the lower extremities by demonstrating a significant correlation between [²⁰¹Tl]- and [^99mTc]-MAA-derived measures (r=0.979). Additional data from scintigraphy investigations also suggest that radiotracer imaging may provide a sensitive approach for non-invasively detecting PAD and perfusion abnormalities in DM patients who present with normal lower extremity hemodynamics (i.e. ABIs). For example, studies have demonstrated that [²⁰¹Tl] scintigraphy of the lower limbs can detect skeletal muscle perfusion defects in asymptomatic diabetic patients with normal ABIs [18] and without claudication [19], and that [²⁰¹Tl]-derived perfusion abnormalities are correlated with clinical manifestations of DM [19]. Furthermore, the first study utilizing [²⁰¹Tl] for 3D SPECT imaging demonstrated significantly higher skeletal muscle perfusion within specific muscle compartments of the lower extremities in healthy subjects when compared to PAD patients [20] while more recent work has demonstrated the ability of hybrid [²⁰¹Tl] SPECT/CT imaging to detect serial changes in regional skeletal muscle perfusion that are associated with active angiogenesis and arteriogenesis [21].

The [^99mTc]-labeled sestamibi and tetrofosmin have emerged as the clinical standard for myocardial perfusion imaging, and the most commonly used radiotracers for evaluating lower extremity skeletal muscle perfusion [6]. [^99mTc]-sestamibi and [^99mTc]-tetrofosmin possess advantages over [²⁰¹Tl] due to a shorter half-life (~6 hr) relative to [²⁰¹Tl] (~73 hr) and higher energy emission (140 keV) relative to [²⁰¹Tl] (78 keV), which enhances count detection, reduces scatter, and results in less attenuation on gamma cameras. Furthermore, both [^99mTc]-sestamibi and [^99mTc]-tetrofosmin redistribute less when compared to [²⁰¹Tl] [22]. Scintigraphy studies utilizing [^99mTc]-sestamibi have been effective at detecting manifestations of PAD, including abnormalities in skeletal muscle perfusion that are associated with upstream arterial occlusions [23]. These studies have revealed sensitivity of 91% and specificity of 94% for diagnosing PAD [24] and have also been shown to significantly correlate with peripheral angiography and Doppler measurements [24]. Within the setting of DM, Celen et al. [25] utilized [^99mTc]-sestamibi imaging to show that DM patients had lower perfusion reserve in response to dorsi/plantar flexion exercise compared to non-DM patients. [^99mTc]-sestamibi scintigraphy has also been shown to be beneficial in more challenging clinical scenarios, such as detecting perfusion defects in asymptomatic patients with mild atherosclerosis and normal Doppler blood flow in the legs [26]. More recently, [^99mTc]-tetrofosmin has also been applied in SPECT/CT imaging studies, where Stacy et al. [27] demonstrated reduced regional calf perfusion under resting conditions in the setting of unilateral peripheral occlusion.

Although numerous radiotracer-based approaches have been applied for evaluation of PAD, research focusing on perfusion imaging in the setting of CLI continues to be an emerging area of investigation due to the critical role that tissue perfusion plays in wound healing for CLI patients. Specifically, recent work has revealed that [^99mTc]-tetrofosmin [28, 29] and [^99mTc]-MAA [29] scintigraphy are useful for evaluating improvements in lower extremity perfusion in response to cell therapy in patients with non-healing foot ulcers. Additionally, recent application of [^99mTc]-tetrofosmin SPECT/CT imaging has demonstrated that SPECT/CT detects perfusion defects within ulcerated regions of the foot in CLI patients [30], serves as a complementary tool to traditional angiographic and hemodynamic measures by evaluating physiological changes in foot perfusion after endovascular revascularization (Figure 1) [31], and potentially possesses prognostic value for predicting amputation-free survival [32]. Thus, emerging research in the CLI population highlights the potential of radiotracer-based imaging for understanding diabetic wound healing, risk for amputation, and the effectiveness of targeted therapies.

Figure 1.

