Quality Control

CT Number Accuracy

Purpose: Verify that CT#/HU of water is consistently 0 regardless of kV.
Method: A water phantom is scanned.
Frequency: Weekly <a class="external" href="http://www.hc-sc.gc.ca/ewh-semt/alt_formats/hecs-sesc/pdf/pubs/radiation/safety-code_35-securite/safety-code35-securite-eng.pdf" rel="nofollow" target="_blank">[2]</a>
Limits: ±4HU

Spatial Uniformity

Purpose: Verify ability of the CT scanner to yield the same CT number regardless of location of the region of interest (ROI) within a homogenous object.
Method: Uniform object is scanned. Mean HU of 5 ROI in center and 4 ROI at periphery are measured and assessed. The diameter of the ROIs should be 10% of the diameter of the phantom.The difference between the mean CT number at the middle and the mean CT number at the periphery of the object is considered.
Frequency: Daily
Limits: ±2HU from established baseline. The baseline uniformity must be ±5HU for water.

Noise

Purpose: Assess quantum mottle. Measure changes in signal-to-noise (S/N) ratio with changes in kVp and mA. Noise is the variation of CT numbers from a mean value in a defined area in the image of a uniform phantom.
Method: Obtaining the mean and SD of CT numbers within a ROI of a homogeneous object (water phantom). The diameter of the region of interest should be 40% the diameter of the uniform phantom.
Frequency: Weekly
Limits: ±10% or ±0.2HU (whichever is larger) from the established baseline. The established baseline should not deviate from the manufacturer specified noise value more than ±15%.

HVL

Purpose: Evaluates effective beam energy determined by kV and added filtration
Limits:

X-ray Tube Voltage (kV)	Half-Value Layer of Aluminum (mm)
60	1.9
70	2.1
80	2.4
90	2.7
100	3.0
110	3.4
120	3.8
130	4.2
140	4.6

<a class="external" href="http://www.hc-sc.gc.ca/ewh-semt/alt_formats/hecs-sesc/pdf/pubs/radiation/safety-code_35-securite/safety-code35-securite-eng.pdf" rel="nofollow" target="_blank">[2]</a>
Frequency: Annually

Scout dose

Purpose: Measure dose for scout image
Limits: ±20%
Frequency: Semi-anually

Scout Accuracy

Purpose: Assure the reference lines on the scout are the area scanned.
Limits: ±2mm in the vertical position. <a class="external" href="http://www.hc-sc.gc.ca/ewh-semt/alt_formats/hecs-sesc/pdf/pubs/radiation/safety-code_35-securite/safety-code35-securite-eng.pdf" rel="nofollow" target="_blank">[2]</a>

Low contrast resolution

Purpose: Measures objects with slight differences in density
Method: Phantom with simulated masses
Limits: Must be able to display objects that are 3mm in diameter with a difference of 1%
Frequency: Monthly

High contrast resolution

High contrast resolution of CT is still much lower than CR/DR and film due to the spacing of the detectors
Purpose: Measures spatial resolution
Method: Phantom with small pins
Limits: ±0.5 lp/mm
Frequency: Monthly

Contrast Resolution

Ability to portray differences between two objects with similar LAC (linear attenuation coefficient) or densities. CT excels at contrast resolution because there are no overlying structures. Contrast resolution is effected by slice thickness, pt. size, detector sensitivity, algorithms used, what image display looks like, kV, mAs.
Method: Contrast disc phantom with varying CT numbers
Frequency: Monthly

CT dose index (CTDI)

Measuring dose (exposure) for head and abdomen
Method: Evaluates radiation distribution within phantom and compares with computer display. Measured with head phantom and body phantom at various pitches
Limits: Must be within 20% of manufacturer display
Frequency: Semi-annually

Table increment

Actual table movement compared to intended (set) table movement
Purpose: Ensures desired volume of patient is scanned
Limits: Must be within ±1mm
Frequency: Monthly

Slice Thickness

Measuring actual slice thickness compared to set slice thickness
Method: Test device containing one or two ramps positioned at an angle to the scan plane. For helical scanning, a test device consisting of a thin disk or band, mounted in a medium should be used. The disc should have a linear attenuation coefficient ≥ aluminum to ensure a high S/N ratio.
Limits: For set slice thickness ≥2mm ±1mm, for set slice thickness <1mm ±0.5 mm. Measured slice thickness must be ±50% from the established baseline thickness.
Frequency: Monthly

CT Number Accuracy

Measuring CT numbers of several known substances (air, water, acrylic, bone, etc.)

Scan Time Calibration

Purpose: Evaluates accuracy and linearity of scan times
Method: Water phantom
Frequency: Monthly

Laser Light Accuracy

Internal and external laser lights are used for patient positioning/landmarking. This is especially important during stereotactic biopsies or interventional procedures. <a class="external" href="http://books.google.com/books?id=3INrAAAAMAAJ&pgis=1" rel="nofollow" target="_blank">[1]</a>
Limits: ±2mm
Frequency: Semi-anually

Linear Length Measurement

Limits: ±1 mm
Frequency: Anually

Angle Measurement

Limits: ±1 mm
Frequency: Anually

References

1. <a class="external" href="http://books.google.com/books?id=3INrAAAAMAAJ&pgis=1" rel="nofollow" target="_blank">Papp, J. (2006). Quality Management in the Imaging Sciences 3rd edition. Mosby Elsevier, St. Louis, MI.</a>
2. Health Canada. (2008). Radiation Protection in Radiology - Large Facilities: Safety Code 35. Retrieved from <a class="external" href="http://www.hc-sc.gc.ca/ewh-semt/alt_formats/hecs-sesc/pdf/pubs/radiation/safety-code_35-securite/safety-code35-securite-eng.pdf" rel="nofollow" target="_blank">http://www.hc-sc.gc.ca/ewh-semt/alt_formats/hecs-sesc/pdf/pubs/radiation/safety-code_35-securite/safety-code35-securite-eng.pdf</a>