equipment AND QUALITY CONTROL

BY MUMBA CHILIMBOYI

EQUIPMENT USED IN NUCLEAR MEDICINE AND QUALITY CONTROL

IONIZATION CHAMBER

DOSE CALIBRATOR

GAMMA SCINTILLATION CAMERA

COMPUTERS

SINGLE-PHOTON EMISSION COMPUTED TOMOGRAPHY (SPECT)

RADIATION MONITOR GEIGER-MUELLER COUNTER

DETECTORS

IONIZATION CHAMBER• Ionization chambers are handheld survey instruments used to measure low or

high exposure rates.

• They have an air or gas-filled chamber but a low efficiency for detection of gamma rays.

• These instruments have a relatively low applied voltage from anode to cathode; as a result, there is no avalanche effect and no dead time problem.

• Ionization chambers typically are useful at exposure rates ranging from 0.1 mR

(2.5 x 10-8 C/kg)/hour to 100 R (2.5 x 10-2 C/kg)/hour.

• A dose calibrator is a special form of an ionization chamber.

DOSE CALIBRATOR AND GAMMA CAMERA

• DEFINITION

• COMPONENTS

• PRINCIPLES OF OPERATION

• QUALITY CONTROL

DOSE CALIBRATOR• The dose calibrator is an ionization chamber used to assay the

amount of activity in vials and syringes• This includes the assay of individual doses before

administration to patients, as required by regulation.• The dose calibrator operates over a very wide range of

activities, from hundreds of kilobecquerels (10s of μCi) to tens of gigabecquerels (up to a curie).•Displayed units are mCI millicurie or Megabecquerel

DOSE CALIBRATOR COMPONENTS1

3

2

OPERATION OF DOSE CALIBRATOR• The chamber is cylindrical and holds a defined

volume of pressurized inert gas (usually argon). • Within the chamber is a collecting electrode. As

radiation emanates from the radiopharmaceutical in the syringe, it enters the chamber and interacts with the gas, causing ionization. • If no voltage is applied to the electrodes, the

ion pairs recombine.• If an electrical differential applied between the

chamber and the collecting electrode is applied it causes the ions to be captured and measured. This measurement is used to calculate the dose contained in the syringe.

QUALITY CONTROL FOR DOSE CALIBRATORS

• The dose calibrator is used to assay the activity administered to the patient, and thus a comprehensive quality control program is necessary. • This program comprises four basic quality control tests:

-Geometry-Accuracy-Linearity-Constancy

THE GEOMETRY PROTOCOL TESTS• This test of the dose calibrator provides the same reading for the same

amount of activity irrespective of the volume or orientation of the sample.• A reading of a certain amount of activity in a 0.5-mL volume is

obtained. The volume is then increased by augmenting the sample with amounts of nonradioactive water or saline and taking additional readings. The subsequent readings should not vary from the original readings by more than 10%. • The geometry test is performed during acceptance testing and after a

major repair or movement of the equipment to another location.

ACCURACY•For accuracy, calibrated sources (typically cobalt-57 and 137Cs) are analysed.•The resultant reading cannot vary by more than 10% from the calibrated activity decay corrected to the day of the test. •The accuracy test should be performed during acceptance testing, annually thereafter, and after a major repair or move.

LINEARITY PROTOCOL TESTS •This test of the dose calibrator operates appropriately over the wide activity range to which it is applied. The device is tested from 10 μCi (370 kBq) to a level higher than that routinely used in the clinic and perhaps as high as 1 Ci (37 GBq). •The activity readings are varied by starting with a sample of radioactivity of Tc-99m at the highest value to be tested (e.g., tens of gigabequerels).

LINEARITY PROTOCOL TESTS Cont.…

• The activity readings are then varied by either allowing the source to radioactively decay over several days or using a set of lead shields of varying thicknesses until a reading close to 370 kBq is obtained. Each reading should not vary by more than 10% from the line drawn through the calculated activity values.• The linearity test should be performed during acceptance

testing, quarterly thereafter, and after a major repair or move.

THE CONSTANCY PROTOCOL TESTS • This tests is the reproducibility of the readings as compared to

a decay-corrected estimate for a reference reading obtained from the dose calibrator on a particular day. • Today’s constancy reading cannot vary from the decay-

corrected reference reading by more than 10%. • The constancy test varies from accuracy in that it evaluates the

precision of the readings from day to day rather than accuracy. • The constancy test should be performed on every day that the

device is used to assay a dose to be administered to a patient.

