How to evaluate incidental extracardiac findings on hybrid PET and SPECT/CT imaging?

Abstract

Cardiac SPECT-CT and PET-CT studies often have clinically actionable non-cardiac pathologies, making thorough interpretation critical. The American Society of Nuclear Cardiology (ASNC) emphasizes importance of reviewing both cardiac and extracardiac regions to ensure that significant incidental findings are not overlooked. Although CT images used for attenuation correction are often low in resolution and lack diagnostic detail, they can still reveal incidental findings that warrant further evaluation with dedicated imaging. In this manuscript, we highlight clinically relevant extracardiac and limited upper abdominal findings that may be of value to the interpreting physicians.

Single-photon emission computed tomography (SPECT) and positron emission tomography (PET) are widely used mainly for the evaluation of coronary artery disease, cardiac amyloidosis, and cardiac infection and inflammation. Modern hybrid systems integrate CT with both SPECT and PET, providing attenuation correction and anatomic localization. As a result, extracardiac findings are frequently encountered during routine cardiac imaging.

For SPECT imaging, the American College of Cardiology provides a class IIa recommendation for the use of attenuation correction to improve image quality and diagnostic accuracy.¹ Additionally, the 2024 European Society of Cardiology guidelines recommend measuring the coronary artery calcium score from unenhanced chest CT (used for attenuation correction) to enhance the detection of coronary artery disease.² Attenuation correction for PET on the other hand is integral to the modality and is typically performed with every study. Accordingly, detecting clinically actionable non-cardiac pathologies on either CT or nuclear images is essential for interpretation of SPECT-CT and PET-CT studies.

From a large retrospective study of 1506 patients, extracardiac findings were identified in 55.1% of patients, with major findings reported in 14.1% of these cases 53.3% being of previously unknown aetiology.³ Similarly, a study investigating PET/CT imaging found a 16.5% incidence of extracardiac findings (67 out of 406 patients), with 1.9% of these representing malignant aetiologies.⁴ Given that age is a well-established risk factor for increased malignancy rates, the detection of such extracardiac findings becomes even more clinically relevant in elderly populations.

The American Society of Nuclear Cardiology advocates for a systematic review of all image data, including extracardiac regions, to ensure comprehensive patient care.⁵ However, the CT images used for attenuation correction often have low resolution and may require confirmation with dedicated cross-sectional imaging. Structured reporting templates further facilitate consistent documentation, allowing for appropriate and timely follow-up.

This series of cases/images aims to highlight common and clinically relevant extracardiac thoracic and limited upper abdominal findings that may be observed during cardiac SPECT-CT and PET-CT imaging and underscore their significance in patient management.⁶

Breast

Both benign and malignant breast lesions may show incidental tracer uptake. Focal asymmetric uptake is highly suspicious of breast malignancy (Figures 1 and 2) while bilateral uptake may be found in non-malignant processes, such as gynaecomastia in males or in lactating females (Figure 3).⁷ Hormonal transitions such as in transgender patients on hormonal replacement therapy can induce glandular proliferation of the breasts with mild tracer uptake (Figure 4). Soft tissue benign masses such as seromas can also be occasionally identified. (Figure 5)

Figure 1.

An elderly female underwent stress Rb PET/CT/CT imaging for evaluation of chest pain, which incidentally demonstrated tracer uptake in the left breast. Corresponding CT attenuation imaging identified a breast mass. Subsequent biopsy confirmed oestrogen and progesterone receptor positive and HER 2 negative (ER/PR+) breast carcinoma.

Figure 2.

A former smoker with hypertension, dyslipidemia, type 2 diabetes, and obesity and known ischaemic heart disease with three-vessel coronary artery disease underwent bypass grafting (internal mammary artery to LAD and venous graft to the RCA). Tc-99m SPECT was performed to evaluate chest pain. Myocardial perfusion showed minimal non-transmural necrosis in the mid-inferior segment and mild ischaemia in the apical, inferoapical, and mid-inferior segments. It also revealed intense uptake in the right breast (arrow), for which biopsy was performed and malignancy was confirmed.

Figure 3.

Tc-99m SPECT/CT performed in a lactating patient revealed increased uptake in the breast region.

Figure 4.

A middle-aged transgender male to female patient on hormonal replacement therapy underwent stress Tc-99m SPECT/CT which revealed increased uptake in the breast tissue.

