Q/A: Nuclear medicine physician makes case for SPECT/CT
TriMed Media asked Ka Kit Wong, MD, a nuclear medicine physician with PRP Imaging, Central West Nuclear Medicine and Ultrasound in Orange, Australia, and a former fellow at University of Michigan Medical Center in Ann Arbor, to discuss the pros and cons of SPECT/CT.
Q: What does SPECT/CT offer that SPECT alone does not?
A: SPECT/CT refers to a relatively novel technology consisting of a gamma camera combined with an inline CT scanner. Functional and anatomic images can be acquired in rapid sequence with the patient in the same bed position, and then viewed using either co-registered side-by-side or fusion image displays. The advantages of SPECT/CT broadly arise from superior anatomical localization of radioactivity and CT-based attenuation correction of SPECT images.
Similar to how 3D SPECT imaging was an improvement over planar imaging, SPECT/CT provides additional information over traditional planar and SPECT resulting in incremental diagnostic value. Functional and anatomic images obtained on separate days can be co-registered using software methods; however SPECT/CT is faster, has superior registration and is more efficient than these older methods.
Q: In what applications might SPECT/CT offer an advantage over SPECT alone?
A: There is growing evidence that SPECT/CT provides additional diagnostic value in significant numbers of patients for endocrine and adrenal imaging, bone scintigraphy, parathyroid scintigraphy and radioiodine imaging of thyroid cancers. SPECT/CT seems to improve lesion localization and characterization, increase reader confidence, change diagnosis and potentially alter patient management when used with these types of studies.
SPECT/CT shows promise for other studies including infection imaging, HIDA (hepatobiliary iminodiacetic acid) studies, gastrointestinal bleeding studies, red cell liver scintigraphy, heat-damaged red cell scans, Meckels scans and many others, which will no doubt be more comprehensively described in the near future. In addition, SPECT/CT can be used to provide CT-based attenuation correction for myocardial perfusion imaging.
Q: Where might SPECT/CT best fit: nuclear cardiology, general nuclear medicine for functional imaging or targeted oncology?
A: SPECT/CT is used by some cardiac centers to provide CT-based attenuation of myocardial perfusion studies. An additional advantage is the ability to use CT to perform calcium scoring in patients referred for myocardial perfusion imaging.
SPECT/CT when available should be applied routinely to nuclear oncologic studies, to provide the highest diagnostic accuracy for staging, restaging and assessing treatment response of neuroendocrine tumors. The superiority of PET/CT to stand alone PET has been extensively proven in the literature with regard to oncology imaging, and the same principles apply to SPECT/CT. The exact role of SPECT/CT may vary for other general nuclear medicine studies ranging from routine use, selected use to evaluate radioactivity on planar images or as a problem-solving tool, depending on future evidence.
Q: Why can you better characterize foci with SPECT/CT?
A: 3D SPECT imaging was an improvement over planar imaging by removing activity from overlying structures, increasing contrast resolution and providing additional anatomical information. SPECT/CT allows very precise anatomical localization of radioactivity, which, in turn, improves characterization of foci as physiological or pathological depending on which organ is involved.
For example, localization of activity to the renal collecting system or gallbladder often confirms activity that represents a physiological route of excretion in many nuclear studies. In addition, although the CT is low-dose, there may be structural abnormalities seen on the CT which point towards a disease process, i.e., localization of uptake to a lung nodule or lymph node. Reader confidence in scan interpretation is increased by this additional knowledge. It is worth mentioning that registration errors may still occur with SPECT/CT due to respiratory, gastrointestinal and patient body movement, as well as due to software used to display co-registered images.