The Gamma Camera

• Equipment used to detect the distribution of radiopharmaceutical within the patient
• The camera must have a heavy lead shielding to attenuate unwanted background gamma radiation

Components:

• Collimator
• Radiation detector (Scintillation crystal, Photomultiplier tubes)
• Electronics (Preamplifier)

NB:

1. HVL of lead for Tc-99m = 0.3 mm

2. Scattered radiation that pass through the collimator will be rejected later by PHA

Collimator:

• Consists of lead

• Average Thickness: 25 mm

• Average Diameter : 400 mm

• About 20000 closely packed circular or hexagonal holes, each is about 2.5 mm in diameter separated by 0.3 mm thick septa

• When radiation is released from the patient, it can exit at any angle and hit the detector in a location that doesn’t correlate with the location of its origin.
• To overcome this, a collimator is used in which only gamma photons that travel parallel to the collimator will be accepted, while those travelling at an angle will hit the septum (usually lead), absorbed and, therefore, not contribute to the image.

N.B. The collimator acts as a lens, its purpose is for spatial mapping. It does not reject scatter.

Features of the collimator

• Parallel hole

-The most common, Used with general purpose camera, Do not change the image size, In air sensitivity are the same at all distances from the collimator face

• Diverging hole

-Used with smaller diameter cameras (mobile cameras), Large FOV (Imaging large organs e.g. lung), Minification of image

NB: allow the use of a small crystal to image a large field of view

• Converging hole

-Holes converge to a point inside the patient, Small FOV (children & imaging small organs), Magnification of image, Spatial resolution deteriorates towards the edge of the field

• Pinhole

-Cone of lead with single hole at its apex, Used with small & superficial organs e.g. thyroid, Magnification & Inversion of image

NB: Divergent & Convergent – Suffer from geometrical distortion (with deterioration of spatial resolution at the edge of the field) – FOV & in air sensitivity differs with distance

Hole formation:

The holes can be created by:

• Crimped lead foil sheets (cheap but the gaps in the septae degrade image contrast)
• Drilling into a lead block (give better image contrast as there are no gaps in the septae, but more expensive)
• Casting from molten lead

Septal thickness:

• The higher the emitted energy of the gamma photons, the thicker the septae need to be to ensure maximum absorption of photons that hit them at an angle and, therefore, better rejection of non-perpendicular photons.
• Parallel hole collimators are classified as low, medium or high energy according to their septal thickness

Collimator sensitivity:

• Fraction of gamma rays falling on the collimator that pass through the holes to the crystal
• About 1%
• Expressed as counts per second / MBq • Sensitivity increase with : – increase number of holes - increase width of the holes – decrease Length of the holes (Shorter holes result in a wider angle of acceptance)
• Sensitivity increase - decrease required patient’s dose

Detector:

1) Scintillation crystal

• Single large crystal, Thickness: 6-12 mm, Diameter: 500 mm
• The crystal is fluorescent i.e. when a gamma photon interacts, it releases light photons (mixture of visible and UV light)
• Encased in aluminium cylinder with one transparent Pyrex face as, it is fragile, hygroscopic and easily damaged by temperature changes.
• Made of sodium iodide + activated by trace of thallium (NaI Tl). The thallium improves the light output.

NB:

• NaI have a high atomic number and density → absorbs about 90% of 99mTc γ rays, principally by the photoelectric process; but only 30% of 131I γ rays.
• Since the gamma photons have vastly greater energy than the light photons, approximately 5000 light photons are produced for every incident gamma photon.
• At 12.5 mm NaI (Tl) crystal and 140 keV approximately 90% of incident photons undergo interactions within the crystal. The figure drops for increasing energy and/or decreasing thickness, so a thinner crystal will have decreased sensitivity.

2) Perspex slab (light pipe / light guide)

• Sits between the scintillation crystal & the photomultiplier tubes
• Flat transparent plate that maximizes transfer of light from the crystal to the PMTs
• Silicone grease is used to ensure good contact between the scintillation crystal, the light pipe and the photomultiplier tubes.

3) Photomultiplier tubes (PMT)

Evacuated glass envelope.

• On one side is a photocathode and on the opposite side is an anode.
• 30-100 PMTs sit behind the scintillation crystal
• Purpose →multiply the small amount of light detected from the scintillation crystal to a large signal.

Mechanism

1. The light photons hit a photocathode at the entrance to the PMT → releases electrons in proportion to the amount of light that hits it (one electron per 5-10 light photons)

2. Electrons are accelerated toward a positive anode, en route electrons impinge on a series of dynodes (electrodes) connected to progressively increasing positive potentials

3. When each electron strikes a dynode it knocks out 3-4 electrons → accelerated to strike the next dynode

4. After 10 stages, the electrons have been multiplied by a factor 410 ≈ 106

5. The total electrons hit the final anode and the current produced forms the signal received by the pre-amplifier.

6. Thus each initial flash of light produces a pulse of charge or voltage large enough to be measured electronically

NB: There are between 10–12 dynodes in the chain and each has a potential difference of about 100 v. This results in a total potential difference across the tube of 1–1.2 kV.

