•Edwards and Kuhl developed the MARK VI, the first emission computed tomography (ECT) device.
•MARK VI consisted of several sodium iodide photon detectors arranged in the rectangular shape around the head of the patient. Tomomatic-32, first SPECT imaging devices, was similar to the MARK VI but had 32 photon detectors.
•Single photon emission computed tomography is a Nuclear Medicine imaging modality, which involves the use of radionuclides injected intravenously into the body,
•To produce a 3D distribution of the gamma rays emitted by the radionuclide,
•Giving physiological information about the region of interest.
NUCLEAR MEDICINE IMAGING
•Whereas other medical imaging modalities depend on external photons interactions (e.g, Absorption, attenuation, scattering) with imaged organs for anatomical images, Nuclear medicine imaging uses metabolic or physiological interactions of radionuclides to produce functional images.
•Nuclear medicine imaging is also called scintigraphy.
DESIGN OF SPECT
•SPECT is the 3D version of the 2D (planar imaging) gamma camera technology.
•It uses one or more gamma camera head rotated around the patient.
•SPECT combines conventional scintigraphic and computed tomographic methods.
•So gives 3D functional information about the patient in more detail and higher contrast than found in planar imaging.
It looks like a CT or MRI system but hardware is different.
•Position logic circuit
•Pulse height analyzer
•Data analysis computer
DESIGN AND INSTRUMENTATION
•Collimators- immediately in front of the detectors- essential to provide positional information, and minimize scatter /false events.
•They select ray orientation.
- •Usually NaI(TI) crystals
- •Converts gamma rays to light photons
- •Converts energy from visible light photons to electrons
- •Magnitude of signal proportional to photon energy
- •Data analysis
- •Processes data- readable image
- •For display
HOW SPECT WORKS ?
•A radiopharmaceutical is injected into the patient’s body.
•It travels into the blood stream, and concentrates in the region of interest.
•There, it decays, emitting gamma rays.
•The gamma rays travel out of the patient’s body and are detected by the gamma camera head of the SPECT machine.
•The gamma rays is collected by the collimators to minimize scatter, and improve image quality.
•The collimated gamma rays hit the crystal detector, usually Sodium Iodide crystals doped with thallium [NaI (Tl)], which converts the energy of gamma rays to visible light.
•As visible light travel through the Photo Multiplier Tubes (PMT), they absorb the light and emit electrons.
•The electrons emitted are used for image formation.
•They are detected by a Positioning and Summing circuit, which decode the body position of the original photon.
•A Pulse Height Analyzer (PHA) decodes the energy of the emitted photon and accept only those that fall within a preset energy window, which is centered at the energy peak of the primary photons, thus scattered photons and their adversive effects are rejected.
•The information is passed on to a digital circuit on a computer, where algorithms are used to reconstruct the image.
•The resultant image gives a physiological state of the organ.
•Hot spots (area of increased uptake) and cold/dark spot or photopenia (area of decreased intake) may indicate pathology, such as arthritis, infections, fractures, tumors.
•A typical SPECT system consist of one or more scintillation cameras that acquire multiple two dimensional planar images around the patient. The projection data are reconstructed into 3D images. The collimator of the scintillation camera has effect on the spatial resolution and detection efficiency of the SPECT system.
•Physical factors such as photon attenuation and scatter affect the quantitative accuracy and quality of SPECT images, and various method have been developed to compensate these image degrading effect.
•Radiopharmaceuticals are the radioactive substances or radioactive drugs for diagnostic or therapeutic interventions.
•It contains a radioactive isotope that can be injected safely into the body.
•SPECT studies use standard radionuclides (eg, technetium-99m or iodine- 123) and simpler imaging instrumentation.
•These standard radionuclides commonly emit gamma- ray photons with energies that are much lower than 511KeV. A typical example is Tc-99m, which emit 140 KeV photon.
PROPERTIES OF RADIONUCLIDE
•A physical half life of few hours.
•Emit gamma rays but no alpha or beta or very low energy photons.
•Emit gamma rays of energy 50-300KeV.
