CT was designed by Godfrey N. Hounsfield to overcome the visual representation challenges in radiography and conventional tomography by collimating the X-ray beam and transmitting it only through small cross-sections of the body.
In 1979, G.N. Hounsfield shared the Nobel Prize in Physiology & Medicine with Allan MacLeod Cormack, a Physics Professor who developed solutions to mathematical problems involved in CT.
1st generation of CT
- Single radiation detector.
- Translate-rotate motion 180 translations with 1°rotation between translations.
- Single image projection per translation.
- 5 min imaging time
- Head Imager only
Contrast resolution of internal structures was unprecedented, images had poor spatial Resolution
2nd generation of CT
- Narrow fan beam.
- Linear detector array (30)
- Translate-rotate movements of Tube-Detector combination
- Fewer linear movements are needed as there are more detectors to gather the data.
- Scan time~20secs per slice.
3rd generation of CT
- Eliminates the lateral movement of the tube-detector system.
- This was the so-called fan-beam or continuous rotation scanner.
- By using a fan beam, the projection of the x-ray beam is in the shape of a fan with an angular spread of between 40°and 55°, enough to encompass the whole of the test object.
- The time to acquire a reconstructed image was reduced to about 5 seconds.
- Scanners of this generation are examples of the implementation of the fan-beam projection system in its purest form.
- ?In this scanner generation, after all the projections have been made for the first image, the table moves and the whole procedure is repeated for the next cross-section of the body
4th generation of CT
- Continuous wide fan beam (500-550)
- Ring of detectors (>2000)
- Rotate(tube)-Fixed(detector)
- The X-ray tube rotates in a circle inside the detector ring
- When the tube is at predescribed angles, the exposed detectors are read.
- Scan time< 2 secs.
5th generation of CT
- X-ray tube is a large ring that circles patient, opposed to detector ring.
- Used for cardiac tomographic imaging “cine CT”
- X -rays produced = high energy electron beam
- No moving parts to this scanner gantry
- It is capable of 50 -millisecond scan times and can produce 17 CT slices/second
- stationary/stationary geometry.
Slip Ring
Allow to transfer of electrical information and power between a rotating device and external components
Spiral or helical CT
- Simultaneous source rotation, table translation and data acquisition.
- Produces one continuous volume set of data for entire region
- Projections are continuously acquired while the patient is translated at a constant speed.
- The scanning mode is called a helical (or spiral) scan because the traveling path of a point on the gantry relative to a fixed point on the patient is shaped like a helix.
Pitch is the patient couch movement per rotation divided by slice thickness
Single-slice CT
- Developed in 1989, based on 3rd generation of CT
- The projection system moved in a spiral around the patient which was called ‘single-slice spiral computed tomography’.
- In the initial phases of the development of spiral CT, the scanners used a detector array in shape of an arc of a circle
- The device was called a single-slice spiral computed tomography scanner or SSCT.
- The single-slice spiral CT works with a rotation of the tube around the patient combined with a smooth displacement of the patient through the gantry.
Multiple detector array
- With multiple detector array scanners, slice thickness is determined by detector size, not by the collimator
- Cone Beam & multiple parallel rows of detectors
The primary difference between single-slice CT (SSCT) and MSCT hardware is in the design of the detector arrays.
- SSCT detector arrays are one dimensional that is, they consist of a large number (typically 750 or more) of detector elements in a single row across the irradiated slice to intercept the x-ray fan beam.
- In MSCT, each of the individual SSCT detector elements in the z-direction is divided into several smaller detector elements, multiple parallel rows of detectors.
There are three types of detector arrays:
- Matrix detectors, which consist of parallel rows of equal thickness
- Hybrid detectors with smaller detector rows in the center
- Adaptive array detectors that consist of detector rows with varying
4-slice CT scanner
- In 1998, 4-slice CT scanner was introduced by several manufacturers
- Four detectors ‘rows’ corresponding to the 4 simultaneously collected slices fed data into 4 parallel data ‘channels’, so that these 4-slice scanners were said to possess 4 data channels.
- Advantages of MSCT include substantially shorter acquisition times, retrospective creation of thinner or thicker sections from the same raw data and improved three-dimensional rendering with diminished helical artifacts.
- For example, the SOMATOM Volume Zoom with a 500 ms rotation time and the simultaneous acquisition of 4 slices offers an eight-fold increase of performance compared to previous 1s, single slice scanning.