[^99mTc]-tetrofosmin SPECT/CT perfusion imaging in a CLI patient before and after lower extremity revascularization. A) Transaxial, (B) sagittal, and (C) coronal views of SPECT/CT images demonstrate improvements in foot perfusion that occur from pre-revascularization (left) to post-revascularization (right). These images represent previously unpublished data.

In addition to scintigraphy- and SPECT-based imaging approaches, various PET imaging techniques have also been utilized for assessing lower extremity skeletal muscle perfusion in the setting of PAD. Depairon et al. [33] performed the first study focused on PET imaging of PAD by using inhalation of [C¹⁵]O₂ and [¹⁵O₂] and revealed asymmetrical patterns of calf muscle blood flow in PAD patients during ankle flexion. The same research team later used the bolus technique of H₂[¹⁵O] and [¹⁵O₂] with PET in patients suffering from severe intermittent claudication to show that PET-derived measures of muscle blood flow under resting conditions and immediately after exercise were closely correlated with the traditional blood flow assessment technique of venous occlusion plethysmography (r=0.71) [34]. Subsequent to these initial PET studies, Burchert et al. [35] were the first to utilize H₂¹⁵O PET imaging to successfully quantify absolute calf muscle perfusion at rest, during dorsi/plantar flexion exercise, and after infusion of prostaglandin E1 in healthy subjects and PAD patients. Several clinical and pre-clinical studies later validated PET perfusion imaging with other more established measures of muscle perfusion [36–38]. For example, in the clinical setting, Schmidt et al. [36] showed that PET-derived measures of calf muscle flow reserve after adenosine infusion were highly correlated with thermodilution-derived values (r=0.82), while Scremin et al. [37] revealed that H₂[¹⁵O] PET imaging possessed similar sensitivity and specificity for detecting calf muscle ischemia when compared to TcPO₂ measurements. While clinical application of PET imaging has proven to correlate well with standard indices of blood flow and perfusion, pre-clinical work by Fischman et al. [38] further demonstrated a significant correlation between PET-derived measures of muscle perfusion and the gold standard technique of microsphere assessment (r=0.99). Ongoing developments with modern day PET imaging systems should bring additional opportunities for non-invasive imaging of PAD and improve the quantitative assessment of lower extremity perfusion in the setting of limb ischemia.

RADIOTRACER IMAGING OF OSTEOMYELITIS AND SOFT TISSUE INFECTION

PAD and CLI are often accompanied by DM in the clinical setting [10], where the combination of peripheral neuropathy and occlusive disease exacerbates the prevalence of diabetic foot ulceration and associated complications [39]. Soft tissue infections and osteomyelitis are potential complications following ulceration, with the latter being particularly related to poor treatment outcomes [40]. Therefore, early diagnosis of both the underlying infection and the type of infection is critical for tailoring treatment plans and improving outcomes. However, diagnosis of osteomyelitis and soft tissue infections is often challenging given that peripheral neuropathy, neuro-osteoarthropathy, and PAD can either mimic or diminish inflammatory findings of osteomyelitis [41]. While MRI is commonly utilized for identifying the presence of foot infections, multiple radiotracer-based approaches have demonstrated good sensitivity and specificity for diagnosing PAD-related lower extremity infections [42].

