CT Scanner Technology

X-ray Beam Geometry

• In fan beam geometry, scattered radiation accounts for approximately 5% of the signal, which is significantly lower compared to other imaging modalities. This is due to the focused beam shape, which reduces the amount of radiation that is scattered in various directions.
• In open beam geometry (conventional projection radiography), the highest detection of scatter occurs, with a scatter-to-primary ratio (s/p) of 4. This is attributed to the broader beam shape, which increases the likelihood of radiation interacting with the surrounding tissues, resulting in scattered radiation.

Characterized by rotate/rotate motion, wide fan beams, and more than 800 detectors, third-generation CT scanners feature a dramatically improved ability to interrogate the entire patient with the x-ray beam, thanks to an increased angle of the fan beam. The detector array forms an arc, allowing for more efficient data collection and improved image reconstruction.

• Rotate/rotate, wide fan beam, and more than 800 detectors.
• The angle of the fan beam was increased to allow the x-ray beam to interrogate the entire patient.
• The detector array forms an arc, eliminating the need for translational motion.
• The multiple detectors in the array capture the same number of ray measurements in one instant as was required by a complete translation in earlier scanner systems.
• Scan time is reduced substantially, with early third-generation scanners delivering scan times shorter than 5 seconds and newer systems delivering scan times of one half second.

Sixth-Generation CT Scanners (Helical)

• The gantry rotates continually, untethered by wires, using slip-ring technology.
• Helical CT scanners acquire data while the table is moving, and the x-ray source moves in a helical pattern around the patient.
• Helical scanning allows the use of less contrast agent.

Reconstruction of Planar Sections

• The raw data from the helical data set are interpolated to approximate the acquisition of planar reconstruction data.
• The interpolated data are used to produce reconstructions of planar sections of the patient.

Seventh-Generation CT Scanners (Multiple Detector Array)

• X-ray tubes designed for CT have impressive heat storage and cooling capabilities, making better use of the x-rays produced.
• When multiple detector arrays are used, the collimator spacing is wider, and more x-rays are used.
• The effect of using too few angular views (view aliasing) is reduced with more views.

Processing of the Data

• The raw data acquired by a CT scanner is preprocessed before reconstruction.
• Correction data are used to adjust the electronic gain of each detector in the array.
• Variations in geometric efficiencies caused by imperfect detector alignments are also corrected.

Interpolation (Helical)

• Before the actual CT reconstruction, the helical data set is interpolated into a series of planar image data sets.
• Helical scanning allows the production of additional overlapping images with no additional dose to the patient.

Dose Considerations

• The CTDI (Computed Tomography Dose Index) is used to estimate the dose delivered to the patient.
• The CTDI100 provides a better estimate of the MSAD (Mean Signal Above Dose) for thin slices.
• The dose in helical CT is calculated in exactly the same manner as it is with axial CT, using the CTDI, but a correction factor is needed when the pitch is not 1.0.

Current Modulation in Computed Tomography

• Scanners capable of modulating the mA during the scan can reduce patient dose with little loss in image quality.
• The mA is reduced during acquisition through the thinner tissue projections, resulting in a reduction in patient dose.