DOSE CALIBRATOR RADIATION PROTECTION

•Lead shielding around the ionization chamber1. Protects the operator2. Reduces the response background radiation

•Sample holder can be cleaned in the event of radioactive contamination of the chamber well

GAMMA CAMERAThe most widely used cameras in nuclear medicine are:1. Simple gamma scintillation (Anger) camera 2. Single-photon emission computed tomography (SPECT) capable

gamma camera.FUNCTION OF GAMMA CAMERA• A gamma camera converts photons emitted by the radionuclide in the

patient into a light pulse and subsequently into a voltage signal.

GAMMA CAMERA SYSTEM

COMPONENTS• The collimator• The scintillation crystal• An array of photomultiplier tubes (PMTs)• Preamplifiers• A pulse height analyzer (PHA)• Digital correction circuitry,• A cathode ray tube (CRT)• The control console. • A computer and • Picture archiving systems(PACs)

GAMMA CAMERA ASSEMBLY

COLLIMATORS• The collimator is made of perforated or folded lead and is

interposed between the patient and the scintillation crystal. It allows the gamma camera to localize accurately the radionuclide in the patient’s body.• Collimators perform this function by absorbing and stopping

most radiation except that arriving almost perpendicular to the detector face.•Most radiation striking the collimator at oblique angles is not

included in the final image.

TYPES COLLIMATORS

As the energy of the radionuclide increases, the best collimator usually has thicker and longer septa. For a given septal thickness, spatial resolution of a collimator increases with septal length but sensitivity decreases.

The two basic types of collimators are pinhole and multihole

SEPTAL PENETRATION AND PHOTON SCATTERING

EFFECT OF SEPTAL LENGTH ON COLLIMATOR SENSITIVITY AND RESOLUTION

EFFECT OF DIFFERENT SOURCE-TO-CAMERA DISTANCES

CRYSTAL AND OTHER PHOTON DETECTOR DEVICES

• Radiation emerging from the patient and passing through the collimator interacts with a thallium activated sodium iodide crystal.• Crystals are made with thallium or sodium activated cesium

iodide or even lanthanum bromide are used. They convert Gamma rays to light.• PMTs situated along the posterior crystal face detect this light

and amplify it to an electrical pulse.

PHOTONMULTIPLIER TUBEs (PMTs)• A photomultiplier tube (PMT) converts a light pulse into an electrical

signal of measurable magnitude. • Localization of the event in the final image depends on the amount of

light sensed by each PMT and thus on the pattern of PMT voltage output. • VOLTAGE AMPLIFIER / VOLTAGE SUPPLY A High voltage supply for the PMT An amplifier increases the size of the pulse.

PULSE HEIGHT ANALYZER

• The basic principle of the PHA is to discard signals from background and scattered radiation and/or radiation from interfering isotopes so that only primary photons known to come from the photopeak of the isotope being imaged are recorded• The PHA discriminates between events occurring in the crystal that

will be displayed or stored in the computer and events that will be rejected.

CONSOLE CONTROLS• Image exposure time is selected by console control • Its usually a preset count, a preset time, or preset information

density for the image accumulation. Other console controls are present for orientation and allow

the image to be reversed on the x- and y-axes. In addition, the CRT image may be manipulated by an

intensity controlHard copy images on film may be obtained directly from the

computer, display digital images on monitors and store the images in a picture archiving system.

OPERATION OF GAMMA SYSTEM Gamma rays emitted from within the patient pass

through the holes of an absorptive collimator to reach the NaI crystal.

On interaction of the gamma ray with the NaI scintillating crystal, thousands of light photons are emitted, a portion of which are collected by an array of PMTs. By taking weighted sums of the PMT signals within the associated computer

The two-dimensional (2D) x and y location and the total energy of the detection event deposited is estimated. If the energy deposited is within a prespecified energy window (e.g., within 10% of the photopeak energy), the event is accepted and the location of the event recorded.

In this manner, the gamma camera image is constructed on an event-by-event basis, and a single nuclear medicine image may consist of hundreds of thousands of such events.

