QA for CT Scan

QA of CT Scanner using water phantom

A good Quality Assurance (QA) program will provide regular testing, prompt interpretation of test data, and faithful record keeping. A current logbook or computer file should be readily available for viewing by any regulatory agency in the event of an unscheduled inspection. In addition to the following tests, daily visual checks should be done on table/gantry movement, cables, cords, operating console, controls, and print system if installed. These visual tests should be documented as well, for future comparison.

           Facilities with equipment under warranty or service contract with an Original Equipment Manufacturer (OEM) or an Independent Service Organization (ISO) must follow the testing and preventive maintenance schedule required by the OEM or ISO to keep the warranty or contract valid. The OEM or ISO testing and maintenance schedule must be included in the manual. Facilities with equipment not under warranty or service contract must follow the testing frequency stated in this guide. Facilities must perform all the QC tests which their manufacturer supplied phantom will allow.

Appropriate quality control testing must be conducted whenever major maintenance (X-ray tube replacement) or a change in equipment operation (software change) occurs.

1. Testing frequency - Each day of operation

Equipment functioning: Each day during equipment warm-up, and before scanning the first patient, the operator must check for any malfunction. The operator must also evaluate the mechanical and electrical safety of the CT system. Malfunctions and unsafe conditions shall be corrected promptly.

CT Number Accuracy of Water, Image Noise, Image Uniformity, and Artifact evaluation shall be performed daily using a common technique setting with a head sized phantom. The average CT number and standard deviation of noise must be performed using a region of interest that is standardized for the QA tests. Image uniformity and artifact evaluation must be done using an appropriate and consistent window level and width.

This is the water phantom used in daily QA carried out by radiographer to test the CT number accuracy of water, image noise, image uniformity and artifact.

CT Number for Water Water filled plastic cylinder (20 cm diameter) Take scan, reconstruct image, place ROI 200-300 pixels in center of field and take measurement. Expected results: CT number of water equal to zero, but range of +/- 3 at center of image is acceptable, and +/- 5 HU at peripheral locations. Cause of failure is usually miscalibration of the algorithm that generates CT numbers. CT number test is Performed Daily. Test an area outside the image representing air for CT number of air, monthly. The acceptable range is -1000 HU, +/-5 HU.

This chart shows the various CT numbers (Hounsfield units) calculated for various tissues and substances based on the density of water. Notice that at the extremes is bone (+1000 HU) and air (-1000 HU). Water has a CT number of zero, which is used to test for the function of the algorithm that calculates CT numbers.

2. Frequency - Monthly

Imaged Slice Thickness or Slice Sensitivity Profile

Performed at each available slice thickness, using an appropriate phantom for both the axial and helical modes of operation.

Slice Positioning Accuracy

Evaluate the accuracy of slice localization lights, the accuracy of slice positioning and the accuracy of table motion with incrementation.

CT Number Scale Accuracy

CT numbers must be evaluated using a phantom containing a variety of materials to evaluate a wide range of CT numbers. The measured values must remain within manufacturer's specifications.

Hard Copy Devices

Hard copy devices and dry image processors must be monitored for consistency in accordance with the manufacturers published recommendations. Many of these devices utilize a SMPTE test pattern, which can be readily used and printed for review.

3. Test frequency - Semi annually

Dose Profile Width

This test must be performed at each available slice thickness. This test can readily be performed using packaged film, which is placed around the surface of the phantom. Alternative methods of evaluation can also be used. Multiple slice units need to be evaluated for over-beaming and adjusted to manufacturer's specifications.

Spatial Resolution

Must be evaluated in the x-y plane using a phantom with suitable objects. The objects will be of different sizes and shapes. In-plane resolution must be evaluated in the axial mode, for a single slice thickness using both standard and high-resolution algorithms. If an appropriate phantom is available, the system must also be evaluated in the helical mode to test for longitudinal resolution with different combinations of collimation and pitch.

Low-Contrast Detectability

Must be performed using a phantom with test objects that will evaluate the system for less than 1% contrast sensitivity.

4. Test frequency - Annually

Patient dose

Each facility shall have available dose measurements based on the most common conditions of operation of their CT units. Since the advent of CTDI FDA, the International Electrotechnical Commission (IEC) has defined a new standard for dose index. This standard is called the CTDI100. CTDI100 utilizes measurements made with a 16cm diameter (head/pediatric body) or a 32cm diameter (body) acrylic phantom. The measurements are made utilizing a 100mm long pencil ionization chamber. Readings are made with the ion chamber in both the center (axial or central dose) position and near surface slots of the phantom (the peripheral dose).

CTDI W, the weighted or blended dose, is calculated by adding together two-thirds of the CTDI 100 peripheral dose with one-third of the CTDI 100 axial or center dose. (CTDI W = 2/3 CTDI 100 peripheral + 1/3 CTDI 100 axial or center

It is important to remember that CTDI W represents an average dose in the x and y planes. With multiple slice helical scanning, the dose is better represented by CTDIVOL which takes into account the effect of pitch and averages over the x, y, and z planes. CTDI VOL represents the integrated dose over the total volume that is irradiated.

CTDI VOL = 1 ¸ PITCH X CTDI W

The resultant measurements should not exceed the following:

Examination	CTDIVOL (mGy)
Adult Head	75
Adult Abdomen	25
Pediatric Abdomen (5 yr. Old)	20

And the measurements shall not exceed:

Examination	CTDIVOL (mGy)
Adult Head	80
Adult Abdomen	30
Pediatric Abdomen (5 yr. Old)	25

5. On Installation of New Tube

All daily and monthly tests must be completed before patient examinations commence. All semi-annual and annual tests must be completed within thirty days of tube replacement.

6. On Installation of New Unit

All daily and monthly tests must be completed before patient examinations commence. All semi-annual and annual tests must be completed within thirty days of a new installation.

Half-Value Layer

The manufacturer's procedure to evaluate HVL shall be followed and the manufacturer's published specifications shall be met.

7. Radiation Protection Survey

A radiation protection survey must be completed before operation to ensure that the structural shielding is adequate to meet the safety requirements.

Scan protocols

Each CT unit shall have posted scan protocols available for reference by the operator. As a minimum, these protocols shall be include the following scan factors: use of oral contrast; use of intravenous contrast; slice thickness; slice spacing; anatomical description of the extent of the field for scanogram; and the technical factors selected for the examination. Technical factors must differentiate between pediatric and adult sized patients as well as anatomical areas that are to be imaged.

The radiologist, CT technologist and medical physicist should work together on establishing the technique (kV and mAs) used on each projection, so that patient dose is minimized. The technique used should always be sufficient, so as to avoid the appearance of clinically unacceptable quantum mottle.