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

Camera head configuration

 

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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Reconstruction

Pack projection 1

 

Back projection 2

 

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

Next page: PET imaging


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