•Ideally emit monoenergetic gamma rays so that scatter can be eliminated by PHA.
•Be easily and firmly attached to the pharmaceutical at room temp.
•Have a very high specific activity.
•Have a very low toxicity.
FACTORS THAT AFFECT SPECT IMAGES
•The quality and accuracy of SPECT images are affected by two factors:
•Physical factors- due to interaction of emitted photons with matter inside the patient.
•Instrumentation factors- related to the SPECT imaging system.
1.Attenuation– Reduction of the number of primary photons passing through a given thickness of material via photoelectric absorption and Compton scatter .
•Photons originating from different depths in patient experience different levels of attenuation.
•Resolution of SPECT detectors degrades rapidly with distance.
2. Scatter- photons that have been scattered before reaching the detector provide misplaced spatial information about the origin of the radioactive source.
The resulting image becomes blurred having low resolution.
•Attenuation (including scatter) results in:
•High image noise
•Low contrast, and
•Reconstruction artifacts and distortions.
COLLIMATION DETECTOR SYSTEM-
Most important component that determines both:
•Spatial resolution (blurring) and
•Sensitivity (detection efficiency).
Higher resolution collimator imply lower sensitivity, and vice versa.
The main challenge in SPECT is finding a balance between resolution and sensitivity.
•Whole body bone
•Studies the brain blood flow and activity pattern.
•Basically show three things:
•Areas of the brain that work well
•Areas that are overactive
•Areas that are underactive
Brain SPECT imaging can show conditions such as tumors/cancer, Dementia- Alzheimer’s disease, stroke, impairments caused by substance abuse, addictions, etc.
•Brain SPECT uses the ability of the radionuclides to cross the Blood Brain Barrier (BBB) and localize in malignant tissues.
•There are 2 types:
•Hydrophilic compounds: they cross the abnormal BBB and localize at pathological sites and not in normal brain tissue.
•Lipophilic compounds: they cross the normal BBB and localize in the normal brain cells.
•For brain SPECT acquisition, a complete 360 degree camera rotation with 64*64*8 matrix is recommended.
•This is the study of CSF flow (CSF: Cerebrospinal Fluid).
•It aids to detect pathophysiological changes in the pathways of Cerebrospinal Fluids (CSF).
•The radiopharmaceutical is injected into the subarachnoid space in the brain or spine (at level between L2-L3 vertebrae).
MYOCARDIAL PERFUSION SPECT
•It evaluates the heart’s blood supply.
•It is also called the cardiac stress rest test.
•Two sets of images showing blood flow (perfusion) are obtained:
1.During period of rest
2.During stress period (after exercise).
•The patient jogs for a few minutes.
•The intensity of the exercise increases about every three minutes to induce maximum stress.
•ECG may be attached to the chest of the patient to monitor heart rate.
•The radiopharmaceutical is injected a second time intravenously shortly before the exercise stops.
•The myocardial perfusion SPECT can check for obstruction of blood flow in heart vessels (Myocardial ischemia)or damage caused by heart attack (Myocardial infarction).
•For most cardiac SPECT protocols, a 180 degree camera rotation with 64*64 matrix size is recommended.
ADVANTAGE OF SPECT
•Improved contrast and reduced structural noise, due to elimination of overlapping structures (compared to planar imaging).
•Localization of defects is more precise and more clearly seen.
•Extent and size of defect is better defined.
•Images free of background.
•SPECT (and PET) provide the only non invasive technique for imaging brain neurochemicals.
•SPECT is available in both developing and developed countries because of lower equipment cost and greater accessibility of SPECT radionuclide.
DISADVANTAGE OF SPECT
•Limited spatial and temporal resolution.
•Relative expensive to build and maintain(compared to CT,MRI).
•Not very effective for patient who just finished exercising.
•Most common artefacts are:
•Star Artefacts: caused by back projection.
•Motion Artefacts: caused by movement of the patient.
•Edge Packing: Increased brightness at edge of crystal.
•Artefacts may also be caused by damaged collimators, metal objects worn by patient, PMT failure, cracked crystals.