Module H Computed Tomography Physics, Instrumentation, and Imaging

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Conventional CT Slice-by-slice One row of detectors Patient moves through gantry according to slice parameters established by the Technologist. 10 x 10 means the couch will travel 10 mm after each 10 mm slice is imaged. Conventional CT is defined as contiguous acquisition

Conventional CT 10 x 5 would be a 10 mm slice would be imaged every 5mm’s. With the above protocol Overlapping Slices cause the tissue within the overlapped area to be irradiated TWICE.

Conventional CT Images acquired slice-by-slice X-ray tube rotates around the patient CT detectors (a single-row) capture the x- ray attenuation measurements. CT couch increments to the location of the next slice location Filtered-back-projection used a reconstruction algorithm

Spiral CT Spiral CT developed in 1989 Also called Helical CT Helical scanning acquires data in volume rather than slice-by-slice X-ray tube travels in a continuous rotational scanning method Slip-ring design

Slip Ring design Electromechanical devices that have circular electrical conductive rings Conductive rings transmit energy across a rotating surface. Slip-ring eliminated the cable wrap-round process of conventional CT Generator is located inside the gantry

Slip Ring designs Slip Ring Disk Conductive rings are positioned in concentric circles to lie within the rotational plane Slip Ring Cylinder Conductive rings are placed along the axis of rotation, forming a cylinder

Brushes Brushes are used to transmit electrical power to the CT scanner components Brushes glide in contact groves along the slip-ring Two types of Brushes –Wire –composite

Wire / Composite Brushes Wire Brush Conductive wire One or more wires arranged so as to function as a cantilever spring 2 brushes per ring are often used to increase communication reliability or current-carrying capacity Composite Brush A block of conductive material used as a sliding contact –Silver-graphite alloy spring designs include: –cantilever –constant- force –compression Two brushes per ring are often used

Low-Voltage / High-Voltage Low-voltage slip ring scanners 480 AC power Low-voltage brushes The slip ring provides power to the high voltage transformer Generator, tube, and other controls rotate continuously High-voltage slip ring scanners AC delivers power to the high-voltage generator, located in the gantry High-voltage generator supplies voltage to the slip-ring High-voltage form the slip ring is transferred to the x-ray tube. In High-voltage slip ring scanners the generator does not rotate

Types of Spiral CT Scanners Single-Slice Volume CT introduced 1990 –Also called Single detector-row spiral CT –Patient is translated at a constant speed through the gantry –Tube makes a continuous exposure (HU) –Faster than conventional CT –Data from patient received by the detectors –Detectors send the data to array processors –Volume data (Not single-image data like in conventional CT)

Volume Data Spiral CT scanners send volume data to the array processor Standard algorithms can not be used for image reconstruction

Single slice volume CT Single slice volume CT have a 1-D detector array because of the single-row of detectors. Fan-beam profile is used in both conventional CT and single slice volume CT’s. There are pre patient and post patient collimators to deliver a constant beam- width at the detectors.

Single slice volume CT Single-slice volume CT scanners: –Analytic reconstruction algorithms were developed to improve the imaging limitations of back projection. Filtered back projection reconstruction algorithms (also called Convolution method) Convolution removes the blurring (see Seeram page , Fig.6-11, 6-12) –Single-slice volume scanning uses 360- degree linear interpolation algorithm (LIA)

Single slice volume In order to improve poor image quality and artifact production, created by the 360- degree LIA, the 180-degree LIA is applied. The 180-degree LIA –Maintains the detail (slice sensitivity and spatial resolution) –Creates more noise in the image

Single slice volume CT In CT NO slices are produced by the scanning process! Images are produced by the Computer. CT is a digital process

Single slice volume CT Advantages Improvements over conventional CT include speed and volume coverage Disadvantages Long exposures create high heat units (HU) High heat units limited the volume of scan area (z-axis) High pitch for volume coverage degraded slice sensitivity profile (detail).

Single slice volume CT Single slice volume CT have a 1-D detector array because of the single-row of detectors. Fan-beam profile is used in both conventional CT and single slice volume CT’s. There are pre patient and post patient collimators to deliver a constant beam- width at the detectors.

Pitch ….defined as the distance of couch top travel, per one revolution of the x-ray tube, divided by the slice thickness or the beam collimation. Normal pitch is 1:1 –The table travels 1cm with a 1cm slice thickness or 5mm with a 5mm slice thickness etc…….

Multi-slice CT scanners Developed late 1990’s Also called Multi-detector-row CT Had 4 detector rows (quad) Cone-beam profile is used in these scanners Detector rows keep increasing….. 8, 16, 32, and 64 Recently developed 128 detector-row CT scanner

Multi-slice or Multi-detector CT scanners Because more detectors are available at any given point in the scanning area, more data can be collected. Therefore, more data can be derived from the anatomy covered

Multi-slice or Multi-detector CT scanners / Pitch The definition of pitch for multi-slice scanners differs from that in single slice CT. Definition varies according to manufacturer. MDCT Multi-Detector Computer Tomography can be: Slice thickness when using single collimation, or Detector-row collimation, or Beam-width at the center of rotation

Multi-slice or Multi-detector CT scanners / Slice Thickness There is a difference in slice-thickness determination for MDCT. The slice thickness is determined by the: Beam-width (BW) Pitch Shape and width of the reconstruction filter in the Z-axis

Detector row and beam collimation relationship d = detector-row collimation D = beam collimation N = number of detector rows d(mm) = D(mm)/N

Image reconstruction for MDCT Several choices based on one of the following: Interlaced Sampling Longitudinal Interpolation by z-axis filtering Fan beam reconstruction

Multi-slice/Multi-detector advantages Greater volume coverage (more detectors collecting data at one time) Allows sub-second scanning (anatomy is over each detector row for less than a second) Greater detail (increased spatial resolution) More efficient use of the beam profile The acquisition of sub-millimeter slices (currently.5mm) Reduction in radiation dose to the patient Greater accuracy in needle localizations for CT guided biopsies) More effective use of IV contrast media

Current applications Pulmonary embolus protocols (.5mm to 1.25mm slices) Trauma protocols with increased volume coverage More detailed and accurate 3-D reformations, surface shading and volume rendering Multiplanar reconstructions Computed Tomography Angiography (CTA) CT Fluoroscopy CT Interactive Cine CT Colonography Virtual Reality CT (colonography, cystography, and endography) Calcium Scoring ECG/EKG Gated Cardiac Studies Coronary Artery CT