SP4:Instrumentation and Quality Assurance Flashcards
How often should the COR check be performed?
(a)At least weekly
(b) Before each patient
(c) Every 6 months
(d) At least monthly
(a) COR stands for center of rotation. This should be checked at least weekly and preferably each day that tomography will be performed to ensure that the axis of rotation is in the center of the computer matrix for all projections and should be determined for each collimator used. While the computer can correct for misalignments, if the COR needs frequent adjustment, service may be needed.
The abbreviation COR stands for:
(a)Circle of rotation
(b) Center of region
(c) Correction of rotation
(d) Center of rotation
(d) COR stands for center of rotation
What is the minimum number of counts that should be obtained in a uniformity correction flood for a SPECT camera?
(a)10 K
(b) 10 million
(c) 30 million
(d) 60 million
(c) Field nonuniformity can cause artifacts in images, and this is a bigger issue in SPECT than in planar imaging because image reconstruction will amplify nonuniformities. While 3–5% nonuniformity is acceptable for planar imaging, SPECT requires 1% or less. Thirty million counts are needed for a flood image with a matrix of 64 × 64. If the matrix is 128 × 128, 120 million counts are required. Flood fields should be made according to the imaging parameters of the exam which will be performed using the system.
Which of the following collimators will magnify an image?
(a)Flat field
(b) Diverging
(c) Converging
(d) High resolution
(c) The converging collimator magnifies the area imaged; the diverging collimator minifies it.
Which of the following must be performed every 6 months at minimum?
(a)COR
(b) Sealed source leak test
(c) Linearity of the dose calibrator
(d) Area survey of hot lab
(b) Sealed sources must be leak checked during an inventory conducted every 6 months. Leak checks are made by wipe tests of the sealed sources.
A dose calibrator must undergo repair or replacement if accuracy or constancy errors are:
(a)Present
(b) Greater than 5%
(c) Greater than 10%
(d) Greater than 20%
(c) According to the NRC, dose calibrators with accuracy or constancy errors >10% must be repaired or replaced.
Accuracy of a dose calibrator must be checked upon installation and then:
(a)Monthly
(b) Quarterly
(c) Semiannually
(d) Annually
(d) Accuracy testing measures the ability of the dose calibrator to assay radionuclides of different energies and should be performed upon installation and annually at minimum. The NRC requires that records be kept for 3 years
Linearity of a dose calibrator must be checked upon installation and then:
(a)Monthly
(b) Quarterly
(c) Semiannually
(d) Annually
(b) Linearity is the ability of the dose calibrator to detect a range of activities. This can be performed over 1 24-h period, using measurements and decay calculations for the isotope, or can be performed quickly using lead shields with a known attenuation. A linearity test should be performed at installation and assessed quarterly thereafter. The NRC requires that records be kept for 3 years.
Constancy of a dose calibrator must be tested:
(a)Daily
(b) Quarterly
(c) Weekly
(d) Every 6 months
(a) Constancy is the ability of the dose calibrator to reproduce measurements of a source over time and should be checked daily. The NRC requires that records be kept for 3 years.
A technologist wishes to evaluate the intrinsic uniformity of a camera used for planar imaging and is preparing a dose of 99mTc. How much activity is sufficient for the image?
(a)2 mCi
(b) 200 uCi
(c) 20 mCi
(d) 10 mCi
(b) Because the collimator is removed for intrinsic uniformity checking, only a small amount of 99mTc is needed. Field uniformity should be checked daily.
The advantages of using a 57Co sheet source over a fluid-filled flood source with 99mTc added include:
(a)It does not have to be prepared each day
(b) Lower cost
(c) Shorter half-life
(d) Higher energy
(a) The cobalt sheet source is more expensive and has a longer half-life, but it does not require mixing as does the fluid-filled source. Insufficient mixing will result in suboptimal flood field images not related to uniformity, and there is also the possibility of contamination with this method.
Geometric dependence of a dose calibrator must be checked:
(a)At installation
(b) Daily
(c) Quarterly
(d) Annually
(a) Geometry testing is performed by measuring the same amount of activity in varying volumes. This should be checked upon installation of the dose calibrator, and the records should be kept for 3 years.
What happens to image resolution as the distance between a patient and a parallel-hole collimator decreases?
(a)Improves
(b) Worsens
(c) Does not change
(a) When the distance between the patient and collimator is decreased, the system resolution improves, not only for parallel-hole collimators but also for diverging, converging, and pinhole collimators.
What happens to the image size as the distance between a patient and a parallel-hole collimator decreases?
(a)Increases
(b) Decrease
(c) Does not change
(c) The resulting image will not change in size with changing patient to collimator distance, but resolution will improve.
What part of an imaging system emits light when it had absorbed a photon?
(a)Photomultiplier tube
(b) Pulse height analyzer
(c) Scintillation crystal
(d) Collimator
(c) When a gamma ray interacts with a scintillation crystal such as a thallium-activated sodium iodide, thousands of electrons are promoted to higher orbits. As they relax and fall back to their original positions, their excess energy is released as light.
Most of the photons emitted from the radiopharmaceutical which has been administered to a patient:
(a)Do not contribute to the final image
(b) Are received by the sodium iodide crystal
(c) Are converted to a voltage signal
(d) Are in the visible spectrum
(a) Many emitted photons will be attenuated in the patient, deflected away from detector, or absorbed in the collimator, so most photons emitted do not contribute to the final image.
To obtain high-resolution images of a small organ, gland, or joint, which collimator will be most useful?
(a)Low-energy parallel hole
(b) Flat field
(c) Diverging
(d) Pinhole
(d) A pinhole collimator produces high-resolution images but suffers form low sensitivity because a very low percentage of the photons incident on the collimator will pass through the pinhole.
Which instrument should be used to determine the location of a 99mTc spill?
(a)Geiger-Muller (GM) survey meter
(b) Portable ionization chamber
(c) NaI well counter
(d) Pocket dosimeter
(a) Geiger-Muller (GM) survey meters are more sensitive than ionization chamber survey meters by a factor of ~10. This allows them to detect spills with very low levels of contamination.
A technologist covers the collimator (which is facing the ceiling) with absorbent paper and obtains a uniformity image using a liquid flood source with added 99mTc. A round cold spot is seen on the image. Subsequently the technologist removes the collimator, turns the camera face down, and obtains another image using a point source placed on the floor. This image appears uniform. What is the most likely cause of the cold spot on the first image?
(a)Collimator defect
(b) Decoupled photomultiplier tube
(c) Subtle crystal crack
(d) Improper energy peaking
(b) The PM tube may have slipped out of position when the detector faced upward and dropped back into place when the detector head was face down, causing a defect that was only observable in the first image.
If two photons strike a sodium iodide crystal at the same time, what will occur?
(a)Neither event will trigger production of light.
(b) The system will perceive only one event which will contain the summed energy of both photons.
(c) Both events will be correctly perceived.
(d) Only the photon closer to the center of the crystal will be perceived.
(b) When photons interact with the crystal simultaneously, or nearly simultaneously, they may be perceived as a single event with the energies summed.
The time after a crystal has absorbed a photon before it is able to respond to another event is called:
(a)Count rate
(b) Distortion
(c) Dead time
(d) Sensitivity
(c) Dead time is the time required after the absorption of a photon before the next photon can be detected.