Clinical Instrumentation

SPECT System

        The SPECT system is a gamma camera mounted on a mechanical system (gantry), which allows it to rotate around the patient on a table that permits the passage of the detector from any angle. The rotation angle is chosen depending on the organ or structure of which to make the image. The bed should be constructed of materials that attenuate little the  gamma radiation in order to minimize the artifacts generated by this effect. At the same time the bed should be narrow to allow a small turning radius of the detector. Another property that should have, is the mobility in different directions: up - down (Z axis),  (Y axis) and eventually left to right (X axis). In some systems the stretcher SPECT is synchronized with the turning movements to  make elliptical as well as circular orbits. In the case of a camera with two detectors, they may be set on opposite sides, 180 degrees (bone scan), at  90 degrees (myocardial spect) or at varying angles.

Parameters of the SPECT system:

        In addition to the ones mentioned under camera system it is important to comment on the following: Response uniformity is one of  the most important parameter of performance of the SPECT system as large differences in response, located in an area of the detector, are converted into circular artifacts when reconstructing the tomographic slices from the different acquisition projections. Sensitivity: One of the most serious problems in SPECT is to achieve a good radioactive counting to determine a high signal/background ratio in each projection. Mechanical, Electronic and Digital Alignment : Center of Rotation (COR). SPECT systems have 3 coordinate references: the mechanical, electronic and the computerized. The Y axis is defined by the longitudinal axis of the table in the mechanical system, by the detector in the electronic and by the center of the matrix of each section on the computerized. The axis of rotation defines the COR, which is the point of intersection of the axis of rotation in the transverse plane when the detector is parallel to the axis and must coincide with the center of the matrix in the computer. When the COR is wrong, the resolution deteroriates. It is the case, for example, when the collimator septa are not parallel to each other or gravity influences the position of the detector head, resulting in lack of parallelism between the detector and the table.

Tomographic reconstruction principles:

        It is possible to reconstruct the image of a distribution of source activity using their projections, acquired at different angles when the detector covers an orbit around the patient. There are different ways to reconstruct the data. To mention only two: a) filtered backprojection method b) iterative method.

        Filtered back projection method: the most common algorithm for tomography. From the standpoint of the computer method, it does not matter if the data comes from X-ray tomography,  a rotating gamma camera, or a Positron Emission Tomography system (PET). The first step is the backprojection. For this, the software analyzes each "row" (x axis) of the different projections to create a picture of the different planes of the object studied. This makes possible to generate activity lines perpendicular to the projection. By doing this procedure for each projection, activity lines intersect and form an image that corresponds to a transverse plane of the object. This image has fuzzy edges with rays coming out of the areas of high activity. This artifact can be partially removed using a filter that is used for modifying each projection before back projection. Basically, this filter produces negative coefficients multiplied by the function in areas of high activity, eliminating rays that are produced during the back projection. The filtering of the image can be done in space X, Y, but this is done much more easily in the space of frequencies. Applying the Fourier transform to the function, it is obtained an increasing function, a ramp, which multiplied point by point to the projected image, increases the amplitude of high frequencies.

        Each image contains components of the background, the signal and noise that can be associated with different spatial frequency (background = low frequency, high frequency = noise). When filtering and depending on the selected filter function, the influence of certain frequency components of the image is removed or enhanced, therefore it better represents the real object. Usually the filters used in practice are a combination of ramp filter (filter "high pass", that must always be applied during the reconstruction and amplifies the noise) with other filters that remove high frequencies (filters "low-pass" to remove noise), low frequencies (filters "high pass" to get rid of the background). There are also filters "bandpass" (accept a range of frequencies) with which the components of background and noise are cleaned, leaving intact the frequency component corresponding to the signal.

Page 4     Home       Index      Clinical Nuclear Medicine       Therapeutic Methods         Previous Page     Next Page