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- EMI scanners used a
pencil beam and sodium iodide (NaI) detectors that moved across the patient (Le., translated) to obtain one projection data set.
-The EMI scanner thus used a translate/rotate acquisition geometry, which is known as a
-Second-generation scanners also use translate-rotate technology but have
multiple detectors and a fan-shaped beam.
-Third-generation scanners use a wide rotating fan beam coupled with a
large array of detectors (rotate-rotate system).
-The geometric relationship between the tube and detectors does not change as it
rotates 360 degrees around the patient.
-Fourth-generation scanners have a
rotating tube and fixed ring of detectors (up to 4,800) in the gantry (rotate-fixed system).
-For single-slice CT scanners, third- and fourth-generation acquisition geometries resulted in similar
patient doses and image quality.
-The advent of MDCT made the manufacture of scanners with a fourth-generation acquisition geometry
-In axial scanning, the table and patient remain stationary while the
x-ray tube rotates through 360 degrees and acquires the necessary projection data.
-At the completion of the x-ray tube rotation, the table is moved a distance (e.g., beam width W),
and the process is repeated.
-A scan length of L will normally require a total of approximately
LlW x-ray tube rotations to cover the anatomic region of interest.
-For some examinations, the table increment distance can be much greater than the
-High-resolution CT in chest imaging may be performed using a
1-mm detector width and a table increment distance of 10 mm.
-A table increment distance greater than the x-ray beam width results in
a sampling of the anatomic region.
-A table increment distance of 10 mm, and a section thickness of 1 mm, will reduce the
patient dose to 10% of the dose for contiguous imaging.
-In helical CT acquisitions, the patient is moved along the
horizontal axis as the x-ray tube rotates around the patient
- The relation between patient and tube motion is called
pitch, defined as the table movementduring each x-ray tube rotation divided by the total x-ray beam width.
- For a 5-mm beam width, if the patient moves 10 mm during the time it takes for the x-ray tube to rotate through 360 degrees, the pitch is
-Image reconstruction is obtained by
interpolating projection data obtained at selected locations along the patient axis.
-Images can be reconstructed at any level and in any increment but have a thickness equal to
the collimation used.
-Reconstructed images can have a greater thickness than the collimation, but cannot be
less than the collimation.
Electron beam CT (EBCT) uses an
electron gun that deflects and focuses a fast-moving electron beam along a 210-degree arc of a large-diameter tungsten target ring in the gantry.
-There is no motion
by the x-ray tube or detector array, which allows images to be obtained in as little as 50 ms with minimum motion artifacts.
-The major advantage of electron beam CT is the
speed of data acquisition, which can freeze cardiac motion.
-Detector widths are normally 0.5 to 0.6 mm, which offers a slice thickness that is comparable
to the (in plane) pixel dimension.
-The beam width in MDCT equals
the number of slices multiplied by acquisition slice thickness (detector width).
- Scanners with N detector rows can simulate scanners that have
N /2 detector rows by adding data from adjacent slices.
-A 64-slice CT (~40-mm beam width) scanner performing an abdominal scan with a length of 32 cm requires
only eight x-ray tube rotations.
-With a 0.3-second rotation time, an abdominal CT scan (eight rotations) can be completed in
A dual-source CT has recently been developed that offers improved
temporal resolution for cardiac imaging.
- Temporal resolution is approximately half of the gantry rotation time for
a singlesource CT scanner.
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