SPECT imaging

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)

 

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Written by radiologists, for radiologists with plenty of diagrams to explain complicated concepts in an easy-to-follow way. 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. More detailed accounts of backprojection and iterative reconstruction is available here.

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
  
  

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