GAMMA CAMERA QUALITY CONTROL•This involves acceptance testing of the device before its initial use and a program of routine tests and evaluations applied on a regular basis. • It is essential that the performance be evaluated regularly to ensure that the images adequately demonstrate the in vivo distribution of the administered radiopharmaceutical and that any quantitation performed with the camera yields values that are accurate and precise

GAMMA CAMERA QUALITY CONTROL SUMMARY

PARAMETER COMMENTDAILY

• UNIFORMITY

• WINDOW SETTING

• FLOOD FIELD; INTRINSIC (WITHOUT COLLIMATOR) OR EXTRINSIC (WITH COLLIMATOR)

• CONFIRM ENERGY WINDOW SETTING RELATIVE TO PHOTOPEAK FOR EACH RADIONUCLIDE USED WITH EACH PATIENT

WEEKLY OR MONTHLY

• SPATIAL RESOLUTION

• LINEARITY CHECK

• REQUIRES A “RESOLUTION” PHANTOM SUCH AS THE FOUR-QUADRANT BAR

• QUALITATIVE ASSESSMENT OF BAR PATTERN LINEARITY

The purpose of quality control is to detect changes in the performance of a gamma camera and below summary:

GAMMA CAMERA QUALITY CONTROL SUMMARY Cont....

PARAMETER COMMENT

ANNUALLY

• SYSTEM UNIFORMITY

• MULTI-WINDOW REGISTRATION

• COUNT RATE PERFORMANCE

• ENERGY RESOLUTION

• SYSTEM SENSITIVITY

• HIGH COUNT FLOOD WITH EACH COLLIMATOR

• FOR CAMERAS WITH CAPABILITY OF IMAGING MULTIPLE ENERGY WINDOWS SIMULTANEOUSLY

• VARY COUNTS USING DECAY OR ABSORBER METHOD

• EASIEST IN CAMERAS WITH BUILT IN MULTICHANNEL ANALYZERS

• COUNT RATE PERFORMANCE PER UNIT OF ACTIVITY FOR EACH COLLIMATOR

EFFECT OF INCREASING THE PATIENT-TO-DETECTOR FACE DISTANCE

ON CLINICAL IMAGES

SINGLE PHOTON EMISSION COMPUTED TOMOGRAPHY (SPECT)•SPECT is a nuclear medicine tomographic imaging technique that uses gamma rays. • It is very similar to conventional nuclear medicine planar imaging using a gamma camera. • It is able to provide true 3D information..

SPECT

SPECT CONT.…•SPECT imaging is performed by using a gamma

camera to acquire multiple 2-D images from multiple angles.• A computer is then used to apply a tomographic

reconstruction algorithm to the multiple projections, yielding a 3-D dataset. •The dataset may then be manipulated to show thin

slices along any chosen axis of the body.

IMAGE RECONSTRUCTION

• SPECT acquire raw data in the form of projection data at a variety of angles about the patient.• Image reconstruction involves the processing of these data to

generate a series of cross-sectional images through the object of interest.

PROJECTION DATA

APPLICATION OF SPECT• Cerebral Blood flow imaging• Myocardial perfusion imaging with thalium-201 or technetium-99

perfusion agents• Imaging of tumors or infections with agents such as gallium-67 or 111

WBCs• Certain Cases of Bone Imaging• Brain studies• Liver/ Spleen Imaging• Renal Imaging

FOURIER TRANSFORM• Image data may be best represented in either spatial (real) or frequency

space.• The mathematician Joseph Fourier noted in 1807 that any arbitrary

signal can be generated by adding a large number of sine and cosine signals of varying frequencies and amplitudes. • The plot of amplitude as a function of frequency is referred to as the

Fourier transform, and it defines the components of the image at each frequency. • The low frequencies provide the overall shape of the object, whereas the

high frequencies help define the sharp edges and fine detail within the image.

APPLICATIONS OF FOURIER TRANSFORM

• The Fourier method is often used on images from astronomy, microbiology, images of repetitive structures such as crystals and so on. • Fourier transform is good for identifying a periodic component or lattice

in an image. • Identifying regular patterns on an image has other advantages like

removing regular dirty spots or noise from an image • Components of higher frequency can be removed to achieve anti-aliasing

effect, i.e. removing ugly zaggy edges.

APPLICATIONS OF FOURIER TRANSFORM Cont.…..

