Quality control of Gamma Camera

By Dr/ Ibrahim Elsayed Saad

242 NMT

WHAT IS QUALITY?

The quality of a practice is to fulfill the

expectations and demands from:

Patient Clinicain

Your self

Quality assurance and quality control

• The concept of quality in the term “quality assurance” expresses the closeness with which the outcome of a given procedure approaches some ideal, free from all errors and artifacts (Whole procedure).

• The term “quality control” is used in reference to the specific measures taken to ensure that one particular aspect of the procedure is satisfactory (Single step in the procedure).

NUCLEAR MEDICINE SERVICE

Primary

service

Secondary

service

Nuclear

medicine

examination

or

treatment

facilities

patient care

waiting time

staff

reporting

competence

experience

optimisation

radiopharmaceuticals

methods

examination technique

instrumentation

etc

etc

QA-PROGRAMME OBJECTIVES

* Improvement in the quality of the diagnostic information. * Use of minimum amount of radionuclide activity to ensure the production of the desired diagnostic information. * Effective use of available resources

QA Medical exposure

• Choice of examination

• Determination of technical parameters

• Optimization of administered activity

• Methods of reducing the absorbed dose

• Quality control of equipment and radiopharmaceutical

• Quality assurance of methods

• Safe routines to avoid misadministration

Gamma Camera

Used to measure the spatial and temporal

distribution of a radiopharmaceutical

Test Schedule for Gamma Camera System

Test Frequency in routine testing

Acceptance Referenc

e

once

per

Quarter

Week

Half year

1-Physical Inspection A

2-Test of Intrinsic

Flood-field Uniformity

A R Q W

3- Test of Intrinsic Spatial

Resolution

A R H

4- Test of Intrinsic Count-rate

Performance

A R H

5- Test of Spatial Linearity and

Spatial Resolution

A R

6- Test of Basic Computer Timing A R H

7- Test of Computer Timing in

Dynamic Acquisition

A R H

8- Test of camera Sensitivity A R W

SOURCES FOR QC OF GAMMA-

CAMERAS

•Point source

•Collimated line source

•Line source

•Flood source

Tc99m, Co57, Ga67

<1 mm

Phantoms for QC of gamma-cameras

•Bar phantom

•Slit phantom

•Orthogonal hole phantom

•Total performance phantom

Phantoms for QC of gamma-Cameras

Physical Inspection • The Purpose of this test is to inspect a camera-

computer system for shipping damage and production flaws.

• Procedure: • 1-Image Display Devices • 2-Image Recording Devices • 3-Electrical Connections, Fuses and Cables • 4-Operation and Service Manuals • 5-Check for controls that are difficult to

operate or are noisy and switches that do not throw securely.

• Inspect keyboards for damage.

Special definitions for quality control

• System: • a term used to refer to the performance of the

camera detector, as it would be used in a clinical environment including components such as a collimator and supporting gantry.

• Energy window: • A range of gamma and X-ray energies, which are to be

accepted and processed. The window is expressed as a range of energies or as a percentage of expected peak energy. When expressed as percentage the peak energy must always be specified, and the window is always symmetrical about the peak energy value.

• Photopeak: • the characteristic energy distribution resulting from

the collection of total photon energy absorbed by the detector.

• Differential uniformity: • The amount of count density change per defined

unit distance when the detector’s incident gamma radiation is a homogenous flux over the field of measurement.

• Integral uniformity: • A measure of the maximum count density

variation over a defined large area of the scintillation detector for a uniform input gamma flux to the Useful Field of View of the camera.

• Pixel: • a picture element used to store a value in a

digital memory. It represents an area defined by dimensions in X and Y directions at a known position defined by X and Y coordinates.

• Scatter: • photons that have lost part of their

energy due to interaction with a medium, such as water, plastic, or tissue.

• Sensitivity: • The observed count rate per unit of

radioactivity at a specified distance. • Spectrum: • a plot of the number of detected gamma

rays versus the measured energy of the gamma rays.

• Energy resolution:

• A term used to characterize the ability of a gamma camera to distinguish between photons of different energies

• Spatial resolution:

• A term that characterizes the scintillation camera’s ability to accurately resolve spatially separated radioactive sources.

• Full Width at Half Maximum (FWHM): • The measure of the spread of a point or

line spread function measured between locations 50 %down on each side from the peak amplitude.