Pre-amplifier

• Converts the current produced at the anode of the PMT to a voltage pulse.
• The amplitude of the voltage pulse is directly proportional to the charge produced at the anode and, therefore, the amount of light received by the PMT, which is proportional to the number of gamma photons that hit the scintillation crystal.

Image Formation

1) Energy calculation (pulse height spectrum)

For each scintillation formed, the calculated absorbed energy (Z value) that caused it depends on the energy of the gamma photon that was emitted from the patient and the proportion of the energy that was absorbed into the crystal.

The gamma photon energy absorbed by the scintillation crystal depends on its interaction with that photon which results in a spectrum of Z values. If:

1. All energy is absorbed: gamma photon interacts with crystal via photoelectric effect

2. Part of the energy is absorbed: photon undergoes one or more Compton interactions

The spectrum has:

• Peak (photopeak) corresponds to the maximum gamma photon energy (for 99mTc = 140 keV) (i.e. photons that have undergone entirely PEE in crystal).
• Compton band (tail) corresponds to photons that have undergone Compton interactions (in the patient, collimator or crystal) and, therefore, have a lower absorbed energy.
• Pulse pile up

The photopeak width:

• Should be very narrow but a variety of factors means that it often isn’t.

NB: This range of energies in the photopeak is due to statistical fluctuations in both:

1. Number of light photons produced in the crystal by each gamma ray photon.

2. Number of electrons produced in the photomultiplier by each light photon.

• Measured as the full width at half maximum (FWHM).

This value is used to calculate the energy resolution of the crystal, which is given as a percentage:

Energy resolution = FWHM (keV) / photopeak energy (keV) x 100

2) Scatter rejection (Pulse height analyser):

• If a gamma photon scatters within the patient’s body (via Compton scatter) → change direction → not hit the detector at a location corresponding to its location of origin.
• Scattered photons should be rejected as they degrade the image contrast and spatial resolution.
• This cannot be done by the collimator and is, therefore, done electronically by a process called energy discrimination.
• A scattered gamma photon will never hit the scintillator with the full energy (i.e. it won’t lie within the peak). Therefore, only gamma photons in the peak can be confidently identified as non-scattered radiation from the patient.
• Acceptance window: Usually a 10% acceptance window is used centered on the photopeak (in case of 99mTC, set around 126 – 154 kev)
• Can be adjusted
• More than one window can be used for radionuclides that have more than one photopeak (e.g. indium-111 has peaks at 172 and 247 keV). This is done by displaying the Z values with a multi-channel analyser that allows more than one window to be set.

NB:

• Causes of pulses with energies higher than photopeak (also ejected)

1. cosmic rays

2. as result of pulse pile up

• Some scattered ɶ rays which lost only 10% of its energy will pass through the window and degrade the image
• In case of Ga 67 or In 111 two or three windows must be set simultaneously

3) Image formation

• Each PMT corresponds to a coordinate on the scintillation crystal.
• This is then mapped out onto a matrix.
• Each time a gamma photon that falls within the acceptable energy window is detected; it is mapped onto its corresponding coordinate within the image.
• Image acquisition is controlled by the user and may be terminated when:

- Preset number of counts obtained (0.5-1 million counts are acquired for each image frame)

- Preset time passed

NB:

Position of the radioactive atom in the body:

1. The horizontal "X" and vertical "Y" coordinates →determined by the hole through which the gamma ray has passed and produce light flash in the crystal

2. The Z-axis (depth) → determined by photon energy of the original γ ray.

4) Image display

• Digital image is displayed upon a monitor with:

- Each pixel corresponding to a memory location in the matrix (X & Y coordinates)

- Brightness / colour scale corresponding to the count number in that location (Z axis)

• Matrix: 128 x 128 of 3mm pixels
• Display can be manipulated and optimised by:

- Smoothing to reduce noise

- Windowing to increase contrast

- Interpolation increases the display matrix relative to the acquisition matrix which spreads the counts and makes the pixels less apparent

- Adding and subtracting images to extract quantified information

NB: Separate images of 2 radionuclides can be obtained in the same time by setting two different PHA windows

Planar Imaging:

Planar imaging is the acquisition of 2D nuclear images, similar to plain x-ray films imaging.

Types:

1) Static

• Used for studies in which the distribution of the radiopharmaceutical is effectively static throughout the acquisition
• Inject → wait → image - Time from injection to imaging depends on the performed study.
• Total time of imaging can be determined by a preset time or a preset number of counts
• Provide information on:

- Organ size, shape and position

- Regions of increased or decreased uptake

Examples: DMSA renal scan, bone scan, lung perfusion scan

2) Dynamic

• Used for studies in which the distribution of the radiopharmaceutical changes rapidly with time
• Inject → image immediately → acquire series of frames over time
• Time between frames varies depending on the performed study
• Provides information on variation of radiopharmaceutical distribution over time

Examples: MAG3 renal scan, gallbladder emptying scan, gastric emptying scan

3) Gated

• Used to study organs with regular physiological motion
• Example: Cardiac gated blood pool imaging
• Acquisition is triggered by the R wave of the ECG. Images are then acquired. When the R wave occurs, again the new images are overlaid onto the images from the previous cardiac cycle.