Nuclear: Gastrointestinal Flashcards
Which is the solid phase of this gastric emptying study?
The left
Note: Solids have a lag time in the beginning where the stomach is grinding up solids into smaller particles. Liquids just immediately start emptying from the stomach.
How long should a pt be fasting prior to a gastric emptying study?
At least 4 hours
What radiotracer is usually used during a gastric emptying study?
Tc-99m sulfur colloid in a standardized meal (4 oz. egg whites, 2 slices of white bread, strawberry jam, and water)
Is the solid phase or liquid phase more sensitive for gastroparesis on a gastric emptying study?
Solid phase is more sensitive
Is attenuation correction important in gastric emptying studies?
Yes, you need to account for the difference in counts when food is in the back of the stomach vs the front of the stomach
Note: You can correct for this by doing both anterior and posterior counts or by using attenuation correction.
What are the pro kinetic drugs that enhance gastric emptying (and should be stopped prior to a gastric emptying study)?
- Metoclopramide
- Tegaserod
- Erythromycin
- Domperidone
Note: These should all be stopped at least 2 days prior to a gastric emptying study to avoid a false negative.
Which medications should be stopped prior to a gastric emptying study?
- Prokinetic agents (e.g. metoclopramide)
- Antikinetic agents (e.g. opioids)
- Anticholingergic/antispasmodic agents (e.g. glycopyrrolate)
Note: These should all be stopped 2 days prior to the exam.
Should ondansetron be stopped prior to a gastric emptying study?
No (serotonin receptor agonists do not interfere with the study)
Should opioids be stopped prior to a gastric emptying study?
Yes, at least 2 days before
What is the most sensitive study for GI bleed?
Tc-99m RBC study (can detect bleeds as slow as 0.1 mL/min)
Note: Mesenteric angiography is the next most sensitive and can only detect bleeds as slow as 1.0 mL/min.
Which portion of the RBC is tagged during a Tc-99m RBC study?
The beta chain of hemoglobin
Note: This is accomplished by first reducing the Tc-99m with a stannous ion (tin).
What is the in vivo method for tagging RBCs with Tc-99m?
- Tin (stannous ion) is injected into the pt and binds to hemoglobin in RBCs
- Tc-99m pertechnetate is then injected, which gets reduced by the tin (stannous ion), allowing it to bind to the hemoglobin
Note: This is a very simple process but results in only 60-80% Tc binding, resulting in a lot of free Tc and a dirty image (poor target to background).
What should you ensure before injecting tin/Tc for the in vivo method of RBC tagging?
That the tubing being used for injections is not heparinized and has not been used recently for IV contrast administration (these will interact with the tin/Tc and ruin the study)
What is the in vivo-in vitro method for tagging RBCs?
- Tin (stannous ion) is injected into the pt and binds to hemoglobin
- After 15-30 min, you pull 3-5 cc blood out of the pt into a syringe containing Tc-99m pertechnetate and a non-heparin anticoagulant allowing the Tc to be reduced and bind to the hemoglobin in the syringe
- After 10 more min, this tagged blood is then reinjected into the pt
Note: This performs better than the in vivo method with ~85% Tc binding (compared to 60-80% for the in vivo method).
In vitro method of tagging RBCs
Blood is withdrawn from the pt and added to a kit with both the tin (stannous ion) and Tc-99m, which is then injected into the pt
Note: This is the best method (results in 98% Tc binding), but is also the most expensive.
How should images be acquired during a tagged RBC scan?
Dynamic imaging (to allow detection of intermittent bleeds and better localization of the origin of the bleed)
Imaging findings of GI bleed during a tagged RBC scan
- Tracer outside the vascular distribution
- Tracer that moves like bowel
- Tracer that increases in intensity over time
If you identify radiotracer in the stomach on a tagged RBC scan, what should you do to confirm it is a true gastric bleed?
Look at the salivary glands and thyroid (if these also show radiotracer uptake, then it is likely artifact from free Tc, not an actual bleed)
What are common causes of false positives on tagged RBC scans for GI bleed?
- Renal/bladder excretion
- Transplant kidney excretion
- Varices/angiodysplasia
- Hemangioma (over liver/spleen)
- Free Tc in the stomach (will also be in thyroid/salivary glands)
Note: This is why it is important to look for bowel motion in the extravasated tracer.
Arrow
Penis (normal uptake)
Long arrow
GI bleed
Note: Radiotracer appears extravascular and follows a curvilinear, bowel-like pattern.
What is the vascular territory of the distal esophagus?
Celiac artery
What is the vascular territory of the stomach?