Figure 5.

A 5.7 × 3.5 × 2.8 cm mass in the right breast with an average attenuation of 16 Hounsfield units, most consistent with a seroma.

Patients with breast implants can have exaggerated anterior wall attenuation artefact which should be recognized especially when CT attenuation correction is not available (Figure 6).

Figure 6.

This middle-aged female with a history of unilateral breast implant status post-radical mastectomy for breast cancer underwent stress Tc-99m SPECT/CT for evaluation of chest pain. A large anterior perfusion defect was noted in both resting and stress images (left panel) which improved with attenuation correction (right panel). Breast implants can cause significant attenuation artefacts that should be recognized, especially in settings where Tc-99m SPECT/CT attenuation correction is unavailable.

Mediastinum

Mediastinal abnormalities are sometimes incidentally noted during imaging for unrelated complaints such as chest pain. For instance, elevated hemidiaphragms and hiatal hernias may be visualized (Figures 7 and 8). Tracer avid anterior mediastinal mass can be seen with thymomas (Figure 9) or intrathoracic goiters with retrosternal extension (Figure 10), the latter was seen extending from the thyroid tissue and eventually required surgical excision. Lesions such as oesophageal adenocarcinomas can also demonstrate uptake and should be further evaluated on CT images (Figure 11) with follow-up dedicated evaluation. Additionally, paraspinal masses may be detected in these patients (Figure 12).

Figure 7.

Patient with chest pain underwent Tc-99m SPECT/CT, which revealed elevated right-sided diaphragm.

Figure 8.

Rb-82 PET/CT in a 85 year old male with a large hiatal hernia.

Figure 9.

Stress Tc-99m SPECT/CT imaging for evaluation of chest pain revealed a 7.9 × 4.6 cm lesions in the right anterior mediastinum. CT attenuation was of the range of 40–50 Hounsfield units, consistent with soft tissue mass. The patient underwent surgical excision with pathology consistent with thymoma.

Figure 10.

Perfusion Tc-99m SPECT/CT imaging performed for chest pain revealed a 12.8 cm × 6.5 cm anterior mediastinal mass with tracer uptake which extended from the thyroid tissue, consistent with goiter, which was surgically resected.

Figure 11.

Patient with a history of gastroesophageal reflux disease found to have tracer uptake on stress Tc-99m SPECT/CT in the distal oesophagus, associated with mural thickening and mass effect on the left atrium, consistent with oesophageal adenocarcinoma.

Figure 12.

A middle-aged woman was found to have a left-sided paraspinal mass measuring 5 × 4 cm on Tc-99m SPECT/CT. Surgical resection revealed the mass to consist of lung tissue with scarring, fat necrosis, and an organizing haematoma.

Pericardium

A photopenic halo adjacent to the myocardium or conformed to the anterior side is a classic finding for a large pericardial effusion (Figure 13) or pericardial cyst (Figure 14), readily confirmed on CT images.

Figure 13.

A photopenic area around the heart on a stress Tc-99m SPECT/CT scan consistent with a large pericardial effusion seen on CT images, was later confirmed with transthoracic echocardiography.

Figure 14.

A case of anterior pericardial cyst, showing as a photopenic area on Tc-99m SPECT/CT.

Lungs

Focal tracer uptake in the lungs warrants further evaluation for malignancy with dedicated chest CT and whole-body PET imaging followed by biopsy if indicated (Figures 15–17).

Figure 15.

64-year-old female referred for evaluation of cardiac sarcoidosis and underwent myocardial perfusion SPECT and [18F]FDG PET/CT. Intense hypermetabolism was observed in the inferior cardiac wall, associated with a moderate perfusion defect on SPECT, consistent with cardiac inflammatory involvement. On the attenuation correction CT, a nodule was detected in the right lung graft (upper left, arrows), not seen in prior studies, which was mildly hypermetabolic on PET (upper right, arrow). Malignancy was ruled out by subsequent core needle biopsy.

Figure 17.

[18F]FDG PET/CY with right middle lung mass biopsy consistent with malignant carcinoid tumour of the lung.

Figure 16.