- Four-slice scanners are the basic system for coronary CT angiography examination.
- With only 250 ms of temporal resolution and gantry rotation of 500 ms made a longer time coronary scan which allows continuous acquisition of data with 4 parallel detectors.
- In addition to smaller scan coverage of 4 × 1.0 mm, resulting in long breath-hold between 30 and 40 seconds which leads to breathing and motion artifacts
16-slice CT scanner
- The installation of MSCT scanners providing 16 data channels for 16 simultaneously acquired slices began in 2002
- In addition to simultaneously acquiring up to 16 slices, the detector arrays associated with 16-slice scanners were redesigned to allow thinner slices to be obtained as well.
- 16-slice scanners have a slightly better spatial resolution and faster gantry rotation (420 ms) compared to 4-slice CT
- The major advantage of 16-slice scanners over 4-slice CT is the larger coverage (16 × 0.75 mm vs 4 × 1.0 mm), resulting in significantly shorter breath hold and less motion artifacts.
64-slice CT scanner
- The 64-slice CT was first introduced with a single x-ray source mounted opposite to a 64-detector-array in the gantry unit and then developed with dual x-ray source (DSCT).
- The x-ray tube and detector array system rotate around the patient to generate tomographic images. To reconstruct a transverse CT image, the gantry requires a rotation of approximately 180°.
- DSCT using two X-ray sources and two detectors at the same time: double temporal resolution, double speed, and twice the power, while lowering dose even further.
A detector module of a 64-slice CT scanner using the double z-sampling technique.
- Due to a periodic motion of the focal spot in the z-direction, two subsequent N-slice readings are shifted by half a collimated slice width (SW Coll/2) at isocenter and can be interleaved to one 2N-slice projection.
- Improved z-sampling is not only achieved at isocenter but maintained over a wide range of the SFOV.
- The simultaneous radial motion of the focal spot in an actual X-ray tube has been omitted to simplify the drawing
The improved z-sampling with the z-flying focal spot technique provides consistent resolution of smaller objects due to a finer sampling scheme
128 & 256-slice CT
- In late 2007, Philips introduced the 128-slice MDCT. A 128 ×0.625-mm detector row system with dual focal spot positions to double the number of slices within the 8-cm (width) z-axis gantry coverage.
- Prospectively ECG-gated cardiac CT typically covers the entire heart in two axial acquisitions over three heartbeats.
- Second generation of 128-slice CT was introduced with dual-source which uses two x-ray tubes with opposing 64 detector arrays mounted 90°from each other.
- However, the volume coverage remains the same; for example, a 128-detector row scanner with two alternating z-focal spot positions can be referred to as 256-slice CT.
- It is important to specify the number of detector rows in z-axis, with or without alternating focal spot positions, and single versus dual source.
320-slice CT
- This hardware (Aquilion One Dynamic Volume CT; Toshiba) currently has the largest z-axis detector coverage. It was released shortly after experiments with a 256-detector row MDCT prototype.
- Each detector element is 0.5 mm wide, yielding a maximum of 16-cm z-axis coverage.
- This configuration allows three-dimensional volumetric entire heart imaging during the diastole of one R-R interval.
640-slice CT scanner
- The Aquilion One Vision Edition is equipped with a gantry rotation of 0.275 seconds, a 100-kw generator and 320 detector rows (640 unique slices) covering 16 cm in a single rotation, with the industry’s thinnest slices at 500 microns (0.5 mm).
- The system can accommodate larger patients with its 78 cm bore and fast rotation, including bariatric and patients with high heart rates.
- Aquilion One Vision Edition also includes Toshiba’s third-generation iterative dose reconstruction software 3-D, which incorporates significant system enhancements by reducing radiation dose compared with conventional scanning.
Photon-counting detector computed tomography (PCD-CT)
- The NAEOTOM Alpha is the world’s first dual-source PCD-CT system for full clinical use and was installed in April 2021 at the University Hospital Zurich, Switzerland. This system uses cadmium telluride (CdTe) as semiconductor material. The X-ray tubes can be operated at voltages up to 140 kVp, the tube current can be set to values between 10 and 1300?mA and the shortest rotation time of the system is 0.25?s.
PCD-CT will push the boundaries of future CT imaging and opens new, currently unpredictable areas of application.
