Nuclear Medicine Renal Imaging Part 1
Review of high-yield concepts pertaining to nuclear medicine board preparation.
Show Notes/Study Guide:
Imaging with which radiotracer classically will show physiologic renal activity—Tc-99m tagged white blood cell scan or Indium tagged white blood cell scan?
Tc-99m tagged white blood cell scans show physiologic renal and GI activity whereas Indium tagged white blood cell scans classically have no renal or GI activity. Therefore, if you see renal uptake on an Indium tagged white blood cell scan one differential consideration is pyelonephritis or other cause of renal inflammation to include post-chemotherapy states. Given the GI activity on Tc but not Indium white blood cell scans, Indium is also preferred for evaluation of bowel infection/inflammation to include settings such as inflammatory bowel disease.
If a Tc99m MDP bone scan shows the renal cortex has more uptake than the osseous structures what entities can be considered?
Hemochromatosis, recent chemotherapy causing renal injury/inflammation, urinary obstruction.
How does horseshoe kidney or renal transplant present on a bone scan?
Lack of renal uptake in expected location. However, look to see the horseshoe kidney or renal transplant in the pelvis. This can be a trap on a multiple choice question to try to get you to incorrectly assess the image as a superscan. With horse shoe kidney you should also have soft tissue uptake and remember a superscan is lack of renal and soft tissue activity.
What is the pattern of normal renal uptake when imaging with Gallium-67?
Gallium-67 shows normal renal uptake at 24 hours but should not show robust renal uptake beyond 48 hours. If you see a Gallium-67 scan with prominent renal uptake on imaging > 48 hours think of renal infection/inflammation.
What are differential considerations when you see prominent renal uptake on a sulfur colloid scan?
Differential considerations include renal transplant rejection, pyelonephritis, congestive heart failure with reduced renal blood flow and filtration, thrombotic processes to include disseminated intravascular coagulation (DIC) and thrombotic thrombocytopenia purpura (TTP).
When considering Tc99m MAG3, Tc99m DTPA and Tc99m glucoheptonate, which of these are filtered by the kidneys and which are secreted?
Tc99m Mag3 is secreted, the others are filtered. Because Tc99m MAG3 is actively secreted and not passively filtered, it has better uptake and allows better assessment in individuals with renal failure.
Are renal scintigraphy images standardly obtained from anterior or posterior projections?
Standard renal scintigraphy projections are obtained posteriorly. Be cautions therefore when stating a finding is in the left or right kidney until you realize you are visualizing the kidneys from a posterior view so right kidney is on your right side which is opposite of usual radiology convention for imaging with CT or MRI. However, exceptions include anterior imaging that may be preferred for settings of pelvic kidneys such as in renal transplantation or horseshoe kidney.
What are the 3 phases of imaging obtained during a MAG3 study?
Blood flow, followed by cortical phase, followed by clearance phase. Remember with MAG3 you are looking for function much more than anatomy. These various phases tell you a lot of specific information about the physiologic function of the kidneys.
What is the normal appearance of the kidney during the flow phase of a Tc99m MAG3 study?
Normal renal appearance on a flow phase of a MAG3 study is prompt aortic activity quickly followed within a few frames by renal artery uptake followed by symmetric renal uptake. The flow phase tells you a lot about whether the kidney is adequately perfused and you expect to see prompt, symmetric renal uptake following injection in the flow phase.
If you see asymmetric renal uptake on a MAG3 study in the flow phase what are some of the top differential considerations?
Thrombosis of the renal artery and/or renal vein, chronic obstruction of the kidney with delayed uptake, acute rejection if a transplanted kidney is imaged and shows delayed uptake in the flow phase, acute renal infection. Note that if both kidneys show abnormal delayed uptake in the flow phase this could be related to a weak radiotracer bolus vs entities such as chronic bilateral high-grade renal obstruction.
Remember also that if both kidneys show prompt uptake and one kidney shows asymmetric increased uptake while the other shows normal uptake one differential consideration is renal artery aneurysm.
True or false: Acute tubular necrosis presents with abnormal renal perfusion?
False. This is a key concept. Acute tubular necrosis and interstitial nephritis have normal renal flow. The problem is not with the arterial system of the kidneys. So, these entities will show normal flow on a MAG3 study.
True or false: differential renal function is typically calculated from flow phase data on a MAG3 study?
False. Differential function should typically be calculated based on the cortical phase data, sometimes also termed the parenchymal phase. This is the phase where the kidney is doing its work of secreting and filtering substances from the blood and this is the phase where calculating relative counts between the kidneys—a surrogate of renal function—can provide information about the differential function of the kidneys. To calculate the differential function you place an ROI around each kidney and have a corresponding ROI for background correction. You want to calculate cortical uptake so ROIs are drawn something like 1 minute after injection so tracer has left the blood into the cortex but is mostly not yet in the renal collecting systems. A well-functioning kidney should have a steep uptake slope in the cortical phase to show it is able to uptake the radiotracer from the blood in a rapid manner.