Patient with focal uptake in the upper left lung lobe on stress Tc-99m SPECT/CT imaging, correlating with an upper left lung spiculated mass which was consistent with non-small cell lung carcinoma after surgical resection (left upper lobectomy and en block resection of left lower lobe wedge).

Diffuse ‘cannonball’ lesions in the lungs often indicate metastatic disease, as illustrated in a patient with metastatic prostate cancer (Figure 18). Pleural effusions can also be readily recognized on CT in these patients (Figure 19). Additionally, 18F-FDG PET can detect metabolic activity within the lung cavities, such as in interstitial lung disease (Figure 20).

Figure 18.

Stress Tc-99m SPECT/CT with multiple bilateral lung nodules/cannonball appearance consistent with known history of metastatic prostate cancer.

Figure 19.

An Octogenarian woman with history of TAVR admitted to the hospital with decompensated heart failure. Work up included Tc-99m SPECT/CT which revealed large right-sided pleural effusion.

Figure 20.

Patient on haemodialysis undergoing pre-renal transplant evaluation. SPECT/CT performed for pre-transplant assessment showed reticular pattern of opacities in the left lower lung lobe, as well as loss of height in a vertebral body. A subsequent [18F]FDG PET scan performed to rule out neoplastic process confirmed the reticular opacities in the left lung with increased metabolic activity, clinically suggestive of autoimmune interstitial lung disease (associated elevated ANCAs). Also note the hypermetabolic vertebral body wedge deformity.

Limited abdomen

Adult polycystic kidney disease patients often have polycystic liver disease with a wide spectrum of findings. Imaging can reveal photopenic areas beneath the diaphragm due to hypofunctional liver tissues, as well as significant organomegaly occasionally displacing bowel loops elevating the diaphragm (Figure 21). Other identifiable findings include isolated hepatic cysts, cirrhosis, and splenomegaly (Figure 22 and 23).

Figure 21.

Polycystic kidney disease can be seen as photopenic areas beneath the diaphragm due to hypofunctional liver tissues and significant organomegaly displacing bowel loops in the upper abdomen.

Figure 22.

Large hepatic cyst in a patient with corresponding photopenia on SPECT/CT.

Figure 23.

60-year-old male with history of NASH related cirrhosis revealing a nodular contour of the liver with abdominal ascites and splenomegaly.

Bones

Incidental osseous uptake of tracer as well as review of the CT images can suggest underlying skeletal pathology, which can range from undetected rib fractures to former bone and/or soft tissue lesions, both benign and malignant. For instance, increased sacral uptake may indicate Paget’s disease, which can be confirmed by CT showing coarse trabecular bone changes consistent with this diagnosis (Figure 24, Video 1).

Figure 24.

The patient underwent stress Tc-99m SPECT/CT imaging for evaluation of chest pain, revealing increased tracer uptake in the clavicle and adjacent sternum, findings suggestive of Paget's disease. Additionally, a CT scan of the abdomen and pelvis performed for an unrelated indication revealed coarse trabeculation of the sacrum, consistent with Paget's disease.

On bone scintigraphy performed for suspected cardiac amyloidosis, rib fractures demonstrate PYP uptake and are easily identified (Figure 25).

Figure 25.

Patient with cardiac amyloidosis and right rib fractures (arrowheads).

Conclusions

SPECT-CT and PET-CT are essential in the evaluation of coronary artery disease, cardiac sarcoidosis, infection and cardiac amyloidosis. Interpretation should include the detection of actionable extracardiac findings. Recognition of these incidental findings is crucial, as they may impact patient management and outcomes beyond the primary cardiac evaluation. To this end, adequate training and experience may be necessary to accurately interpret and provide differential diagnosis and recommendation to minimize the healthcare burden while maximizing the clinical benefits.

Funding

No funding was available for this review.

Lead author biography

Dr Simrat Kaur is an Advanced Cardiac Imaging Fellow and Chief Fellow at the Cleveland Clinic Foundation, holding the Braun Endowed Fellowship. She completed her residency and cardiology fellowship training at the Cleveland Clinic. Dr Kaur’s research focuses on advanced multimodality cardiac imaging, structural heart disease, and cardiac amyloidosis. She is committed to advancing cardiovascular research and clinical practice through her scholarly work and has authored several book chapters and peer-reviewed publications. Dr Kaur is an active member of the American College of Cardiology’s Cardiovascular Diseases in Women Committee.