•There are other techniques associated with Fourier transform:

convolution theorycorrelation, samplingReconstructionimage compression, and more.

RADIATION MONITOR-GEIGER-MUELLER COUNTER• Geiger-Mueller (GM) counters are handheld,

very sensitive, inexpensive survey instruments used primarily to detect small amounts of radioactive contamination. • The detector is usually pancake shaped, it may

also be cylindrical. • The detector is gas-filled and has a high applied

voltage from the anode to the cathode. This causes one ionization to result in an “avalanche” of other electrons, allowing high efficiency for detection of even a single gamma ray.

TYPES OF RADIATION DETECTORSBasically three types of radiation detectors are used in nuclear medicine

Gas detectorsScintillatorsSemiconductors

GAS DETECTORS• A gas radiation detector is filled with a volume of gas that acts as the sensitive

material of the detector. • In some cases, it is air and in others it is an inert gas such as argon or xenon,

depending on the particular detector. • Electrodes are located at either end of the sensitive volume. • The detector circuit also contains a variable voltage supply and a current detector. • As radiation passes through the sensitive volume, it causes ionization in the gas. • If a voltage is applied across the volume, the resulting ions (electrons and positive

ions) will start to drift, causing a measureable current in the circuit. The current will last until all of the charge that was liberated in the event is collected at the electrodes.

• The resulting current entity is referred to as a pulse and is associated with a particular detection event.

SCINTILLATION AND SEMICONDUCTOR DETECTORS

• Some crystalline materials emit a large number of light photons upon the absorption of ionizing radiation.This process is referred to as scintillation, and materials are referred to as scintillators. •As radiation interacts within the scintillator, a large number

of excitations and ionizations occur. On deexcitation, the number of light photons emitted is directly proportional to the amount of energy deposited within the scintillator.

SCINTILLATION AND SEMICONDUCTOR DETECTORS Cont.…

• Thermal energy can lead to a measureable current in some semiconductor detectors such as GeLi, even in the absence of radiation, and thus these semiconductor detectors must be operated at cryogenic temperatures. • On the other hand, semiconductor detectors such as cadmium telluride

(CdTe) or cadmium zinc telluride (CZT) can operate at room temperature. CdTe and CZT do not have the excellent energy resolution of GeLi, but at approximately 5%, it is still significantly better than that of sodium iodide.

CONCLUSION• Radiation detection and counting is the corner stone of nuclear

medicine. • Detectors of all types—gas detectors, scintillators, and semiconductors

—are used every day in the nuclear medicine clinic. • Some are used for ancillary purposes that support the clinic, such as

those used in the context of radiation protection. • Others are used to specifically acquire biological data for a particular

clinical purpose. Most notably, the gamma camera is used to obtain images of the in vivo distribution of the administered radiopharmaceutical from which the patient’s physiology or function can be inferred to further define the patient’s medical picture.

Conclusion Cont….• A rigorous quality control program must be maintained for all equipment used in

the nuclear medicine clinic to ensure the integrity of the data obtained from the patient. The quality control program for the gamma camera includes acceptance testing and tests that need to be performed on a routine basis. The nuclear medicine image acquired with the gamma camera provides a snapshot of the patient’s in vivo radiopharmaceutical distribution from a certain view and at a particular point in time. These images also can be acquired as a dynamic (time-sequence) study or in conjunction with a physiological gate such as the ECG. • Regions of interest can be drawn about specific features to provide regional

quantitation or TACs of dynamic processes. • Finally, nuclear medicine instrumentation continues to evolve, including the

development of devices designed for a specific clinical task such as breast imaging. It is expected that this development will continue in the years ahead.

REFERENCES• Essentials of nuclear medicine imaging; Fred A.Mettler 2012• The Requisites Ziessman - Nuclear Medicine, 4th ed. FRED A.

METTLER• Camera Systems. Publication 1141. Vienna, Austria: International

Atomic Energy Agency; 2003.• IAEA Comprehensive Clinical Audits Of Diagnostic Radiology

Practices A Tool For Quality Improvement• Nuclear Medicine Instrumentation Lecture Notes:2005; Ghoorun

S• Operational Levels In Radiopharmacy For Realigning Our

Profession With International Guidelines Richard E. 2015• Chandra R. Nuclear Medicine Physics: The Basics. 7th ed.

Philadelphia: Williams & Wilkins; 2011.

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