Celiac artery
What is the vascular territory of the duodenum?
1st part: Celiac artery
2nd-4th parts: SMA
Which portions of the small bowel are supplied by the SMA?
Everything except the first part of the duodenum
What is the vascular territory of the large bowel?
Colon proximal to splenic flexure: SMA
Colon distal to splenic flexure: IMA
Proximal rectum: IMA
Distal rectum: Internal iliac artery
Why don’t people use Tc-99m sulfur colloid for GI bleed scans anymore?
- Multiple blind spots (sulfur colloid goes to the liver and spleen obscuring the splenic and hepatic flexures)
- Fast clearance (need to do the scan within 30 min of injection)
What radiotracer is used for a Meckel scan?
If not actively bleeding: Pertechnetate (because it is taken up by gastric mucosa, which is commonly present in symptomatic Meckel diverticula)
If actively bleeding: Tc-99m RBC
Note: Only 10-30% of Mockers diverticula have gastric mucosa (but these are the ones most likely to bleed).
What preparation can be done to improve a Meckel scan?
- Give pentagastrin to enhance gastric mucosa uptake of pertechnetate
- Give an H2 blocker (e.g. cimetidine or ranitidine) to slow the secretion of pertechnetate from gastric mucosa
- Give glucagon to slow gastric motility
Common causes of false positives on a Meckel scan
Bowel irritation (e.g. recent endoscopy, laxative use)
Common causes of a false negative on a Meckel scan
- Recent in-vivo labeling of RBCs (resulting in free Tc contamination)
- Recent barium study
Pertechnetate
Meckel diverticulum (with heterotopic gastric mucosa)
Pertechnetate
Meckel diverticulum (with heterotopic gastric mucosa)
Can you perform HIDA scintigraphy on a pt with hyperbilirubinemia?
Yes, but you need to increase the radiotracer dose or use a different tracer (e.g. DISIDA or BROMIDA)
Note: Bilirubin over 5 mg/dL will increase the number of non-diagnostic exams.
What radiotracer is used for HIDA scintigraphy?
Tc-99m labeled analog of imilodiacetic acid (there are many different types)
How long should a pt be fasting prior to HIDA scintigraphy?
At least 4 hours (so the gallbladder is ready to fill), but no more than 24 hours (so the gallbladder isn’t too full to prevent more filling)
Can you perform HIDA scintigraphy on a pt who has been fasting for over 24 hours?
Yes, but then you need to give CCK before the study (to empty the gallbladder enough to allow it to fill with tracer)
What indicated a normal HIDA scintigraphy?
- Prompt tracer uptake by the liver (within 5 min)
- Tracer visible in the bile ducts, gallbladder, and bowel (within 60 min)
Acute cholecystitis
Note: Gallbladder does not fill.
Arrow
Rim sign (a curved area of increased activity along the gallbladder fossa due to regional hepatic hyperemia adjacent to the gallbladder)
Note: This suggests a more angry gallbladder (seen in 20% of gangrenous cholecystitis).
Gallbladder starts being visible at 3 hours…
Think chronic cholecystitis
Note: Gallbladder filling is considered delayed if it takes more than 1 hour for it to be visible on HIDA scintigraphy.
Gallbladder ejection fraction of 25% following CCK stimulation…
Think chronic cholecystitis (or calculus cholecystitis) if gallbladder ejection fraction is < 30%
What dose of what medication(s) should be used when evaluating gallbladder ejection fraction?
- 0.02 microgram/kg CCK given over 60 min
- 0.02-0.04 mg/kg morphine given over 30-60 min
HIDA scintigraphy with 24 hour delay
Biliary atresia (pt is an infant)
Note: No tracer in the biliary tree OR bowel. Contrast is starting to be excreted via the kidneys. If there was tracer in the bowel, you would think neonatal hepatitis (not biliary atresia).
What medications can cause biliary obstruction on HIDA scintigraphy?
- Chlorpromazine
- Erythromycin
- Birth control (estrogens)
- Anabolic steroids
- Statins
What is main reason HIDA scintigraphy would be done on an infant
To discern between biliary atresia (no bowel tracer) and neonatal hepatitis (tracer in bowel within 24 hours)
What medication is often given prior to HIDA scintigraphy in an infant to increase accuracy?
Phenobarbital 5mg/kg for 5 days prior to HIDA scintigraphy (“5 for 5 keeps the liver alive”)
Note: This upregulates the cytochrome system allowing the liver to take up more tracer.
HIDA scintigraphy findings of biliary atresia
- No visualization of biliary ducts/gallbladder
- No tracer in bowel within 24 hours
Bile leak
How should you modify HIDA scintigraphy when looking for a bile leak?