Three-phase bone scintigraphy is a common radiotracer-based technique used for diagnosis of foot osteomyelitis in PAD patients that applies [^99mTc]-labeled diphosphonate radiotracers such as [^99mTc]-methylene diphosphonate (MDP), [^99mTc]-diphosphono-1,2-propanodicarboxylic acid (DPD), and [^99mTc]-hydroxymethane diphosphonate (HDP), where a positive scan for osteomyelitis shows increased uptake in all three phases: early local hyperperfusion (blood flow phase), soft tissue hyperemia (blood pool phase), and increased bony uptake (bone phase) [43]. Bone scintigraphy has demonstrated high sensitivity (~90%) but poor specificity (~50%) for diagnosing osteomyelitis due to non-specific radiotracers that also target osteogenesis, and owing to the inherently low spatial resolution of scintigraphy [41]. To overcome the limitation of low spatial resolution, hybrid SPECT/CT imaging of radiolabeled diphosphonates has emerged in the last decade for non-invasive evaluation of foot infections. For example, Horger et al. [44] compared [^99mTc]-DPD SPECT/CT scans to bone scintigraphy in patients with suspected bone infection and found that SPECT/CT enhanced the diagnostic performance of three-phase bone imaging by reducing the number of false positives and equivocal results. Additionally, SPECT/CT imaging of sequential dual-isotope [^99mTc]-HDP and indium-111 [¹¹¹In]-labeled leucocytes has also been shown to improve the diagnosis and localization of infection when compared to conventional approaches such as CT, scintigraphy, and MRI [45, 46].

Along with imaging of radiolabeled diphosphonates, radiolabeled white blood cells (WBCs) have also been extensively utilized for imaging of soft tissue infections and osteomyelitis due to the role that WBCs play in mediating inflammatory responses and their innate ability to migrate to sites of infection. Early studies utilizing [¹¹¹In]-labeled WBC scintigraphy demonstrated that this approach possessed good sensitivity (79%) and specificity (78%) for detecting osteomyelitis [47], offered higher specificity for diagnosing osteomyelitis when compared to three-phase bone scintigraphy [48, 49], and allowed for serial monitoring of responses to antibiotic treatment [50]. In addition to [¹¹¹In] labeling of WBCs, multiple strategies for [^99mTc] labeling of WBCs have emerged for imaging of infection. For example, [^99mTc]-hexamethylpropylamineoxine (HMPAO)-labeled WBCs with scintigraphy have demonstrated diagnostic accuracy of 88% [51] and 93% [52] for detecting osteomyelitis, and this approach has also revealed higher sensitivity for diagnosing osteomyelitis when compared to MRI (92% vs. 78%) [53]. [^99mTc]-HMPAO labeled WBCs have also been utilized for 3D imaging with SPECT, which has shown that [^99mTc]-HMPAO-WBC SPECT imaging assists in characterizing the extent of infection and differentiates between soft tissue infection and osteomyelitis [54]. The addition of co-registered anatomical CT images with WBC targeted SPECT images has also demonstrated prognostic value of hybrid SPECT/CT imaging for predicting clinical outcomes in diabetic patients with foot infections [55], sensitivity for determining osteomyelitis remission after antibiotic treatment that ranges from 90–100% [56, 57], and excellent sensitivity (100%) and specificity (91.5%) for detecting relapse after the discontinuation of antibiotic treatment [58].

Using scintigraphy [59, 60] and SPECT/CT imaging [61], studies have also utilized [^99mTc]-labeled monoclonal anti-granulocyte antibodies to target specific antigens expressed on the surface of granulocytes for assessment of soft tissue infections and osteomyelitis, which has demonstrated sensitivity, specificity, and accuracy levels that are similar to [¹¹¹In]-WBC imaging [60]. Harwood et al. [62] specifically utilized [^99mTc]-Sulesomab, an anti-granulocyte antibody Fab’ fragment, for scintigraphy and found that [^99mTc]-Sulesomab possessed significantly greater sensitivity than conventional WBC scintigraphy (92% vs. 79%) and radiography (90% vs. 62%) in the evaluation of foot infections. Although labeled monoclonal anti-granulocyte antibodies have demonstrated potential utility, several limitations of this approach exist that have limited its widespread adoption. Specifically, these antibodies possess high molecular weights (thereby limiting diffusion into infection sites) and a long plasma half-life, are cleared by the reticuloendothelial system (resulting in high uptake in liver and bone marrow), and require image acquisitions 6 hours after tracer injection to optimize target-to-background ratio for image quantification purposes [63].

