![]() In contrast a SPECT scan, which is an emission scan, would tell us where a pharmaceutical is distributing throughout the body – thus providing functional information.įor a PET acquisition, a patient is placed within a ring of detectors. Of course we can recall that a CT scan, a transmission scan, would give us information about the attenuation properties of the object being imaged – thus providing anatomical information. A CT scan would work very similarly, with respect to image acquisition and reconstruction, except the photons reaching the detector would be coming from an x ray generating source at the other side of the patient (a transmission scan). This gif specifically illustrates a SPECT acquisition, where the information about the biodistribution of a radioactive tracer is being emitted from within the patient (through photon emission) and virtually registered by the rotating detectors (an emission scan). In CT and SPECT imaging, a sinogram is generated by rotating detectors around a patient, and storing the detected projection profiles at each angle in the sinogram, as depicted in the gif above. (3) Filtered sinograms are back projected into image space (the back project in filtered back projection) ![]() (2) Data is filtered (the filter in filtered back projection) (1) Forward projection (data acquired and forward projected into sonogram space) The main steps involved in a filtered back projection image acquisition include: This website shows animations of FBP reconstruction. While iterative reconstruction is more complex, it has advantages in that it is capable of dealing with noise and other practical issues by incorporating their expected impact/uncertainty into the reconstruction process. Now that computing power is getting more accessible, many vendors are incorporating iterative reconstruction techniques into their systems. It is much faster, simpler, reproducible, and linear (performs uniformly across environments). They run by repeating (ITERATING) two distinct steps: (1) Expected projections are calculated by forward projecting data (using system matrix), and is based on activity distribution estimation from the previous iteration, and (2) the current image estimate is compared to the raw acquisition and updated so as to maximize the likelihood it is the “correct” image estimation.įBP has been the reconstruction algorithm traditionally used for medical imaging. Iterative reconstruction: these algorithms involve a feedback process that permits sequential adjustments of an estimated image so that its virtual acquisition corresponds to the raw acquisition.The mathematics of FBP are based on the central slice theorem, but are not discussed here. Analytic reconstruction: on this website we focus on an image reconstruction technique called filtered back projection (FBP).There are two main types of mathematical algorithms for image reconstruction: analytic reconstruction (filtered back projection) and iterative reconstruction. Reconstructed (3d) images can be rendered and displayed in many useful ways: (this webpage focuses on analytic reconstruction) Images can be reconstructed using analytic or iterative reconstruction. Usually, sinograms are much smaller than listmode files, but less flexible for complex reconstruction In a sinogram, every detected event can be sorted and stored using the angle and offset charactoristic to its detection. Raw tomographic data can be acquired using CT, PET, SPECTĭata stored (in computer) as either as raw detector signal output (listmode), or sinograms (pictured above) which are angle specific histrograms of detected events. Since this page was created by a nuclear medicine specialist, the animations were designed with respect to emission tomography (PET, SPECT), yet the concepts are also appropriate for CT. It is with this principle that we are able to acquire 3d images in medical imaging modalities: Computed Tomography (CT), Positron Emission Tomography (PET), Single Photon Emission Tomography (SPECT). When multiple 2d projection images are acquired of an object from many angles, one can use mathematical tools to reconstruct a 3d representation of that object. The main principle of image reconstruction is this: Users are welcome to use images for any non-commercial use PETīasic 3d image reconstruction does not need to be a complicated topic. All gif images will play as animations when opened with modern internet browsers. In addition, several sets of images have been generated, and users may download zipped folder here ( PET, CT, Phantom). Gif animations can be downloaded by simply right clicking and choosing “save as”. Once correctly loaded, gif animations should display at approximately 10 frames per second. ![]() Specific loading time will depend on your internet connection speed. NOTE - this page contains 8 gif images (of a few MB each) and may take several minutes to fully load. ![]()
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