Single photon emission computed tomography (SPECT) is the method of obtaining cross-sectional nuclear images (similar to CT in x-ray imaging).
- Single photon:
- SPECT uses single gamma photon detection that are produced by gamma photon decay
- c.f. PET which uses the simultaneous detection of the two gamma photons that arise from positron decay
- Emission:
- Radioactivity used to create image is emitted from patient rather than transmitted through patient from an outside source as is done in x-ray imaging
- Computed tomography:
- Slices are imaged that can be reconstructed into 3D data
SPECT can be used to image any radiopharmaceutical in which:
- The distribution does not change significantly during the image acquisition time (20-40 minutes)
- Acquisition time long enough for sufficient amount of gamma photons to be collected
Equipment
Camera / detector
a. Single head gamma camera
- Rotated around the patient during image acquisition
- Long image acquisition times
- No longer commonly used
b. Multiple head gamma camera
- Dual head, large field of view camera
- Housed on a gantry with slip ring technology that can rotate the cameras around the patient
- The cameras can be positioned in either an H-configuration or an L-configuration relative to each other
Hybrid SPECT/CT
- X-ray source and x-ray detector array placed between the gamma camera heads
- Anatomical CT and functional SPECT images then fused
- CT information can also be used to correct for attenuation in the SPECT images
Gantry
Needs to have:
- Accurately aligned centre of rotation
- Constant rotational speed
- Detectors aligned parallel to axis of rotation
Collimator
Important to use high resolution collimator
- Maximise spatial resolution throughout depth of the patient
- Reduce image distortion during reconstruction
Hole direction
- Parallel holes
- Hole and septae size as uniform as possible
- Non-parallel holes
- Can only be used with circular (i.e. not body contouring) orbits e.g. heads
Collimator type
- Fan beam collimator
- Used for brain imaging
- Utilises magnification – uses more of the detector field of view to collect the image data
Ensure smallest camera-patient distance but maintain safe distance
- Infra-red beams fitted to collimator face that enable automatic body contouring to minimise the detector-patient distance and optimise image quality
- Fitted with pressure sensitive safety devices to prevent any contact between the collimators and the patient
Patient table
- Needs to be comfortable due to long image acquisition times
- Low attenuation of gamma photons to allow photons to pass through and enable 360 degree acquisition
Image acquisition
Matrix
- Determines maximum resolution and image noise (counts per pixel)
- Modern dual headed system = 128 x 128 matrix
- To reduce image noise (at expense of resolution):
- Increase slice thickness
- Smoother reconstruction filters
- Display slice data in 64 x 64 pixel matrix
Views
- 20-40 sec per projection frame
- Heads rotated in continuous or ‘step and shoot’ mode
Minimising artefacts
- Minimise patient movement
- Injection site (very high count density) should be kept out of the field of view
- Arms above heads for chest and abdominal imaging to remove radiation attenuation and minimise patient-detector distance
Specific scans
- Cardiac SPECT
- Heart is located off centre
- Imaged over 180 degrees from LPO to RAO projection with heads in L mode to reduce detector distance and attenuation in tissues
- ECG gating used to demonstrate cardiac wall motion (8 frames per cardiac cycle acquired into 64 x 64 matrix for each projection angle)
Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.
Reconstruction
1. Creation of 1D profiles from each projection angle
- As the camera rotates around, it creates a 1 dimensional view of the measured radioactivity for each angle.
2. Filtering of profiles
- Compromise between noise reduction (degree of smoothing) and preservation of image (resolution)
- Smoothing usually defined by cut-off or critical frequency of the filter (maximum spatial frequency present in image)
3. Processing of data to create reconstructed slice image
- Back projection or iterative reconstruction of the filtered profiles create the reconstructed slice image. For more detailed accounts of backprojection and iterative reconstruction, please read Acquiring an image part 2.
Correcting for attenuation:
- Analytical method: apply algorithm that assumes uniform attenuation and then adjusts pixel counts depending on distance from camera and edge of patient
- Direct measurement: CT data can be used to calculate an attenuation map and adjust the pixel counts according to this
