CAMRT - Canadian Association of Medical Radiation Technologists Imaging Equipment and QC Questions and Answers — Questions and Answers
Question 1: An MRT observes a thin, perfectly straight, dark line appearing in the exact same location on every image acquired from a specific direct radiography (DR) flat-panel detector. This occurs regardless of the patient or anatomical part being imaged. What is the MOST likely cause of this artifact?
- Grid cut-off due to improper alignment
- A row of defective detector elements (pixels) (Correct answer)
- Patient motion during the exposure
- A scratch on the surface of the collimator mirror
Correct answer: A row of defective detector elements (pixels)
A consistent, straight line appearing in the identical position on all images from a specific detector points to a hardware failure within the detector itself. A row of defective or 'dead' pixels will fail to capture data, resulting in a line artifact that cannot be corrected by changing technique or patient positioning.
Question 2: According to Health Canada's Safety Code 35, the quality control test for collimator beam alignment must ensure that the misalignment between the perimeter of the light field and the x-ray field does not exceed what value?
- 5% of the Source-to-Image Distance (SID)
- 1 cm at any SID
- 2% of the Source-to-Image Distance (SID) (Correct answer)
- 0.5% of the Source-to-Image Distance (SID)
Correct answer: 2% of the Source-to-Image Distance (SID)
Health Canada's Safety Code 35 specifies that for radiographic equipment, the misalignment between the x-ray field and the light localizer field must not exceed 2% of the SID. This tolerance ensures accurate collimation, which is critical for radiation protection and image quality.
Question 3: A quality control test of an Automatic Exposure Control (AEC) system involves making several exposures of a phantom of varying thicknesses while keeping kVp and mA constant. The primary goal of this test is to verify that the AEC can maintain consistent:
- Spatial resolution
- Minimum response time
- Backup timer function
- Image receptor exposure (Correct answer)
Correct answer: Image receptor exposure
The fundamental purpose of an AEC system is to terminate the exposure once a preset amount of radiation has reached the detector, ensuring consistent image receptor exposure regardless of patient thickness. This QC test directly evaluates the AEC's ability to compensate for changes in attenuation by adjusting the exposure time.
Question 4: During a daily quality control check on a CT scanner, a water phantom is scanned. The mean CT number for a region of interest (ROI) placed in the center of the phantom should be approximately:
- -1000 Hounsfield Units (HU)
- 0 Hounsfield Units (HU) (Correct answer)
- 100 Hounsfield Units (HU)
- 50 Hounsfield Units (HU)
Correct answer: 0 Hounsfield Units (HU)
The Hounsfield Unit (HU) scale is calibrated by definition such that water has a value of 0 HU and air has a value of -1000 HU. Daily QC tests verify the stability of this calibration, and the measured value for water should be 0 HU, with a very small tolerance (typically ± 3-4 HU).
Question 5: Which of the following QC tests is performed WEEKLY by a technologist on a digital mammography unit to ensure consistent image quality and artifact detection?
- Phantom image quality test (Correct answer)
- Compression force check
- Half-value layer (HVL) measurement
- Display monitor calibration
Correct answer: Phantom image quality test
The weekly phantom image quality test is a critical component of a mammography QC program. The technologist images a standardized phantom (e.g., an ACR phantom) to ensure the system can consistently visualize small, low-contrast objects like fibers, specks, and masses, and to check for any new artifacts.
Question 6: An MRI technologist observes a series of sharp, linear artifacts across an image, perpendicular to the frequency encoding direction. The artifact disappears after the technologist ensures the scanner room door is fully sealed. This phenomenon is known as a:
- Aliasing artifact
- Chemical shift artifact
- Zipper artifact (Correct answer)
- Motion artifact
Correct answer: Zipper artifact
A zipper artifact is caused by extraneous radiofrequency (RF) signals leaking into the scan room and interfering with the MR signal. These artifacts often appear as a line or series of lines across the image. A common cause for this leak is an improperly sealed scanner room door, which is part of the room's RF shielding.
CAMRT - Canadian Association of Medical Radiation Technologists Imaging Equipment and QC Questions and Answers
An MRT observes a thin, perfectly straight, dark line appearing in the exact same location on every image acquired from a specific direct radiography (DR) flat-panel detector.
This occurs regardless of the patient or anatomical part being imaged.
What is the MOST likely cause of this artifact?