• Full Width at Tenth Maximum (FWTM): • The measure of the spread of a point or

line response function measured between locations 90 % down on each side from the peak amplitude.

• Differential linearity: • The amount of positional distortion

or displacement over a limited distance

• Useful Field of View (UFOV): • The area of the detector, which is

used for imaging gamma rays and x-rays. It is defined by a dimensioned figure supplied by the manufacturer.

• Central field of view (CFOV): • The area defined by scaling all linear

dimensions of the useful field of view (UFOV) by a factor of 75%.

• Intrinsic: • A term used to describe performance

characteristics of a scintillation camera that exclude external variables, which affect these specifications such as collimators or display devices.

• Extrinsic: • A term that is used to describe the total

system performance including photons attenuated throw passing into the collimator, so that it includes all external factors such as collimator.

EnergyWindow Peaking

15 % window 25 % window 35 % window

140 KeV 137 KeV Wrong setting

of energy window

1) Test of Intrinsic Flood-field Uniformity

• The purpose of this test is to test the combined intrinsic response of a camera-computer system to a spatially uniform flux of incident gamma radiation over the camera field-of-view.

• Materials used are: • Point source consisting of 10-20 MBq (0.3-

0.5 mCi) Tc-99m, giving a count rate not greater than 30 000-50 000 c /s with a 20% PHA window.

• Source mounting for point source.

• Procedure • 1. Remove the collimator from the detector

head. Align the head and the source mounting. • 2. Mount the source in the source mounting

with distance equal to 5 times the average detector diameter, which is (220 cm).

• 3. Centre a 20% PHA window on the Photopeak (Check of Energy Calibration of PHA).

• 4. Acquire an image on the cameras display device at a preset count of 15 x 10 6 using a 512X512 matrix.

• 6. Record the collection time for the camera. • 7. Remove the source and mount the collimator.

• Data Analysis: • Determine the maximum (Max) and minimum (Min)

counts in the pixels lying within the UFOV and the CFOV. The integral uniformity, IU, is then given by:

• IU = 100 * Max - Min Max + Min • Determine for each row or column of pixels in the

X and Y directions within the UFOV and the CFOV, the maximum count difference in any 6 contiguous pixels. Determine the highest value of this maximum count difference in the sets of rows and columns. The differential uniformity, DU, is then given by:

• DU =100* Hi ' Low Hi + Low

2) Test of Intrinsic Spatial Resolution

• The purpose of this test is to test the combined intrinsic spatial resolution of a camera- computer system in terms of the full width at half-maximum (FWHM) and full width at tenth-maximum (FWTM).

• Procedures: • 1-The collimator is removed from the detector face. • 2- Position the intrinsic-resolution phantom ( slit

phantom) and then adjust the source on the source holder to be in front of the detector face with distance equal to 5 times the average detector diameter, which is (220 cm).

• 3-The acquisition parameters for this test is adjusted as follows:

• a)15000 K. counts. • b)512X512X16 acquisition matrix. • c)20 % PHA window on the photopeak which is 140

KeV for Tc-99m.

• 4-Acquire an image for the camera device at the preset count of 15 x 10 6

• 5-By using the software provided on the processing computer connected to the camera, by which we can draw a line on a single slit of the slits in the phantom image, this will give us a profile which indicate to the peak from which we can calculate the FWHM in Pixels. And then by knowing the matrix size used while acquiring the image we can calculate the FWHM in millimeters.

• 6-Then the estimation of the intrinsic spatial resolutions is done in terms of FWHM of the line spread function.

• counts

•

• Pixels

• At acceptance testing, the calculated values of FWHM in the X and Y directions should be compared with the manufacturer's worst-case values.

• At routine testing, the calculated values should be compared with the reference values.

• The same procedures for testing the intrinsic resolution is prepared .

• The resulted peaks for the lines separating the lead sheets is plotted on a graph and then the spatial distortion (linearity) is measured.

3) Test of Intrinsic Spatial Linearity

Test of Intrinsic Spatial Linearity

• Count

• pixels

Test of Intrinsic Spatial Linearity

• Then the calculation of the distance between each adjacent peaks is done, and calculating the difference between the maximum distance and the minimum distance this will indicate the shifting between the parallel slits, this is directly the spatial linearity.