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Principles of CT: Multislice CT^*

Department of Radiation Therapy and Medical Physics, Hartford Hospital, Hartford, Connecticut

View larger version (57K): [in this window] [in a new window]   FIGURE 1.  (Left) SSCT arrays containing single, long elements along z-axis. (Right) MSCT arrays with several rows of small detector elements.

View larger version (35K): [in this window] [in a new window]   FIGURE 2.  Flexible use of detectors in 4-slice MSCT scanners. (A) Groups of four 1.25-mm-wide elements are linked to act as 5-mm-wide detectors. (B) Inner 8 elements are linked in pairs to act as 2.5-mm detectors. (C) Inner, adaptive-array elements are linked to act as 5-mm detectors (1 + 1.5 + 2.5) and, together with outer, 5-mm elements, yield four 5-mm slices. (D) The 4 innermost elements are linked in pairs to form 2.5-mm detectors (1 + 1.5), which along with the two 2.5-mm detectors, collect data for four 2.5-mm slices.

View larger version (36K): [in this window] [in a new window]   FIGURE 3.  Diagrams of various 16-slice detector designs (in z-direction). Innermost elements can be used to collect 16 thin slices or linked in pairs to collect thicker slices.

View larger version (32K): [in this window] [in a new window]   FIGURE 4.  Diagrams of various 64-slice detector designs (in z-direction). Most designs lengthen arrays and provide all submillimeter elements. Siemens scanner uses 32 elements and dynamic-focus x-ray tube to yield 2 measurements per detector.

View larger version (90K): [in this window] [in a new window]   FIGURE 5.  Section of 16-slice detector with scatter removal septa. Septa are sufficiently deep to eliminate nearly all scatter. Note smaller elements (0.625 mm, in this example) in center of array and larger (1.25-mm) outer elements. Also note dead spaces (lighter lines) between elements.

View larger version (39K): [in this window] [in a new window]   FIGURE 6.  Cone beam effects in SSCT and MSCT. (A) In SSCT, divergent, cone-shaped x-ray beam irradiates different tissues—yielding different attenuation measurements—for parallel opposed rays, sometimes causing streaking for thicker slices. (B) Wider beams of MSCT accentuate cone beam shape and lead to rays that do not even lie within same plane. Cone beam reconstruction algorithms are generally required. opp. = opposite.

View larger version (35K): [in this window] [in a new window]   FIGURE 7.  z-Spacing in helical CT. (A) Minimum z-spacing equal to d/2 (d = slice thickness) is achieved in SSCT with pitch of 1 and interpolation between interleaved parallel opposed rays. (B) With pitch of 1 in MSCT, parallel opposed rays overlap rather than interleave, giving z-spacing equal to d. (C) z-Spacing equivalent to that in SSCT is achieved with pitch that overlaps one slice thickness but results in double irradiation of some tissue. Reduced z-spacing can also be achieved with other pitches. det = detector; rot = rotation.

View larger version (46K): [in this window] [in a new window]   FIGURE 8.  z Filtering. (A) For MSCT scans configured to acquire thicker slices (e.g., 5 mm), slices are interpolated as for SSCT. (B) For MSCT scans with small detector collimation, numerous measurements are obtained within slice plane (up to 13, in this example) to form thicker slices. Combining many measurements to form thicker slices is referred to as z filtering.

View larger version (26K): [in this window] [in a new window]   FIGURE 9.  Geometric dose efficiency. (A) If MSCT detectors configured to acquire four 2.5-mm slices are irradiated with 10-mm-wide x-ray beam, as specified for SSCT, outer 2 slices will receive lower intensity and yield higher image noise. (B) To compensate, MSCT beams are widened to use only inner, nonpenumbra regions. Penumbra regions that were partially used in SSCT are discarded in MSCT, leading to reduced dose efficiency.

View larger version (90K): [in this window] [in a new window]   FIGURE 10.  Coronal images formed from off-axis reformatting. (A) Thicker slices collected by SSCT lead to poor-quality, less efficacious off-axis images. (B) Thinner slices collected by MSCT lead to high-quality reformatted images, often with resolution equivalent to that within slice plane. collim = collimation.