Helical scanning

The next level beyond the step-and-shoot method is the helical-scan approach. The conveyor belt of a heftcal-scan system moves continuously at a uniform speed and is synchronized with the rotating disk that holds the X-ray source and detectors. The synchronization ensures that by the time a bag has advanced by... 

The benchtop Cyclone Plus quantitative radiometric phosphor imager (PerkinElmer Life and Analytical Sciences) operates with the same storage phosphor and scanning process except that a helical scanning mechanism is used with the flexible phosphor screen loaded into a cylindrical carousel that spins at 360 rpm. This allows the instrument to be more compact and lower in cost than the BAS-5000, in which the phosphor screen is kept on a flat plane during scanning. 

The two basic modes of MDCT data acquisition are axial and spiral (helical) scanning. 

Helical scanning is performed as with conventional 64-detector-row CT. The images generated during one examination are displayed on the monitor in the form of preliminary reconstructions in real time. In a second computational step, the image data is reconstructed as a volume data set with variable slice thick-... 

The increase in effective detector width is associated with a proportionate increase in the effect of over ranging in the helical scan mode, and therefore, increased proportion of the overall radiation exposure from the first and last half rotation that does not contribute to image reconstruction. This effect is more significant when smaller volumes are imaged, e.g., when examining children, where this effect may account for up to 50% of the overall dose. The only effective means to reduce this excess radiation is the use of an active colh-mator in front of the X-ray tube, which is commercially available. 

One major disadvantage of axial acquisition techniques is that a functional cine sequence cannot be reconstructed. This may preclude the evaluation of noncoronary cardiac structures. The helical scan will therefore remain first choice for additional valvular or ventricular imaging. 

In the helical CT era, a fixed scan delay for the pancreatic parenchymal phase of 40 s was recommended. With an acquisition time of approximately 20 s for a helical scan of the pancreas, good enhancement of the pancreas was guaranteed for most of the patients. However, with the evolution of MDCT scanners, which has considerably shorted the acquisition time of a pancreatic CT scan to not more than 3-5 s for modern scanners, a fixed scan delay may be suboptimal to catch the phase of maximum enhancement. Schueller et al. (2006) compared fixed scan delays of 40 s with an individual scan delay... 

Urinary tract Bilateral flank pain and progressive oliguria developed over 3 weeks in a 47-year-old woman who took sulfadiazine for toxoplasmosis retinitis [183 ]. Only in a second CT scan (an nnenhanced helical scan with very low attennation for stones) was urolithiasis detected snlfonamide crystals were found in the urine. 

Magnetic tape media are divided into two categories (Onodera et al., 1996a) i.e., particulate media where magnetic particles are dispersed in a polymer matrix with some additives and coated onto the polyethylene terephthalate (PET) substrate, and thin film media in which monolithic magnetic thin films are deposited onto the substrate in a vacuum, which is discussed in this chapter. For magnetic tape helical scan systems, the tap>e is driven by a pinch roller and a tension (0.2 - 0.5 N) is applied. 

Fig. 3. Tape path for helical scan tape systems.

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