Single Photon Emission Computed Tomography –SPECT

What is SPECT?

        SPECT is short for single photon emission computed tomography. As its name suggests (single photon emission) gamma rays are the sources of the information rather than X-ray emission in the conventional CT scan.

Introduction

        Emission Computed Tomography is a technique where by multi cross sectional images of tissue function can be produced , thus removing the effect of overlying and underlying activity. The technique of ECT is generally considered as two separate modalities. SINGLE PHOTON Emission Computed Tomography involves the use single gamma ray emitted per nuclear disintegration. Positron Emission Tomography makes use of radio isotopes such as gallium-68, when two gamma rays each of 511KeV, are emitted simultaneously where a positron from a nuclear disintegration annihilates in tissue.

 Brain Imaging 

        This figure is a transverse SPECT image of the brain. The hot spots present in the right posterior region are seen clearly using SPECT. SPECT examines cerebral function by documenting regional blood flow and metabolism.

Positron Emission Tomography (PET)

        The distribution of activity in slices of organs can be obtained in a more accurate way using PET. In the simplest PET camera two modified sophisticated cameras called Anger cameras are placed on opposite sides of the patient. This increases the collection angle and reduces the collection times which are the limitations of SPECT .In PET, radiopharmaceuticals are labeled with positron emitting isotopes. A positron combines rather quickly with an electron. As a result the two gamma quanta are emitted almost in opposite directions .In PET scanners, rings of gamma ray of gamma ray detectors surrounding the patient are used. Each detector interacts electronically with the other detectors in the field of view. When a photon arrives within a short time frame, it is clear that a pair of quanta was generated and that these were created somewhere along the path between the detectors. Conventional PET tomography makes use of standard filtered back projection techniques used in computed tomography and SPECT.

Back Projection 

                The main reconstruction step involves a process known as ‘Back Projection’. As the original data was collected by only allowing photons emitted perpendicular to the camera face to enter the camera, back projection smears the camera bin data from the filtered sonogram back along the same lines from where the photon was emitted from. 

Conclusion

        It is reasonable to speculate about a constant by perhaps a slower rate of increase of clinical applications of SPECT. It is safe to conclude that SPECT has reached the stage where it will be a valuable and also an unavoidable asset to the medical world.