Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

\- -, ffIffIffIffIffI\\\,\\ , \\fIffIffIffIffIfcontouring, using sensors on the camera heads to determine their proximity to thepatient.In brain SPECT, it is usually possible for the camera head to orbit with a muchsmaller radius than in body SPECT, thereby producing images of much higher spatialresolution. Unfortunately, in older cameras, the large distance from the physicaledge of the camera head to the useful portion of the detector often made itimpossible to orbit at a radius within the patient's shoulders while including thebase of the brain in the images. These older systems were therefore forced to imagethe brain with an orbit outside the patient's shoulders, causing a significant loss ofresolution. Most modern SPECT systems permit brain imaging with orbits withinthe patient's shoulders.Transverse Image ReconstructionAfter the projection images are acquired, they are usually corrected for axis-of-rotationmisalignments and for nonuniformities. (These corrections are discussedbelow; see Quality Control in SPECT.) Following these corrections, transverseimage reconstruction is performed using either filtered backprojection or iterativemethods.As described in Chapter 13, filtered backprojection consists of two steps. First,the projection images are mathematically filtered. Then, to form a particular transverseimage, simple backprojection is performed of the row, corresponding to thattransverse image, of each projection image. For example, the fifth row of each projectionimage is backprojected to form the fifth transverse image. A SPECT studyproduces transverse images covering the entire field of view (FOY) of the camera inthe axial direction from each revolution of the camera head or heads.Mathematical theory specifies that the ideal filter kernel, when displayed in frequencyspace, is the ramp filter (Fig. 22-4). However, the actual projection imagescontain considerable statistical noise. If they were filtered using a ramp filter kerneland then backprojected, the resultant transverse images would contain an unacceptableamountof statistical noise.