Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Chapter 4: Computers in Medical Imaging 6710(10votts)V5(0 volts)No SignalA10 20 30Time (msec)V105(0 decimal) (5 decimal) (10 decimal)000 0 o 1 0 1 1 0 1 020Time (msec)V520Time (msec)20Time (msec)FIGURE 4-2. Analog and digitalrepresentation of numericaldata. A: Three analog voltagepulses, similar to those producedby a photomultipliertube attached to a scintillator.The height of each pulse representsa number. B: These samenumbers represented in digitalform. C: A continuously varyinganalog signal, such as thatfrom the video camera in a fluoroscopysystem. The height ofthe signal at each point in timerepresents a number. 0: Thevalues of this signal, sampled atthree points, represented indigital form.when signals are amplified, attenuation losses, and electronic noise-small strayvoltages that exist on circuits and become superimposed on the data. The more thedata are transferred, the more distorted they become. On the other hand, datastored or transferred in digital form are remarkably immune to the accumulation oferror because of signal distortion. These distortions are seldom of sufficient amplitudeto cause a 0 to be mistaken for a 1 or vice versa. Furthermore, most digital circuitsdo not amplify the incoming data, but make a fresh copy of it, thus preventingdistortions from accumulating during multiple transfers.