Nuclear Medicine Physics: Collimators Part 1
Review of collimators for radiology board exams and nuclear medicine board exams including the ABR Core Exam. Essential details of the physics of parallel hole, converging/diverging, and pinhole collimators are discussed.
Nuclear Medicine Physics: Collimators
Nuclear Medicine Collimators for Radiology Board Review
Although there are at least 5 basic collimator designs, the good news is that I think you only need to know 3 of these for board examinations: parallel hole collimators, converging/diverging collimators, and pinhole collimators.
1. Parallel hole collimators—these have separate collimators for low, medium, and high energy imaging
a. Low-energy all-purpose (LEAP)
i. Have large holes allowing for high sensitivity (lots of counts can enter) but lower resolution
1. There is a trade-off in nuclear imaging between sensitivity and resolution
2. Sensitivity requires that you get as many counts as possible in the time allotted
3. Maximizing sensitivity requires that you allow some scatter to enter the collimator which lowers resolution
b. Low-energy high-resolution (LEHR)
i. As in the name, this is the high-resolution collimator for use with low energies
1. LEHR collimators obtain higher resolution than LEAP collimators by making the holes in the collimator smaller and elongating the holes in the collimator. This makes it harder for scattered gamma rays to reach the scintillation crystal. Many non-scattered gamma rays can still make it.
2. By making the holes smaller, you have more of them within the same collimator field of view and therefore given better spatial resolution.
ii. Use LEAP and LEHR collimators for Tc99 (140 keV), Thal201 (multiple energies, but over 90% are less than 81 keV).
c. Medium energy
i. These have thicker septa than a low-energy collimator which is necessary to reduce septal penetration when imaging more energetic radiopharmaceuticals
ii. Septal penetration has a star pattern and is most seen when imaging I131 with a low or medium energy collimator. Septal penetration is very commonly tested on board examinations.
iii. Use a medium energy collimator for Ga67, In111.
1. Gallium67 keV photopeaks are approximately 100, 200, 300, 400 keV
a. About 65% of energy is around 200 keV or below, using the medium energy collimator is a compromise as Ga67 spans low to high energies
2. Indium111 keV photopeaks are approximately 175 and 250 keV
d. High energy
i. Even thicker septae than a medium energy collimator
ii. For board purposes you use this for I131 only, 364 keV
1. Again, know what septal penetration looks like if you image I131 with a low or medium energy collimator
2. Converging/diverging collimators
a. Converging collimator
i. Key to answer board questions is to remember that the collimators converge toward the person/body part being imaged
ii. This makes the imaged area appear larger because as gamma rays leave the body part being imaged, they enter the holes that while converging towards the patient, by extension, diverge towards the crystal. So, the rays diverge and are more spread out on the crystal face, therefore making the image on the crystal larger.
b. Diverging collimator
i. The collimators diverge toward the person/body part being imaged
ii. This will allow a larger field of view to fit on a smaller crystal, so called “minification”. The image on the crystal will be smaller because of passing through the collimator.
iii. This decreases sensitivity but increases resolution.
c. Key points:
i. The reference point to which a collimator converges or diverges is the object being imaged NOT the crystal. A converging collimator converges toward the object being imaged. A diverging collimator diverges toward the object being imaged.
ii. Converging collimators make an image larger
iii. Diverging collimators increase the field of view beyond that of the crystal size
3. Pinhole collimator
a. Magnifies images of small organs like thyroid, a joint, scrotal imaging
b. Image is inverted on crystal
c. Pinhole provides the most magnification but smallest field of view
d. Pinholes maximize resolution at expense of sensitivity because there is a very small opening for gamma rays to enter
