Review of Non-Technetium Radiopharmaceuticals
1. Radiopharmaceuticals with approximate 3-day half-lives, multiple energies, all cyclotron produced
a. Gallium 67 (NOT 68—that is a PET agent)
i. half-life of 78 hours
ii. Not exact but I remember the multiple energies as 100, 200, 300 and 400 keV—4 major energies, majority are 100 and 200 so…
iii. Use medium energy collimator
iv. Mechanism: Iron analogue
v. Testable trivia:
1. Gallium is one of few agents that have lacrimal gland uptake so if you see this thing Gallium first, Gallium: liver uptake > spleen uptake
2. Know that Ga67 is SPECT and Ga68 is PET (Ga68 is positron emitter)
3. Key uses: spinal osteomyelitis—better sensitivity than tagged WBC scan—and pulmonary infection such as PCP in HIV (look for diffuse lung uptake).
vi. Critical organ: colon
b. Indium 111
I. half-life 67 hours
ii. 173 and 247 keV—I round these to 175 and 250 for ease of remembering
iii. Medium energy collimator
iv. Mechanism: not exactly a mechanism—it’s a very inert, very safe, very non-reactive agent so you can inject it basically anywhere including in CSF via LP for cisternography for normal pressure hydrocephalus
v. Testable Trivia: InWBC, better for adults compared to TcWBC better for kids: has less GI activity than Tc so is preferred for IBD evaluation
vi. In111 is used for Octreoscan for carcinoid tumor
vii. Critical organ: depends what it’s tagged to but for core know that In-DTPA for cisternography is the spinal cord, InWBC is the spleen
c. Thallium 201
i. half-life 73 hours
ii. 135 and 167 keV
iii. Testable trivia: decays to Hg201—that is the majority of what you image, so you inject Tl201 but actually mostly image the daughter Hg201
1. Also means you get low quality images due to low energy, x-ray production
iv. Low energy collimator (69 and 81 keV mercury)
v. Mechanism: potassium analogue: thereby images viability because if it is in a cell it means there is a sodium potassium pump that is functional and by extension the cell is alive
1. Key use: cardiac viability: if thallium redistributes into a cell it means the cell can take up a potassium like substance through active transport and is therefore viable
2. Key use in CNS: Kaposi sarcoma shows thallium uptake, radiation necrosis does not
3. So what about toxoplasmosis? Key is to remember that Thal201 requires a HUMAN NaK transporter so parasites don’t have this—no uptake. Viable tumors have these so you see uptake in things like lymphoma, Kaposi sarcoma.
2. Iodine agents
a. I131
i. half-life 8 days (can remember about 1 week)
ii. 365 keV—so I remember about 1 week (7 days close to 8 days) for half-life and 1 year (365) for energy. “1 week and 1 year” gives you the half-life and energy.
iii. This is the only SPECT/gen nuclear agent that uses a HIGH energy collimator
iv. This is reactor produced—byproduct from nuclear reactions
v. Mechanism: iodine transport: selectively goes to thyroid with only minimal salivary/GI activity
vi. Critical organ: thyroid
vii. Testable trivia: has a beta particle whereas I123 does not
1. I131 works for both imaging (lower doses) and therapy (higher doses), know basics of thyroid uptake and scan
2. Energy is so high it can penetrate septa at high concentrations or if you use a low or medium energy collimator, and…
3. Septal penetration has a star pattern on the image—if you don’t know what this looks like look it up online or in a book because it is a highly testable imaging finding and common imaging artifact question
4. Need to stop breast feeding completely if receive I131 given unacceptable risk to ablate your baby’s thyroid.
b. I123
i. 1half-life 13 hours (remove the 2 from 123 and you are left with 13)
ii. 159 keV—closer to Tech 140 keV so gives better images than I131 at lower doses (shorter half-life by far)
iii. Medium energy collimator
iv. Cyclotron produced so more expensive than I131
v. No beta particle so no therapy
vi. Testable trivia: Know that I123 provides better images than 131 due to more optimal energy (closer to ideal 140 keV of tech) and shorter half-life with no beta particle which allows you to inject a higher dose for imaging purposes than is possible with I131
vii. For both I131 and I123 use Lugol’s solution (potassium iodide) to block the thyroid if you are using a pharmaceutical tagged to radioactive iodine
1. I123/I131 MIBG in kids—no benefit in radiating/ablating the thyroid when you are evaluating a neuroblastoma so treat with Lugol’s prior to imaging
2. Remember MIBG uses I123/I131 and Octroscan uses In111
viii. Unlike I131, with I123 you can resume breast feeding in 2-3 days
3. Xenon 133
a. Physical half-life 125 hours/5.3 days (don’t really need to remember this value—just know it is long)
b. Biological half-life of 30 seconds! (do remember this number). Key is really long physical half-life but really short biological half-life.
c. 81 keV—very low
d. Low energy collimator
e. Critical organ is trachea/airways
f. Testable trivia:
i. Energy is so low that in a VQ scan you MUST do the Xenon portion first or else the higher energy Tc-MAA scan will wash out the image due to downscatter even if you set the pulse height analyzer to 81 keV.
ii. Short effective half-life means you only have enough time for a single view (usually posterior view) so a quick way to tell a Xenon ventilation scan from a Tc-DTPA scan is with Xenon you only get a single projection, with Tc you get oblique and lateral views as well
iii. But with Xenon, unlike Tech, you can also get wash in and washout because you only need to image in one view so you start imaging just before inhalation, keep imaging during equilibrium and washout
iv. Testable trivia:
1. Air trapping—retained uptake in a portion of the lung on delayed ventilation images: COPD
2. If you see activity in right upper quadrant on a xenon scan, think fatty liver (hepatic steatosis). This is such a common question that if the stem mentions anything about xenon and shows an image, I look immediately at the RUQ to see if there is hepatic uptake. Th mechanism for this uptake is that Xenon is fat soluble.
3. If using xenon, you need to use a trap as this could otherwise diffuse throughout the NM imaging suite. Need to store the trap 10 physical half-lives which in this case is about 55 days to allow to decay to background. Need negative pressure room as well.