In addition to imaging of labeled antibodies, gallium-67 [⁶⁷Ga] citrate has also been utilized to a small extent for imaging of foot infections with scintigraphy and SPECT/CT to evaluate suspected osteomyelitis [64–66]. While [⁶⁷Ga] citrate SPECT/CT imaging in combination with bedside bone puncture has recently demonstrated utility in the diagnosis of foot osteomyelitis and has assisted with guiding antibiotic treatment in some clinical cases [66], earlier studies revealed that [⁶⁷Ga] citrate binding is non-specific to neutrophil and macrophages [67] and that [⁶⁷Ga] imaging offers less diagnostic value when compared to conventional [¹¹¹In]-labeled WBC methods [64, 65]. Thus, as is the case with [^99mTc]-labeled antibodies, [⁶⁷Ga] citrate imaging currently remains a minimally utilized approach for evaluating lower extremity infections in PAD and CLI patients.

While numerous studies have utilized scintigraphy and SPECT/CT, PET/CT imaging possesses some advantages for the evaluation of foot infection, such as faster image acquisition times and superior spatial and temporal resolution of PET. 2-deoxy-2-[18F]fluoro-D-glucose ([¹⁸F]-FDG), which is a glucose analog that accumulates in metabolically active cells, such as inflammatory and infectious cells [68], is commonly utilized for PET imaging of diabetic foot infection and osteomyelitis (Figure 2). [¹⁸F]-FDG is an attractive PET radiotracer for imaging of infection due to its strong connection to macrophage infiltration; however, [¹⁸F]-FDG uptake is not necessarily specific to infection and may be retained in the setting of other pathologies such as malignancies, inflammation, and regenerating and traumatic processes [41]. Therefore, multiple studies have compared [¹⁸F]-FDG PET/CT to other more conventional imaging modalities for evaluating soft tissue infection and osteomyelitis of the foot, with varying degrees of diagnostic accuracy being reported. For example, Keidar et al. [69] demonstrated that co-registration of [¹⁸F]-FDG PET with CT images successfully differentiated between osteomyelitis and soft tissue foot infections, and additional work by Kagna et al. [70] revealed that [¹⁸F]-FDG PET/CT imaging had high sensitivity (100%), specificity (93%), and accuracy (96%) for diagnosing osteomyelitis. However, other research teams have reported that MRI was superior to [¹⁸F]-FDG PET in the detection of osteomyelitis [71, 72], and that serial [¹⁸F]-FDG PET imaging had lower sensitivity (43% vs. 86%), specificity (67% vs. 100%), and diagnostic accuracy (54% vs. 92%) for detecting osteomyelitis compared to [^99mTc]-WBC scintigraphy [73]. The variable diagnostic performance of [¹⁸F]-FDG PET imaging that has been reported to date may be due to the inherently low spatial resolution of PET imaging, particularly when it is not co-registered with high-resolution anatomical CT images, and/or the lack of specificity of [¹⁸F]-FDG for targeting osteomyelitis as opposed to global inflammation/infection/metabolism. Thus, while radiotracer imaging of osteomyelitis and soft tissue infection remains an active area of investigation in the PAD and CLI patient population, questions still remain regarding the role of [¹⁸F]-FDG PET imaging in the standard clinical imaging paradigm. A complete summary of the various radiotracer-based techniques for diagnosing osteomyelitis, along with their associated sensitivity and specificity values, are provided in Table 2.

Figure 2.

[¹⁸F]-FDG PET/CT imaging of soft tissue infection in the foot of a diabetic CLI patient. A) Transaxial view of [¹⁸F]-FDG PET/CT images demonstrate increased radiotracer uptake that is localized to (B) the site of the diabetic wound infected with methicillin-resistant Staphylococcus auereus (MRSA) bacterium. These images represent previously unpublished data.

Table 2.

Sensitivity and specificity of various radiotracer techniques for diagnosing lower extremity osteomyelitis.