Get extended delayed images
How should you modify HIDA scintigraphy when looking for biliary atresia?
- Get extended delays to at least 24 hours
- Usually give phenobarbital for 5 days prior to study
Bile leak
Note: Persistent collection of tracer in the hepatic hilum.
60 min image
Delayed clearance of blood pool (heart with tracer), suggestive of hepatic dysfunction (e.g. hepatitis)
HIDA scintigraphy shows no bowel activity on 6 hour image despite normal hepatic uptake and clearance of blood pool…
Think extra hepatic bile duct obstruction
HIDA scintigraphy findings of acute cholecystitis
No gallbladder activity after 4 hours (or after 1 hour with morphine administration, which should increase pressure to fill gallbladder by reducing sphincter of Oddi function)
How does recent morphine administration affect HIDA scintigraphy?
It will delay bowel activity (by causing the sphincter of Oddi to contract)
Note: This can actually cause the gallbladder to fill more quickly due to back pressure.
If you see bowel activity but no gallbladder activity, what should you do?
Administer morphine to see if the back pressure can help visualize the gallbladder
What type of mass classically shows up hot on a Tc-99m sulfur colloid liver scan?
Focal nodular hyperplasia
Note: In real life, only 40% of these are hot (30% are neutral and 30% are cold).
Arrow, Tc-99m sulfur colloid
Think focal nodular hyperplasia (one of the only liver masses that often appears hot on sulfur colloid imaging)
How big should sulfur colloid particles be for a Tc-99m sulfur colloid liver scan?
0.1 - 1.0 micrometer
Note: Any larger and the spleen with trap them more (or if really big they will get stuck in pulmonary circulation); any smaller and they will get trapped more in the bone marrow.
What liver lesions are sulfur colloid cold and gallium hot?
- HCC
- Hepatic abscess
What liver lesion is sulfur colloid cold and xenon hot?
Fatty infiltration (xenon is lipophilic)
What liver lesion is sulfur colloid cold and tagged RBC hot?
Cavernous hemangioma
What is the normal distribution of radiotracer uptake for a Tc-99m sulfur colloid scan?
- Liver (85%)
- Spleen (10%)
- Bone marrow (5%)
What is “colloid shift” in the setting of a Tc-99m sulfur colloid scan?
When there is unusually high uptake in the spleen or bone marrow due to unusually low uptake in the liver (e.g. hepatic dysfunction, cirrhosis, diffuse liver mets, etc.)
What are common causes of increased bone marrow activity on a Tc-99m sulfur colloid scan?
- Colloid shift from reduced hepatic uptake (e.g. cirrhosis, diffuse liver mets, portal hypertension)
- Bone marrow activation
Tc-99m sulfur colloid
Splenosis
Note: No spleen uptake with subsequent increased bone marrow uptake (think splenectomy/splenosis). Tracer uptake in a pelvic mass should make you think splenosis (or prostate cancer).
Tc-99m sulfur colloid
Think portal hypertension
Note: Hepatosplenomegaly with colloid shift (more tracer in the spleen than in the liver).
Differential for this Tc-99m sulfur colloid
Diffuse pulmonary uptake (usually no pulmonary uptake):
- Excess aluminum in the colloid
- Colloid shift (reduced hepatic uptake due to diffuse liver disease)
- Primary pulmonary pathology (increased pulmonary macrophages)
Differential for renal uptake on a Tc-99m sulfur colloid scan
- Heart failure (most common)
- Rejection of a renal transplant (colloid entrapment in the fibrin thrombi of the microvasculature)
- Microthrombi (DIC or TTP)
- Cocksackie B virus infection
What should you look for on a Tc-99m sulfur colloid study to suggest contamination with excess aluminum?
Diffuse lung uptake (normally there should be no lung uptake)
Note: Lung uptake can also be due to colloid shift (hepatic dysfunction).
What nuclear imaging study can be done to show that a liver lesion is a hemangioma?
Tc-99m RBC scan
How big does a hepatic hemangioma need to be to show up on a Tc-99m RBC scan?
At least 1.5 cm
Tc-99m RBC
Hepatic hemangioma
Note: This could also be an angiosarcoma (look on the flow and blood pool phases to differentiate: hemangiomas will only show up on delayed images, but ansiosarcomas will be visible on all phases).
What radio tracers are used to locate ectopic splenic tissue?
- Tc-99m sulfur colloid
- Heat-damaged Tc-99m RBCs
Does hepatic steatosis alter FDG uptake in the liver?
No
