Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

FIGURE 10-12. A: An isometric plot of a simple image of three circles ofvarying contrast (different heights on this display). B: The same image isshown after the blurring influence of an imperfect imaging system occurs.The input to the imaging system has sharp, crisp edges; the imperfect resolutionproperties of the imaging system smoothes out (blurs) edges inthe image.The PSF describes the blurring properties of an imaging system. If you put ina point stimulus to the imaging system, you get back the PSF. But an image is justa large collection of individual points, and if you expose the imaging system to, forexample, a medical image, the PSF acts to blur each and everyone of the millionsof point inputs that comprise the image. Figure 10-12 illustrates (using isometricdisplay) an image consisting of three circular regions of various intensity (intensityshows up as height in the isometric display), before and after the blurring influenceof the imaging system. The process of breaking up an input image into its constituentpoint stimuli, individually blurring each point using the PSF of the imagingsystem, and then adding up the net result is a mathematical operation calledconvolution. Convolution describes mathematically what happens to the signalphysically. Convolution is discussed in other chapters and is a recurrent theme inmedical imaging.There are many different mechanisms in radiologic imaging that cause blurring.The spatial resolution of an image produced by any optical device (e.g., a 35-mmcamera) can be reduced by defocusing. When an x-ray strikes an intensifYingscreen, it produces a burst of light photons that propagate by optical diffusionthough the screen matrix. For thicker screens, the diffusion path toward thescreen's surface is longer and more lateral diffusion occurs, which results in abroader spot of light reaching the surface of the screen and consequently moreblurring (Fig. 10-13A).Motion is a source of blurring in all imaging modalities. Motion blurring canoccur if the patient moves during image acquisition, or if the imaging systemmoves. Cardiac motion is involuntary, and some patients (e.g., pediatric) have difficultyremaining still and holding their breath during an imaging study. The bestway to reduce motion blurring is to reduce the image acquisition time.In tomographic imaging modalities such as CT, MR!, and ultrasound, slicethickness plays a role in degrading spatial resolution. For anatomic structures thatare not exactly perpendicular to the tomographic plane, the edges of the structurewill be blurred proportional to the slice thickness and angle of the structure withrespect to the tomographic plane (Fig. 10-13 B).