Radiotracer	Modality	Sensitivity	Specificity	Reference
99mTc-diphosphonate	Scintigraphy	75–100%	38–56%	43, 48, 49
99mTc-diphosphonate	SPECT/CT	78–94%	47–86%	44, 45
111In-WBC	Scintigraphy	75–100%	67–89%	47, 48, 62, 64
99mTc-HMPAO-WBC	Scintigraphy	46–88%	67–100%	51–54, 73
111In-WBC	SPECT/CT	87%	68%	45
99mTc-HMPAO-WBC	SPECT	96%	100%	54
99mTc-diphosphonate + 111ln-WBC	Scintigraphy	93–100%	66–80%	45, 64
99mTc-diphosphonate + 111ln-WBC	SPECT	93%	77%	45
99mTc-diphosphonate + 111ln-WBC	SPECT/CT	95%	94%	45
99mTc-antigranulocyte antibodies	Scintigraphy	29–93%	67–85%	59, 60, 71
99mTc-antigranulocyte antibodies 99mTc-antigranulocyte antibodies	SPECT	100%	78%	61
	SPECT/CT	100%	89%	61
99mTc-Sulesomab	Scintigraphy	92%	56%	62
67Ga citrate	Scintigraphy	100%	40%	64
67Ga citrate	SPECT/CT	88%	94%	66
18F-FDG	PET	29–81%	67–93%	71–73
18F-FDG	PET/CT	100%	93%	70

FDG, fluorodeoxyglucose; HMPAO, hexamethylpropylamineoxine; WBC, white blood cell.

RADIOTRACER IMAGING OF CHARCOT FOOT

Another lower extremity complication associated with DM and PAD is the development of Charcot neuropathic osteoarthropathy (CN), often referred to as Charcot foot, which is characterized by local inflammation in the early phase of the condition, followed by destruction of the skeletal architecture of the foot and ankle in the later phases [74]. In addition to disrupting the skeletal architecture of the foot, CN also increases the risk of secondary ulceration and infection, thereby making it a major risk factor for lower extremity amputation in PAD patients [74]. The diagnosis of CN can be challenging due to conditions such as osteomyelitis, which often exist concurrently in the setting of CLI. Imaging techniques such as radiography and MRI are often utilized to evaluate CN, but radiography possesses low sensitivity and specificity (below 50%) for diagnosing the early inflammatory stages of CN [75] while MRI has difficulty in differentiating between CN and osteomyelitis [76].

Radiotracer-based approaches can be used for detecting the initial inflammatory cascade of events that occur prior to skeletal morphology changes associated with CN, thereby assisting with early diagnosis of CN. For example, [^99mTc]-MDP bone scintigraphy has been found to possess excellent sensitivity (near 100%) for diagnosing CN and can assist with ruling out osteomyelitis in the setting of CN [77]. While bone scintigraphy alone has revealed benefits in CN, studies utilizing a combination of both [^99mTc]-MDP bone imaging with either [¹¹¹In]-WBC or [^99mTc]-WBC imaging have demonstrated sensitivity of approximately 93% [78, 79] and specificity ranging 83%−97% [78–80] for differentiating between cases of soft tissue and bone infection in patients with CN, while also revealing higher specificity than conventional radiography or MRI in the diagnosis of CN [81]. In addition to the combined method of bone/WBC scintigraphy, another common dual-isotope approach for distinguishing CN from osteomyelitis is the combination of [^99mTc]-sulfur colloid bone marrow scintigraphy and [¹¹¹In]-WBC scintigraphy [82–84], which is based on the principle that both radiotracers will be retained in bone marrow, but the presence of osteomyelitis will stimulate additional [¹¹¹In]-WBC uptake while also suppressing sulfur colloid uptake. Thus, studies have shown that uptake of both radiotracers in the bone marrow is indicative of CN, whereas a scan that is positive for [¹¹¹In]-WBC is suggestive of osteomyelitis. The reported accuracy of this combined WBC/bone marrow imaging approach has been excellent (88–98%), and this method serves as the gold standard for detecting the presence of foot infection versus osteoarthropathy [82, 83].

