Technical Principle of SPECT (Single Photon Emission Computed Tomography)

SPECT is a nuclear medicine technique used to create a three-dimensional representation of the distribution of an administered radio-pharmaceutical.

 SPECT cameras detect only radio-nuclides that produce a cascaded emission of single photons.

 SPECT radio-nuclides do not require an on-site cyclotron. However, the isotopes of Tc,TI,In,and Xe are generally not found in the body.

 For example, it is extremely difficult to label a biologically active pharmaceutical with Tc-99m without altering its biochemical behavior.

 Presently, SPECT has been used mainly in the detection of tumors and other lesions, as well as in the evaluation of myocardial function using TI-201.

 However, certain pharmaceuticals have been labeled with iodine and technetium and provide information on blood perfusion within the brain and the heart.

 The largest category of SPECT systems uses a single gamma camera mounted on a specialized mechanical gantry that automatically rotates the camera 360° around the patient.

 SPECT systems acquire data in multiple projections at increments of two or more degrees.

 In limited-angle systems, the camera is moved a limited number of times, usually six.

 From the sequence of projections, an image is reconstructed by an algorithm called filtered

back projection.

 After non-target data are mathematically filtered for each view, the reconstructed, three-dimensional image is derived from back projection, which comprises the multi-angled, two-dimensional views and projects them onto a computer monitor.

 The projection data are combined to produce transverse (also called axial or trans-axial) slices.

 Sagittal and coronal image slices can also be produced through mathematical manipulation of the data.

 SPECT systems with multiple camera heads are also available.

 In a dual-head system, two 180° opposed camera heads are used, and acquisition time is reduced by half with no loss in sensitivity.

 A triple-head SPECT system further improves sensitivity. Some suppliers also offer variable-angle dual-head systems for improved positioning during cardiac, brain, and whole-body imaging. Imaging times can be decreased by using another SPECT configuration—a ring of detectors completely surrounding the patient.

 Although multiple camera heads reduce acquisition time, they do not significantly shorten procedure/exam time because of factors such as patient preparation and data processing.

 Several approaches are being investigated to improve SPECT sensitivity and resolution.

 Novel acquisition geometries are being evaluated for both discrete detector and camera-based SPECT systems.

 Kuhl [1976] recognized that (Fig(a)) the use of banks of discrete detectors could be used to improve SPECT performance.

 The system (Fig(b)) developed by Hirose et al. [1982] consists of a stationary ring of detectors. This system uses a unique fan-beam collimator that rotates in front of the stationary detectors.

 Another approach using a multi-detector brain system (Fig(c)) uses a set of 12 scintillation detectors coupled with a complex scanning motion to produce tomographic images.

 An advantage of discrete detector SPECT systems is that they typically have a high sensitivity for a single slice of the source.

 However, a disadvantage has been that typically only one or at most a few non-contiguous sections could be imaged at a time.

 In order to overcome this deficiency, Rogers et al.[1984] described a ring system that is capable of imaging several contiguous slices simultaneously.

 Camera-based approaches (Fig(d)) for SPECT have the advantage of generating true three-dimensional images of the entire organ of interest.

 An obvious method to improve the sensitivity of these systems is to use more than a single camera (Fig(e)).

 Lim et al. [1985] have developed a triangular configuration using three scintillation cameras. A high-sensitivity system is illustrated in (Fig(f)) which consists of an annular crystal combined with a rotation collimator.

 However, this device is limited to brain imaging whereas the triangular system is suitable for both brain and body imaging.

 Figure.21.6 illustrates a camera-based SPECT (Jaszczak et al, 1979). A pallet, designed to minimize gamma ray attenuation, supports the patient between the two scintillation cameras.

 The camera separation is radially adjustable from 22 to 66 cm detector surface-to-surface.

 This adjustment range permits the collimators to be in close proximity to the patient for both body and brain scans.

 The data are collected with continuous gantry motion during a 360° rotation.

 Acquisition times may be varied from 2 to 26 minutes. Angular samples are stored in 2° frames.