CT Sim QA Procedures

Goal of CT QC

Ensure the best possible CT image quality with minimal patient dose

QA program in CT must meet accreditation standards of ________ along with vendor specifications

American College of Radiology (ACR)

Quality control testing must also be completed after major repairs and before the first clinical scan after the repair. Examples of repairs include:

Replacement or repair of x-ray tube, generator, collimator assembly or x-ray detectors

Acceptance testing

Performed on new equipment

Routine testing

Performed on existing equipment on an ongoing basis

3 basic principles of QC

• Test performed on a regular basis

• Test measurements should be documented on a designated form

  • Can use QA data form provided by the manufacturer but each facility can design their own forms

• Indicate if results are within specified guidelines

Equipment used in CT QA tests

Phantom (along with test protocols to ensure optimal operation is within established tolerances)

LAP IsoMark (test the gantry laser and BB alignment)

Laser check device (check the room’s external lasers)

Tube conditioning

A process which allows the x-ray tube to come to its normal operating temperature after a pause in system operation of more than 10 hours

High-contrast (spatial) resolution

Minimum distance between two objects that allows them to be seen as separate and distinct

Low-contrast resolution

Subtle differences in tissue densities from one region of anatomy to another

Image uniformity

The ability of the CT scanner to yield the same CT number regardless of the location of the ROI within a homogenous object

Noise

Represents the portion of the CT image that contains no useful information

Produces a “salt-and-pepper” appearance or grainy quality in the image

What is the primary factor limiting low-contrast resolution in CT?

Image noise

Sources of noise in a CT image

Object size, reconstruction algorithms, detector efficiency, and artifacts

As noise increases on an image, the edge definition of anatomic borders and subtle differences in attenuation between tissues ________

Decreases

Methods to minimize noise

Tube warm-ups

Daily system calibrations

Pitch (spiral pitch ratio)

Describes the relationship between the patient couch movement through the CT scanner and the x-ray beam width. Changing pitch may cause overlap or gaps in beam width during table movement

CT numbers represent the ________ within the body according to their atomic number and physical density

Attenuation values of different structures

Daily tolerance limit of water

0±5 HU

HU of air

-1000

HU of adipose tissue

-100 (slight range of -50 to -100 HU)

HU of water

0

HU of muscle

50 (range of 10 to 50 HU)

HU of bone

1000

Alignment light accuracy test (lasers)

Set device up to lap zero

Load preset beams and physically confirm lasers adjust to the correct position

Are CT lasers movable or fixed?

Movable

Laser/image/treatment isocenter coincidence daily tolerance for non-SRS, non-SBRT, and SBRT

2mm

Laser/image/treatment isocenter coincidence daily tolerance for SRS

1mm

Who performs laser/image/treatment isocenter coincidence daily checks

Therapist