The gamma camera is the equipment used to detect the distribution of radiopharmaceutical within the patient
- Radiation detector
- Scintillation crystal
- Photomultiplier tubes
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. Those travelling at an angle will hit the septum (usually lead), be absorbed and, therefore, not contribute to the image.
N.B. The collimator acts as a lens to reject photons that have a path that means they do not hit the camera in a location that corresponds to their original location i.e. its purpose is for spatial mapping. It does not reject scatter.
Features of the collimator
- Parallel hole – these are the most common.
- Diverging hole – for a minified image
- Converging hole – for magnifying the image
- Pinhole – single-hole collimator for magnifying images of small objects e.g. thyroid
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 (these give better image contrast as there are no gaps in the septae, but are more expensive)
- Casting from molten lead.
The higher the energy of the emitted 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.
|Classification||Photon energy (keV)||Septal thickness (mm)||Radionuclide|
- The crystal is fluorescent i.e. when a gamma photon interacts it releases light photons (mixture of visible and UV light)
- Single crystal of sodium iodide with a small amount of thallium (NaI(Tl)). The thallium improves the light output.
- 6-13 mm thick
- Hermetically sealed in aluminium can
Perspex slab (light pipe)
- This sits between the scintillation crystal and the photomultiplier tubes
- Silicone grease is used to ensure good contact between the scintillation crystal, the light pipe and the photomultiplier tubes.
Photomultiplier tubes (PMT)
- 30-100 PMTs sit behind the scintillation crystal
- The purpose of these is to multiply the small amount of light detected from the scintillation crystal to a large signal.
- The light photons hit a photocathode at the entrance to the PMT.
- The photocathode releases electrons in proportion to the amount of light that hits it.
- The electrons are attracted to the electrodes (dynodes) which have an increasingly positive charge along the PMT. This accelerates the electrons. As they accelerate, they gain kinetic energy resulting in multiple electrons being released from the dynode for each electron that hits it. This serves to multiply the original signal.
- The total electrons hit the final anode and the current produced forms the signal received by the pre-amplifier.
This 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.
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.
- All energy absorbed: gamma photon interacts with crystal via photoelectric effect
- Part of the energy absorbed: photon undergoes one or more Compton interactions
The spectrum has a peak (photopeak) that corresponds to the maximum gamma photon energy (for 99mTc this is 140 keV). The Compton band corresponds to photons that have undergone Compton interactions and, therefore, have a lower absorbed energy.
The photopeak should be very narrow but a variety of factors means that it often isn’t. The width of the photopeak is 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
If a gamma photon scatters within the patient’s body (via Compton scatter) it will change direction and, therefore, will not hit the detector at a location corresponding to its location of origin. It is important to reject these scattered photons 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 gamma photon that scatters within the patient 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.
Usually a 20% acceptance window is used centred on the photopeak. The acceptance window can be adjusted and 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 made possible by the Z values being displayed with a multi-channel analyser that allows more than one window to be set.
Key point: Compton band
Gamma photon energies within the Compton band can be due to:
- Unscattered photons that have undergone Compton interactions with the crystal
- Scattered photons that have undergone Compton interactions within the patient
Unfortunately these are indistinguishable and so energy discrimination will remove both the lower energy unscattered and scattered signals.
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 on to its corresponding coordinate within the image.
Image acquisition is controlled by the user and may be terminated when:
- Preset number of counts obtained
- Preset time passed
The digital image is displayed upon a monitor with each pixel corresponding to a memory location in the matrix and the brightness / colour scale corresponding to the count number in that location.
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
- Series of holes separated by lead septae
- Rejects non-parallel gamma photons that do not hit the gamma camera in a location corresponding to their location in the patient
- Does not remove scatter
- Parallel: most common
- Diverging: to minify image
- Converging: for magnify image
- Pinhole: single-hole for magnifying small objects e.g. thyroid and tear ducts
- Crimped lead foil sheets: cheap but gaps in septae degrade the image
- Drilled lead block: no septal gaps so better image contrast but more expensive
- Casting from molten lead
- Low energy: max keV 150, septae 0.3 mm, for 99mTc
- Medium energy: max keV 300, septae 1 mm, for Indium-111
- High energy: max keV 400, septae 2 mm, for 131I
- Single crystal of sodium iodide with thallium. 6-13 mm thick
- Gamma photon hits – releases light photons (visible and UV light)
- With silicone grease ensures good contact between scintillation crystal and PMTs and spreads light across several PMTs
Photomultiplier tubes (PMT)
- 30-100 PMTs
- Multiply signal
- Light photon hits photocathode
- Releases electrons
- Electrons accelerated between dynodes of increasingly positive charge. Multiple electrons released per electron that hits the dynode
- Electrons hit final anode. Current produced forms signal that pre-amplifier receives
- Converts current from anode into voltage pulse
- Gamma photon interactions with crystal:
- Photoelectric – full energy absorbed by crystal
- Compton – proportion of energy absorbed by crystal
- Calculated absorbed energy (Z value) spectrum with photopeak at maximum radioisotope energy
- Energy resolution = FWHM
- Gamma photons that undergo Compton scatter in patient have lower energy
- Scatter electronically rejected via energy discrimination. Acceptance window around photopeak rejects gamma photons that have undergone Compton scatter
- Also reject gamma photons that are not scatter from patient but have undergone Compton scatter in crystal
- More than one acceptance window can be set