Topic 7. Gamma Camera (I)

• General Comments

• Basic Principles of the Anger Camera

• Types of Gamma Cameras

General Comments

• Why γ rays? (penetrating through the body, easily stopped by lead, β emission or Auger electrons can not get out of body)

• Why NaI(Tl) detector (reasonable compromise between efficiency and cost etc. )

• Historically, γ ray imaging started from matrix detectors of late 40s to rectilinear scanner, and to the Anger scintillation camera of late 50s which is the most used today.

Basic Principles of the Anger Camera

• System Components

• Detector System and Electronics

• Collimators

• Event Detection in Gamma Cameras

System Components

• Collimator

• NaI(Tl) crystal

• Light Guide (optical coupling)

• PM-Tube array

• Pre-amplifier

• Position logic circuits (differential&addition etc.)

• Amplifier (gain control etc)

• Pulse height analyzer

• Display (Cathode Ray Tube etc).

NaI(Tl) Crystal Assembly

Detector System and Electronics(1)

• Typical detector in Anger camera: NaI(Tl) crystal with 1.25cm thick x 30-50 cm in diameter (thinner for low energies, 6mm)

• Thinner crystal is preferred for Anger camera in order to get better intrinsic resolution therefore better image (sacrifice intrinsic efficiency)

Detector System and Electronics(2)

• Optical coupling materials (silicon fluid, grease, or lucite light pipes) are placed between the NaI(Tl) crystal and the array of photomultiplier tubes --called light guide or pipe

• Array of PM tubes (37,61,75 or 91, round, hexagonal or square shapes) arranged in hexagonal pattern

Detector System and Electronics(3)

• Part of the signal processing circuitry (preamplifier, pulse height analyzers, amplifier, pulse pile-up rejection etc.) is attached to each PM tube and sealed in a light-tight protective housing

Position Circuitry(1)

• The photomultiplier tubes are divided into horizontal halves to obtain X+ and X- signals and vertical halves to obtain Y+ and Y- signals.

• Four summing matrix circuits are used to sum up for x+,x-,y+ and y- signals from each TM tubes where each of these signals is the product of signal amplitude and position factor.

• A separate summing circuit is used to sum up a total signal Z from all PM tubes (signal amplitude only, no position factor)

Light Sharing Between PM Tubes

Position Circuitry(2)

• The radiation position is then determined by X=k(X+-X-)/Z and Y=k(Y+-Y-)/Z where k is a scale constant, Z is the total signal amplitude and proportional to the incoming radiation energy.

• The positional signals X and Y must be normalised by total signal Z because X and Y themselves depend on the both signal and positional factors (different radiation energy gives different signal amplitude at the same position)

PM Tubes and Signal Positions

PMT Energy Window Correction

Pulse Height Displays

• Pulse height analyzer is used to analyse the Z signal and if accepted, signal will be displayed on the monitor (CRT etc.) at the position determined by X and Y.

• Two kind of display modes can be used to display the energy spectrum, namely, Z pulse display and multi-channel analyzer display.

Collimators

• Absorptive collimation is used for most γ ray image formation (inefficient method for utilisation of radiation)

• Four basic collimator types are used with Anger camera and similar camera-type imaging device: pinhole, parallel hole, diverging and converging collimators.

Parallel-Hole Collimator

Pinhole Collimator(1)

• A cone shape lead with a small pinhole of a few millimeter in diameter, about 20-25 cm from the pinhole aperture to the detector.

• The image is inverted and could be magnified or minified depend on the object position: I/O=f/b where I and O are image and object sizes respectively, f is the distance between the pinhole and the detector, b is the distance between the object and the pinhole.

Pinhole Collimator(2)• The size of the imaged area changes with

the distance between the object and the collimator b: D’=D/(I/O) where D is the diameter of the detector and D’ is the images area.

• Image size changes with the distance between the object and the collimator b, therefore, image is distorted in 3 dimension.

Parallel Hole Collimator• Parallel holes are drilled or cast in lead

• Sept is the walls between holes and its thickness is chosen to prevent γ rays from crossing from one hole to the next.

• Image is the same size as the source distribution to the detector.

• Slant-hole collimator is a titled parallel holes collimator.

Diverging Collimator• Diverging from the collimator face towards the object.

• The converging point is about 40-50cm behind from the collimator.

• Image is minified: I/O=(f-t)/(f+b) where f is the distance between the front of the collimator and the converging point, t is the thickness of the collimator and b is the distance between the object and the front of the collimator.

• Useful image area becomes larger as the image becomes more minified. Image size depends on the object distance b (image has distortion).

• Useful for small detector to image large organ.

Converging Collimator• Holes converge to a point in front of the collimator (about

40-50 cm from the collimator)

• Images are magnified & non-inverted if the objects are placed between the converging point and the collimator surface: I/O=(f+t)/(f+t-b). (image distortion due to b dependence)

• Images are inverted & magnified if the objects are placed between the converging point and twice the convergence length and an inverted & minified image beyond that distance (not often used).

• Useful for using large detector to image small organs

Parallel vs Converging Collimators

Gamma Camera Detection Events

Image Display and Recording Systems(1)

• Persistence CRT: displays light spots that do not fade immediately

• Non-persistence CRT: display images for film decording

• Polaroid film is convenient in use but expensive. It is a positive type and has limited range of optical densities (limit both contrasts and latitude, the useful exposure range)

• Transparency film is a negative film type (darker for greater exposure) and has better contrasts and latitude than Polaroid film

Image Display and Recording Systems(2)

• Polaroid cameras often have three separate lenses, with different lens aperture opening, to provide three deferent densities on a single film simultaneously.

• Laser film printers are now replacing the old film making practice in nuclear medicine.

Stationary or Mobile

• Anger camera can be sued for static or dynamic imaging

• Stationary cameras are designed to be at a fixed location while mobile camera has wheels.

Types of Gamma Camera

Types of Gamma Camera

Types of Gamma Camera

Scanning Camera• Scanning Anger cameras are used for

whole body imaging

• Either detector or patient support table may be set to move

• Diverging collimator may be used to cover entire width of the patient’s body.

• Whole body images can be printed on a single film.

Whole Body Bone Scan

Dynamic Sequence of Planar Images