Along with scintigraphy, [¹⁸F]-FDG PET imaging has emerged in recent years for evaluation of CN, where cases of CN commonly present as low-grade diffuse [¹⁸F]-FDG uptake that is distinguishable from both the normal foot that has low uptake, as well as from cases of osteomyelitis that demonstrate focal intense ¹⁸F-FDG uptake [85]. Several studies have demonstrated superior accuracy of [¹⁸F]-FDG PET compared to MRI in the diagnosis of Charcot lesions (94–95% vs 75–79%) [85, 86]. PET/CT imaging of [¹⁸F]-FDG has further demonstrated higher specificity (100%) than MRI (63.6%) for the diagnosis of osteomyelitis in patients with chronic CN [87]. In addition to comparison with MRI, PET/CT imaging has also revealed that [¹⁸F]-FDG uptake can occur without the presence of concurrent bony abnormalities, thus demonstrating an inflammatory origin associated with CN (Figure 3) [88]. Furthermore, [¹⁸F]-FDG PET/CT imaging has shown potential for quantifying and non-invasively monitoring serial changes in the inflammatory process associated with CN [89].

Figure 3.

[¹⁸F]-FDG PET/CT imaging of a patient with Charcot foot. Sagittal and axial views of (A) CT images, (B) [¹⁸F]-FDG PET images, and (C) fused [¹⁸F]-FDG PET/CT images of a patient with Charcot joint disease. Although diffuse [¹⁸F]-FDG uptake was present, no Charcot-related bony abnormalities are revealed by CT imaging, thus revealing a potential underlying inflammatory origin of Charcot disease prior to the presence of structural alterations. This research was originally published in Clinical Nuclear Medicine [88], © 2011 Wolters Kluwer Health, Inc.

EMERGING CLINICAL APPLICATIONS FOR RADIOTRACER IMAGING OF PAD AND CLI

While a number of physiological targets have been established for decades to non-invasively assess PAD and CLI, novel radiotracers and applications for SPECT and PET are continuing to emerge that could eventually play a role in the standard clinical paradigm for evaluating PAD (Table 1). Specifically, imaging of atherosclerosis has recently garnered attention, with PET imaging of [¹⁸F]-FDG being the most commonly utilized radiotracer for assessing the inflammatory status of peripheral artery plaque [90]. Studies have revealed an increase in the prevalence of [¹⁸F]-FDG uptake within lower extremity arterial plaque with increasing age [91–94], a positive association between arterial uptake of [¹⁸F]-FDG and measures of arterial stiffness [95], and the ability of [¹⁸F]-FDG PET/CT imaging to non-invasively monitor reductions in inflammation within atherosclerotic lesions in response to lifestyle interventions [96] and medical therapy [97]. While [¹⁸F]-FDG has shown potential for assessing vascular inflammation, carbon-11 [¹¹C]-acetate is another radiotracer that has demonstrated feasibility for evaluating mechanisms of atherosclerotic plaque development by allowing for assessment of fatty acid synthesis within the vessel wall, thereby representing a potential approach for evaluating intra-plaque metabolic activity [98]. Additionally, [¹⁸F]-sodium fluoride (NaF), which targets active calcium deposition during the later stages of plaque development, has demonstrated value for assessing plaque burden in the femoral arteries [99] and has shown a significant correlation to both calcified plaque burden and the presence of multiple cardiovascular risk factors [100]. Although standard imaging techniques for imaging of atherosclerosis (i.e. ultrasound, angiography, and optical coherence tomography) provide high spatial resolution imaging of plaque anatomy, SPECT and PET applications for molecular imaging of atherosclerotic plaques continue to expand and offer novel insight into disease progression and cardiovascular risk.

Table 1.

Radiotracers for clinical evaluation of peripheral arterial disease and critical limb ischemia.

