Nuclear Medicine: HIDA Scans for the ABR Core Exam Part 1
Question and answer review of HIDA scans for the ABR core exam. Find additional free radiology podcast reviews and free radiology board preparation materials at www.theradiologyreview.com.
HIDA Scans for the ABR Core Exam
All HIDA radiotracers are analogs of what?
Iminodiacetic acid which is very close to lidocaine in chemical structure.
HIDA tracers mimic what physiologic process?
Bilirubin uptake, transport, and secretion.
If a patient has hyperbilirubinemia, what implication does this have for performing a HIDA scan?
You must inject a higher dose of tracer due to competition with the bilirubin for uptake, transport, and secretion.
How lung must a patient fast prior to a HIDA scan?
At least 4 hours but no longer than 24 hours
If a patient has fasted longer than 24 hours, what must we do prior to performing a HIDA scan?
Give CCK to cause gallbladder constriction to empty the gallbladder prior to HIDA scan. IF you image without giving CCK the gallbladder is already completely full the HIDA tracer will not be able to enter the gallbladder, and you will get non-visualization of the gallbladder on imaging which would give you the false appearance of cystic duct obstruction/acute cholecystitis.
What percentage of patients with acute cholecystitis have cystic duct obstruction?
About 95% of patients with acute cholecystitis have an obstructed cystic duct.
What is the normal sequence of uptake/excretion that you expect to see on a HIDA scan?
1. Hepatic uptake and blood pool clearance
2. Excretion into biliary ducts
3. Excretion into bowel
4. Excretion into gallbladder *note in some cases you may see gallbladder prior to bowel
The gallbladder normally shows uptake on a HIDA scan within how many minutes?
60 minutes
How many minutes after injection of radiotracer should you typically see hepatic uptake?
5 minutes
What is the rim sign and what does this mean in terms of cholecystitis?
Rim sign looks like a curved are of uptake around the gallbladder fossa due to focal pericholecystic hepatic uptake. This results from reactive hepatic inflammation and hyperemia due to a very inflamed and typically gangrenous gallbladder. On board exams the rim sign suggests that gangrenous cholecystitis is present.
What is the cystic duct sign and what does this mean in terms of cholecystitis?
The cystic duct sign is when you see a small focus “nub” of activity in the proximal cystic duct with no activity distally. This means the cystic duct is obstructed and acute cholecystitis is present.
Is CCK administration used to evaluate for acute cholecystitis, chronic cholecystitis, or both?
CCK administration is used to evaluate for chronic cholecystitis. CCK is not part of an acute cholecystitis evaluation. You only administer CCK once you see gallbladder filling and the purpose of it is to cause gallbladder constriction and emptying so you can calculate a gallbladder ejection fraction.
What are manifestations of chronic cholecystitis on a HIDA scan?
1. Delayed filling of the gallbladder with no filling at 1 hour but fills by 4 hours
2. Gallbladder ejection fraction < 30% following CCK administration
What is the dose of CCK that is administered for a HIDA scan?
0.02 MICROgrams/kg over 1 hour
When morphine is used during a HIDA scan, what is the dose administered?
0.02 to 0.04 MILLIgrams/kg over 30-60 minutes.
*Note MICROgrams of CCK and MILLIgrams of morphine
What does it mean if you see the liver on a HIDA scan, but you do not see the biliary tree?
Lack of visualization of the biliary tree has been named the “liver scan sign” and this is an indication of acute obstruction of the common bile duct.
What do you look for to identify that there is persistent blood pool activity on a HIDA scan?
Persistent cardiac activity during the initial 60 minutes of imaging is a top sign of persistent blood pool activity. Always check for cardiac activity on HIDA scans to evaluate for this.
What does it mean on a HIDA scan if you have persistent blood pool activity with some liver uptake but no bowel activity?
This is a sign of hepatitis and hepatic dysfunction
*If you have no bowel activity but persistent blood pool this is hepatitis/hepatic dysfunction
*If you have no bowel activity but blood pool clears normally this is common bile duct obstruction
How long must you wait on a HIDA scan to see gallbladder activity before you call acute cholecystitis?
4 hours
What if you don’t want to wait 4 hours, is there anything you can do to speed this up?
Give morphine to promote gallbladder visualization
What is the mechanism whereby morphine augments gallbladder visualization on a HIDA can?
Morphine constricts the sphincter of oddi which then increases backpressure in the biliary tree to promote gallbladder filling
What opioid does not constrict the sphincter of oddi?
Demerol (meperidine)
After how many minutes is morphine typically administered for a HIDA scan?
You can administer morphine after 30-60 minutes if the gallbladder is not visualized. You then need to image at least 30 more minutes before you would call gallbladder non-visualization and acute cholecystitis. Note an expedited protocol has been proposed whereby you give morphine at the time of initial radiotracer injection, but I don’t think the boards are likely to ask about this.
Is there anything else you may consider administering along with morphine if you are performing morphine-augmented imaging?
A booster of radiotracer (50% of the original dose) can optionally be given at time of morphine to make sure that you have enough tracer in the liver for biliary secretion and gallbladder visualization. A potential false negative scan for acute cholecystitis can happen if, when you give morphine, there is no longer sufficient tracer in the liver to subsequently fill the gallbladder.
