QUALITY CONTROL IN CT SCAN

Quality Control program

are designed to ensure that the CT system is producing the best possible image quality using the minimal radiation dose to the patient.

CT technologists and medical physicists

Responsibility for performing and documenting quality control tests is often shared between

CT technologist

typically perform and record routine quality control tests

medical physicists

testing done by ___ is typically annual or semiannual

medical physicis

is required to obtain necessary dosimetric data

Alignment of table to gantry,

Gantry tilt,

Positioning of the patient support (longitudinal), 

Axial patient positioning accuracy, 

Collimation test (slice width)

Mechanical tests: (5)

Gantry Tilt

The accuracy of displayed gantry tilt can be assessed by supporting envelope-wrapped x-ray film at the gantry end of the patient table

Gantry Tilt.

The film must be held vertically (e.g. by taping to a Perspex block), so that it is parallel to the sagittal plane and intersects scan and coronal planes at right angles

maximum superior gantry tilt, maximum inferior gantry tilt, 0º gantry tilt

three axial exposures are made using the same film

ruler or tape measure

A ___ placed alongside the table, can be used to check that the degree of couch movement indicated on the gantry agrees with the actual distance moved.

70- 80 kg

A load of approximately ____ should be placed on the table in order to simulate the weight of a patient

table top

gantry

The test should be performed twice: by driving the ___ both away (forward) from and towards (backlash)the ___.

simple mechanical test

In helical scanning, it is not sufficient to use a ___ because the distance imaged depends on couch speed and scanner software

Perspex test object

One method of assessing imaged distance accuracy is to use a ___ containing two small radio-opaque markers, separated by a fixed distance

Scan Projection Radiography (SPR)

20 cm

The test object is scanned in ____ mode and a helical run is planned to start at the first marker and to end at a distance of ____ from the first marker If couch travel is accurate during the helical scan, the markers should be clearly seen on the first and final images of the series

markers

If couch travel is accurate during the helical scan, the ___ should be clearly seen on the first and final images of the series.

Axial Patient Positioning Accuracy

Axial patient positioning accuracy/Agreement between internal and external scan plane lights

Axial Patient Positioning Accuracy

Use the envelope-wrapped film recommended for that measurement, however, a piece of paper or card can also be use

external scan plane ligh

Axial Patient Positioning Accuracy

The wrapped film is placed flat on the table and illuminated by the 

Axial Patient Positioning Accuracy

The position of the light is marked on the film envelope and the table is moved automatically to the scan plane

internal light

Axial Patient Positioning Accuracy

If the distance between the internal and external lights is correct, the ___ should now coincide with the mark on the film envelope

Coronal And Sagittal Plane Lights

A long, thin object, with a high CT number relative to air, such as the ‘lead’ in a pencil or a straightened paper clip, can be used as a marker to perform this test

Coronal And Sagittal Plane Lights.

The marker is supported above the patient table and aligned, using the indicating lights, so that it is positioned at the isocentre, parallel to the z-axis and perpendicular to the scan plane

x = 0, y = 0

Coronal And Sagittal Plane Lights

If indicating lights are accurately aligned to the coronal and sagittal planes, the marker should appear as a dot at exactly ___ on the axial image

Slice Width (Tomographic Section Thickness)

Axial mode measurements

Catphan

contains an insert suitable for this test

Dose Profiles (Irradiated Slice Width).

Expose envelope-wrapped x-ray film, supported in air at the isocentre, at each of the slice width settings

Dose Profiles (Irradiated Slice Width)

Once developed, optical density profiles may be plotted using a scanning microdensitometer or the width measured using a ruler

Dose Profiles (Irradiated Slice Width)

To provide an accurate dose profile, a calibration curve can be applied to convert optical density profiles into dose profiles, from which irradiated slice widths may be derived

Accuracy of X-Ray Tube Voltage (kV),

Linearity of X-Ray Tube Current (mA),

Accuracy of Irradiation Time (t), 

Consistency of Radiation Output, 

Radiation Quality (Total filtration – HVL)

Tests For High Frequency Generators (5)

minimum, maximum, mid range

When more than 3 selections of a CT condition of operation are available then measurement should be carried out at least for ___, ___ and one ___ value of the CT condition of operation.

Radiation Dose Test

Measurement of Computed Tomography Dose Index

CTDI in Perspex Phantoms

The body phantom placed on the patient table and the head phantom is supported in the head rest

CTDI in Perspex Phantoms

Phantoms are aligned centered at the scan isocenter

CTDI in Perspex Phantoms

The ion chamber is inserted into either the central or one of the peripheral cavities of the phantom (all other cavities being filled with Perspex rods

central CTDI

Dose measurements at the center are used to calculate the

Peripheral CTDI

is measured in at least four positions around the phantom, so as to achieve a true average

Test phantoms

are used for the purposes of calibration and evaluation of the performances of CT scanners (supplied by manufacturer)

CT number Uniformity, CT number Linearity, Noise, Low contrast resolution, Spatial resolution

Image Quality Parameters (5)

CT Number Accuracy

depends on tube voltage, filtration, object thickness/attenuation

CT Number Uniformity

It relates to the fact that a CT number of each pixel in the image of an homogeneous object should be the same over various regions

Physical Parameters Of CT Image

Image quality. May be expressed in terms of physical parameters such as uniformity, linearity 

CT Number Linearity

It concerns the linear relationship between the calculated CT number and the linear attenuation coefficient of each element of the object

Noise

Is the local statistical fluctuation (standard deviation) of CT numbers of an homogeneous Region Of Interest (ROI)

Noise.

It strongly affects the low contrast resolution

Noise

_____ is dependent on the radiation dose

high contrast resolution

determines the minimum size of detail visualized in the plane of the slice with a contrast >10%

reconstruction algorithm,

detector width,

effective slice thickness,

object to detector distance, 

X-ray tube focal spot size, 

matrix size

high contrast resolution is affected by (6)

Ionization chamber/ Semiconductor based type survey meter

Water Phantom

Radiation Leakage Levels And Radiological Protection Survey Instruments and Accessories required: 

Ionization Chamber/Semiconductor based survey meter

is used to measure the radiation levels

Near control console (operators position),

Outside patient entrance door,

Behind window (if any), 

Patient waiting area 

Provide The Values Of Maximum Radiation Level (mR/hr) at following places: (4)