Physiological Target	Radiotracer	Modality	Reference
Skeletal muscle perfusion	24Na	Geiger meter	9–11
	133Xe	Geiger meter	11, 12
	131I-MAA	Geiger meter	13, 14
	99mTc-MAA	Scintigraphy	17, 29
	201Tl	Scintigraphy	15–20
	201Tl	SPECT	21, 22
	99mTc-sestamibi	Scintigraphy	23–26
	99mTc-tetrofosmin	Scintigraphy	28, 29
	99mTc-tetrofosmin	SPECT	27, 30–32
	15O2	PET	33, 34
	C15O2	PET	33
	H215O	PET	35–38
Osteomyelitis & infection	99mTc-diphosphonate	Scintigraphy	43, 49, 50, 60
	99mTc-diphosphonate	SPECT	44
	99mTc-diphosphonate & 111In-WBC	SPECT	45, 46
	111In-WBC	Scintigraphy	47–50, 64, 65
	99mTc-HMPAO-WBC	Scintigraphy	51–53, 73
	99mTc-HMPAO-WBC	SPECT	54–58
	99mTc-antigranulocyte antibodies	Scintigraphy	59, 60
	99mTc-antigranulocyte antibodies	SPECT	61
	99mTc-Sulesomab	Scintigraphy	62
	67Ga citrate	Scintigraphy	64
	67Ga citrate	SPECT	65, 66
	18F-FDG	PET	69–73
Charcot foot	99mTc-MDP	Scintigraphy	77
	99mTc-MDP & 111In-WBC	Scintigraphy	78, 80
	99mTc-MDP & 99mTc-HMPAO-WBC	Scintigraphy	79
	99mTc-sulfur colloid & 111In-WBC	Scintigraphy	82–84
	18F-FDG	PET	85–89
Atherosclerosis	18F-FDG	PET	91–97
	11C-acetate	PET	98
	18F-NaF	PET	99, 100
Peripheral neuropathy	13N-ammonia & 6-18F-fluorodopamine	PET	101
Skeletal muscle metabolism	18F-FDG	PET	102
Ischemic tissue viability	18F-FDG	PET	103

FDG, fluorodeoxyglucose; HDP, hydroxymethane diphosphonate; HMPAO, hexamethylpropylamineoxine; MAA, macroaggregates of albumin; MDP, methylene diphosphonate; WBC, white blood cell.

Aside from targeted imaging of lower extremity vascular physiology, the feasibility of radiotracer-based imaging of sympathetic innervation in the feet has also been assessed in diabetic patients with peripheral neuropathy. Specifically, Tack et al. [101] applied dual isotope [¹³N]-ammonia and 6-[¹⁸F]-fluorodopamine PET imaging to investigate the relationship between perfusion and sympathetic innervation in the diabetic foot and found impaired flow-corrected 6-[¹⁸F]-fluorodopamine uptake that was suggestive of regional sympathetic denervation in the setting of peripheral neuropathy. Additional PET imaging investigations have also utilized [¹⁸F]-FDG for evaluating skeletal muscle metabolism, which has demonstrated impaired calf muscle glucose uptake in PAD patients with intermittent claudication [102], as well as decreased [¹⁸F]-FDG uptake in the setting of non-viable ischemic foot tissue [103]. While various radiotracer-based approaches are emerging for novel physiological evaluation of PAD in the clinical environment, additional research in pre-clinical models of limb ischemia, which are outside the scope of this review, continue to reveal potential for other molecular imaging strategies, such as targeted imaging of lower extremity skeletal muscle angiogenesis [104], which may one day play a role in the clinical evaluation of PAD and CLI.

SUMMARY

Compared to other clinical imaging tools that primarily measure vascular anatomy and hemodynamics, radiotracer-based imaging plays a unique role in the physiological evaluation of the lower extremities and continues to evolve and demonstrate potential in the setting of PAD and CLI. Considering the complex and multifactorial nature of many PAD- and CLI-related complications, such as lower extremity ischemia, osteomyelitis, and Charcot neuropathic osteoarthropathy, ongoing advancements in hybrid SPECT/CT, PET/CT, and PET/MR imaging systems should facilitate development of novel molecular imaging strategies that are capable of high sensitivity targeting and detection of the underlying pathophysiology associated with PAD and assist with the non-invasive serial monitoring of responses to medical treatment.