Single photon emission tomography (SPECT)

        Single photon emission tomography is a nuclear medicine tomographic imaging technique using gamma rays. It is very similar to conventional nuclear medicine planar imaging using a gamma camera, but is able to provide true 3D information.

        Apart from some basic models and those intended only for whole body studies, most stationary and some mobile gamma cameras can perform SPECT, a nuclear medicine technique used to create a three-dimensional representation of the distribution of an administrated radio - pharmaceutical. SPECT cameras detect only ratio-nuclides that produce a cascaded emission of single photons

         SPECT radio-nuclides do not require an on-site cyclotrons. However, the isotopes of Tc, TI, In and Xe are not normally found in the body. For example, it is extremely difficult to label a biologically active pharmaceutical with Tc-99m without altering its biochemical behaviour. 

           Presently, SPECT has been used mainly in the detection of tumours and other lesions, as well as in the evaluation of myocardial function using T1-201. However, certain pharmaceuticals have been labelled 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 360 degree around the patient. SPECT systems acquire data in a serious of 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 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.

 

                   The technique needs delivery of a gamma emitting radio isotope (a radionuclide) into the patient, normally through injection into the bloodstream. On occasion, the radio isotope is a simple soluble dissolved ion, such as an isotope of gallium. Most of the time, though, a marker radioisotope is attached to a specific ligand to create a radioligand, whose properties bind it to certain types of tissues. This marriage allows the combination of ligand and radiopharmaceutical to be carries and bound to a place of interest in the body, where the ligand concentration is seen by a gamma camera.

Collimator used to collimate gamma rays (red arrows)

in a gamma camera

   

  Principles

        Instead of just "taking a picture of anatomical structures", a SPECT scan monitors level of biological activity at each place in the 3-D region analyzed. Emission from the radionuclide indicate amounts of blood flow in the capillaries of the imaged regions. In the same way that a plain X-ray is a 2- dimensional (2-D) view of 3-D distribution of a radionuclide.

        SPECT imaging is performed by using a gamma camera to acquire multiple 2-D images (also called projections), from multiple angles. A computer is then used to apply a tomographic reconstruction algorithm to the multiple projections, yielding a 3-D data set. 

        This data set may then be manipulated to show thin slices along any chosen axis of the body, similar to those obtained from other tomographic techniques, such as magnetic resonance imaging (MRI). X-ray computed tomography (X-ray CT) and positron emission tomography (PET).

         SPECT is similar to PET in its use of radioactive tracer material and detection of gamma rays. In contrast with PET, the tracers used in SPECT emit gamma radiation that is measured directly, whereas PET tracers emit positrons that annihilate with electrons up to a few millimeters away, causing two gamma photons to be emitted in opposite directions.

        A PET scanner detects these emissions "coincident" in time, which provides more radiation event localization information and, thus, higher spatial resolution images than SPECT (which has about 1 cm resolution). SPECT scans are significantly less expensive than PET scans, in part because they are able to use longer-lived and more easily obtained radioisotopes than PET.

         Because SPECT acquisition is very similar to planer gamma camera imaging, the same radiopharmaceuticals may be used.

        If a patient is examined in another type of nuclear medicine scan, but the images are non-diagnostic, it may be possible to proceed straight to SPECT by moving the patient to a SPECT instrument, or even by simply reconfiguring while the patient remains on the table.

       To acquire SPECT images, the gamma camera is rotated around the patient. Projections are acquired at defined points during the rotation, typically every 3-6 degrees. In most cases, a full 360-degrees rotation is used to obtain an optimal reconstruction. The time taken to obtain each projection is also variable, but 15-20 seconds is typical. This gives a total scan time of 15- 20 minutes.

         Multi-headed gamma cameras can accelerate acquisition. For example, a dual-headed camera can be used with heads spaced 180 degrees apart, allowing two projections to be acquired simultaneously, with each head requiring 180 degrees of rotation. Triple head cameras with 120 degree spacing are also used.

      Cardiac gated acquisition are possible with SPECT, just as with planer imaging techniques such as multi gated acquisition scan (MUGA). Triggered by Electrocardiogram (ECG) to obtain differential information about the heart in various parts of its cycle, gated myocardial SPECT can be sed to obtain quantitative information about myocardial perfusion, thickness and contractility of the myocardium during various parts of the cardiac cycle and also to allow calculation of left ventricular ejection fraction, stroke volume and cardiac output.

Applications

        SPECT can be used to complement any gamma imaging study, where a true 3D representation can be helpful, such as tumor imaging, infection (leukocyte) imaging, thyroid imaging or bone scintigraphy.

       Because SPECT permits accurate localization in 3D space, it can be used to provide information about localized function cardiac or brain imaging.

             * Myocardial perfusion imaging

             * Functional brain imaging