4A – Computed Tomography
Computed tomography (CT) uses a series of x-ray scans to produce cross-sectional (tomographic) images of the interior of the object.
- Slices of the body (usually perpendicular to the long axis of the body) are imaged.
- The collection of these image slices gives the full 3D image.
- There is no blurring resulting from the superposition of overlying or underlying structures.
- Accurate quantitative measurements of the attenuation coefficient versus position are obtained, and this helps to distinguish one tissue from another.
- Spatial resolution is not as good as with conventional x-ray images. - The derivation of the attenuation coefficients from measured
trans-mittances requires a computer to propcess the measured data. - The medical applications with CT include detection of hard and
soft tumours, cysts, blood clots, injured or dead tissue, and other morphological conditions.
Brief history
CT was invented in 1972 by British engineer Godfrey Hounsfield of EMI Laboratories, England and by South Africa-born physicist Allan Cormack of Tufts University, Massachusetts. It has been suggested that the success of The Beatles provided funds for EMI to support research and build early models for medical use.
- The first CT scanner developed by Hounsfield took several hours to acquire the raw data for a single scan or slice and took days to reconstruct a single image from this raw data.
- The first medical scaner “Siretom” was built in 1974.
- The first clinical CT scanners were installed between 1974 and 1976 (dedicated to head imaging).
- “Whole body” systems with larger patient openings became avail-able in 1976.
Principle
- Consider an x-ray beam of intensity I0. - The measured intensity at the detector is
Id = I0exp
− d0 μ(s) ds
(1) - Given a measurement of Id and knowledge of I0, Eq. (1) can be
re-arranged to yield the projection measurement gd:
gd = − ln Id I0 (2) = d 0 μ(s) ds (3)
- In an actual CT system where there are many detectors, the ref-erence intensity I0 must be measured for each detector as a
cali-bration step.
- The source and detector pair is translated across the object. - The set of measured intensities gives one intensity profile, Id(ρ),
and from there the corresponding projection profile
g(ρ) = − ln ⎛ ⎝Id(ρ) I0 ⎞ ⎠ (4)
- The scanning gantry is rotated through a certain angle, and an-other linear traverse takes place. In practice, the increment in angle is small, typically ≤ 1◦, giving 180 profiles altogether.
- In general, the more projection profiles that are obtained, the better the quality of the resulting image.
The projection at an orientation θ is denoted by g(ρ, θ). - the xy axes define the coordinate system
- the origin of the xy coordinates is the rotation centre of the scan-ning gantry
- the g axis passes through the the origin of the xy coordinates - θ denotes the angle which the ρ axis makes with the x axis - for 1◦ increments, there are 180 measured projection profiles:
g(ρ, 0◦), g(ρ, 1◦), g(ρ, 2◦), · · · , g(ρ, 179◦)
- the projection data are processed by a computer to give the re-constructed image ˆf (x, y)
- A computer algorithm is required to compute the matrix of at-tenuation coefficients from the set of projection profiles to give a single 2D image.
- Typical CT slices are between 0.5 to 1 mm, with images of size 512× 512.
- The full 3D image is obtained by stacking a series of such 2D images.
An example of CT scanning parameters: Slice thickness (mm): 1
Image matrix size : 512× 512 Display field of view (mm): 500× 500 Image pixel size (mm): 1.024× 1.024
Scanning-gantry Designs
The CT scanning gantry is the doughnut-shaped part of the CT scan-ner that houses the equipment necessary to produce and detect x-rays in order to create a CT image.
The evolution of gantry design over the years has led to - shorter scan time
- higher quality images
- reduction in radiation exposure
It is instructive to examine the earlier generations of the CT scanner gantry.
First-generation(1G) gantries
The first generation scanning mechanism is known as “traverse and index”.
- The tube and detector are mounted on a frame. - A pencil x-ray beam traverses the slice linearly.
- At the end of one traverse, the frame indexes (rotates) through 1◦ (typically) and the traverse is repeated.
- With a 1◦ rotation, there are 180 traverses and 180 profiles mea-sured. The entire scanning procedure takes about 4 or 5 minutes per slice.
- Image quality is acceptable, but this system has the following drawbacks:
* relatively slow, hence low patient output * poor quality image
* patient has to remain immobile for this relatively lengthy pe-riod, hence use is limited to brain scans
Second-generation (2G) gantries
These have the following features:
- Narrow-angle fan beam, multiple detectors (up to 30). - Translation steps as in the first-generation design.
- Scanning gantry rotates around patient typically in 10◦ steps. - With a 10◦ fan beam, it is possible to take 10 profiles (at 1◦
inter-vals) with each traverse.
By indexing through 10◦ before taking the next set of profiles, it is possible to obtain a full set of 180 profiles by making only 18 traverses. At the rate of 1 s for each traverse, the scanning system could operate in the range of 18-20 s per slice and cross-sections of any part of the body is possible.
Third-generation (3G) gantries
The main improvements are:
- Pure 360-degree rotational system. - Wide-angle (20◦–50◦) fan beam.
- Multiple-detector array (about 800 detectors) - Scan times can be as low as 500 ms per slice.
Fourth-generation (4G) gantries
This offers some improvements on the third-generation design: - A 360◦ ring of up to 4800 fixed detectors.
- Rotating x-ray source, stationary detector ring.
- Fan beam increased slightly so that the detectors on the leading and trailing edge can be monitored and the data adjusted to allow for variation in detector performance (thus reducing ring artifacts in the reconstructed image).
- Scan times of approximately 500 ms per slice are possible.
The evolution from first to second to third generation scanners in-volved radical improvement with each step. In subsequent genera-tions (fourth, fifth, sixth and seventh), the improvement with each generation is less marked.
Fifth-generation (5G) gantries
These scanners, also called electron beam CT (EBCT), were developed specifically for cardiac tomographic imaging.
- They are capable of 50-ms scan times and can produce CT movies of the beating heart.
- A conventional x-ray tube is not used; instead, a large arc of tungsten encircles the patient and lies directly opposite to the detector ring. X-rays are produced as a high-energy electron beam strikes the tungsten.
- The electron beam is steered so that it strikes the tungsten targets.
Sixth-generation (6G) gantries (helical CT)
- The development of high-voltage slip rings (for the x-ray tube) has enabled third and fourth-generation scanners to continuously rotate about the patient.
- The patient can be moved at a uniform speed in or out of the scan plane, and the x-ray beam will describe a spiral path through the patient.
- A 60cm torso scan will take about 60s, a 24cm lung study about 12s.
Seventh-generation (7G) gantries
- A thick fan beam is used in conjunction with multiple (up to 640) detector arrays, where each detector array acquires a separate axial CT image.
- This is known as multislice CT (MSCT) or multidetector CT (MDCT).
- Allows imaging of multiple slices in a single rotation.
- It is used in investigations that require rapid acquisition of high-resolution images, e.g. imaging of the heart and coronary arteries.
Cone-beam CT
- In conebeam CT (CBCT), a cone-shaped beam is used to obtain a high-resolution image of the patient.
- In CBCT, and entire volumetric data set can be obtained with a single rotation of the gantry.
- It is typically used for head and neck and dentomaxillofacial imag-ing.
]
