GAMMA CAMERA

INTRODUCTION

●It is an electronic device that detects the gamma rays which are emitted by radio pharmaceauticals.

●It was developed by Hal O Anger in late 1950s. That’s why it is also called Anger camera.

•Gamma camera is  composed of radiation detectors called gamma camera  heads, which are encased in metal and plastic.

•It is attached to a round circular donut  shaped gantry.

•The patient lies on the examination table  which slides in between the parallel gamma camera heads.

•Sometimes, the  gamma camera heads are oriented at a 90 degree angle and are placed over the patient’s body.

•All circuits and motors related to movement ( longitudinal, rotational, up & down) of gamma camera are placed in gantry.

COMPONENTS

●Collimator

●Detector

●PMT tube

●Preamplifier

●Amplifier

●Position and summing circuit

●PHA

●Computer and display

COLLIMATOR

●The multihole collimator is a lead disc of 25mm thick and 400mm diameter

●It has 20,000 circular or hexagonal holes of diameter 2.5mm with septa of 0.3mm

●Gamma rays passes from collimators and reaches to crystals

●The rays which are perpendicular to the crystals only those rays intracts to it

●The rays which are not perpendicular, those rays are absorbed by the collimators

●Some rays can pass through the collimators but of less energy. Those rays are eliminated by PHA

TYPES OF COLLIMATOR

●Parallel hole collimator

●Diverging hole collimator

●Converging hole collimator

●Pinhole collimator

PARALLEL HOLE COLLIMATOR

●These collimators have thousands of holes which may be in round, square, rectangular or hexagonal shape

●Septa must be suffecient thick to absorb the photons incident upon them. Hence, thick septa collimators are used while using radiopharmaceuticals.

●No collimators gives the expected spatial resolution and effeciency and there is always compromise between them

●Collimator to object distance do not affect the image size but degrade spatial resolution with increasing collimator to object distance.

DIVERGING COLLIMATOR

●It is used with small diameter camera to get larger FOV and is used to image large organ like lungs

●This type of collimators minifies the image

CONVERGING COLLIMATOR

●It is used to get less FOV

●It minifies the image

●It is useful in children and to the examination of small organs

PIN HOLE COLLIMATOR

●It is cone of lead with single hole of few mm diameter

●It gives magnified but inverted image of superfecial organs like Thyroid

●If hole is large, greater will be the sensitivity and require only less radionuclide with lesser patient dose

●This reduces the spatial resolution which is the limitation of gamma camera

PMT – PHOTO MULTIPLIER TUBE

•The crystals are coupled to a no. of PMT tubes

•Gamma Camera contains 37 -91 PMT.

•Each PMT is evacuated glass envelope having photocathode

•It absorbs light and emit photoelectrons

•The photoelectrons passes through a series of dynodes and produces additional electrons

•All photoelectrons are accelerated towards the anode to create charge pulse.

•PMT converts the light photons into the electrical signal

•Amplitude of pulse which are produced by PMT Tube is proportional to the amount of light it receives.

•Each PMT is provided with a preamplifier to amplify the signal

PRE – POST AMPLIFIER

●Preamplifiers are attached above the PMT tubes.

●The amount of charge which is given by PMT tubes is very small. Due to which a very sensitive amplifier is  needed to amplify the signal. This type of amplifier is  generally called a pre-amplifier.

●After the preamplifier an amplifier is located to further amplify the signal.

POSITION AND SUMMING CIRCUIT

●The pulses from pre amplifier are fed to the position and summing circuit

●The position circuit receives the pulses from individual preamplifier after each photon interaction in crystals

●It produces an X and Y signal respectively

●X and Y signal together specify the position of interaction in the plane of crystal

●The summing circuit adds the pulses from the individual preamplifier  to produce a Z pulse

●Amplitude of Z pulse is proportional to the total energy deposited in the crystal

●The Z pulse is fed to the PHA

PULSE HEIGHT ANALYZER

•PHA is an instrument that accepts electronic pulses of varying heights from particle and event detectors

•It digitizes the pulse heights, and records the number of pulses of each height in registers or channels

•Recording a pulse-height spectrum or pulse-height distribution is used for later pulse-height analysis.

•PHA gives the logic pulse. The pulses which are within the preset range of energy, only those rays are passed through the PHA

•The pulses which are higher or lower than preset value, those pulses are rejected.

•PHAs are used in nuclear and elementary particle physics research. It is a specific modification to multichannel analyzers

•It is also integrated into particle counters or used as a discrete module to calibrate particle counters.

COMPUTER AND DISPLAY

●Finally, a processing computer is used to deal  with the incoming projection data and processes it  into a readable image of the 3D spatial  distribution of activity within the patient.

●The computer may use various methods to  reconstruct an image, such as filtered back  projection or iterative reconstruction.

●The pulses are digitalized by ADC

●X, Y and logic pulse of Z are fed into the computer, which records the Z pulse as a count in the memory location , corresponds to X, Y coordinates

●The image is displayed on the monitor screen

●The brightness of each pixel is proportional to the number of counts, in turn to gamma rays originated from. The patient. Thus, images are build up by using 50,000 to 1 million counts per image frame

●If counts are too long or short, the monitor screen would appear as uniformly bright or grainy

●CRT, Scalar, Timer are used as display

FLOW DIAGRAM OF GAMMA CAMERA

APPLICATION OF GAMMA CAMERA

●It is used to locate cancerous tumours, minor  bone fractures, abnormal functioning of organs and other  medical problems .

●Iodine-131 is used to detect thyroid (a gland that absorbs Iodine) problems.

●Technetium-99 is used to find tumours in the body.

●Gamma camera gives structural and functional image of body  organs.

●Bone scan.

●Myocardial Perfusion

●Lungs scan.

●Kidney function.

●Thyroid uptake

●Whole body scan.

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