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ct 3

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In 1972, the EMI scanner was the
first CT scanner introduced into clinical practice.
- 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 x-ray tube and detector were
rotated 1 degree, and another projection was obtained (rotation).
-The EMI scanner thus used a translate/rotate acquisition geometry, which is known as a
first-generation system.
- The CT scanner generation defines the
acquisition geometry.
-An old generation is not (necessarily)
inferior.
-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
cost prohibitive.
-All current MDCT systems use
third-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.
-A single-slice CT scanner generates
one slice per 360-degree x-ray tube rotation.
-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
section thickness.
-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.
-Sampling in this manner risks missing lesions but
greatly reduces doses.
-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 x-ray beam central ray follows a
helical path during the CT scan.
- 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
2.
- Increasing pitch reduces
the scan time and patient dose.
-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.
-EBCT is also known as
fifth-generation CT or ultrafast CT.
- The x-ray beam produced is collimated to
traverse the patient and strike a detector ring.
-Two detector rings permit the
simultaneous acquisition of two image sections.
-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.
-Images of the whole heart can be acquired in
O.2 sec (eight images).
-Serial images of a given section can be acquired every
50 ms (cine mode).
-The advent of MDCT, as well as (fast) dual-source CT scanners, is now
rendering EBCT obsolete.
An N-slice MDCT scanner generates
N projections at each position of the x-ray tube.
-In axial mode, one complete rotation of the x-ray tube will generate
N slices.
-Acquisition slice thickness is determined by the
detector width.
-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).
-In helical mode, different classes of
interpolation algorithms are used by different vendors.
-Common modes of interpolation are
linear and z-filtering.
-Use of linear interpolation algorithms restricts the choice of pitch to
a few fixed values.
-Use of z-filtering offers
much greater flexibility in the choice of pitch.
- Scanners with N detector rows can simulate scanners that have
N /2 detector rows by adding data from adjacent slices.
-The number of x-ray tube rotations is given by the
scan length (L) divided by the beam width.
-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.
-A 360-degree x-ray tube rotation takes between
0.3 and 2 seconds.
-Longer rotation times are used to
increase mAs.
Slow rotation times are common in head CT where motion is
minimal.
-Fast rotation times are used in body imaging to
reduce motion artifacts.
-With a 0.3-second rotation time, an abdominal CT scan (eight rotations) can be completed in
2.4 seconds.
A dual-source CT has recently been developed that offers improved
temporal resolution for cardiac imaging.
-The scanner has two
x-ray tubes and two detector arrays.
-Both acquisition systems are mounted on a rotating gantry with angular offset of
90 degrees.
-One detector array covers a field of view of
50 cm (fan angle 52 degrees).
-The second detector array has a smaller FOV of
26 cm (fan angle 27 degrees).
-Gantry space limitations restrict the size of the
second detector array.
- Gantry rotation time is
0.33 second.
-Two 80-kW generators power
each x-ray tube.
-Partial scans (half scans) are used for
electrocardiographically gated CT image reconstruction.
- Temporal resolution is approximately half of the gantry rotation time for
a singlesource CT scanner.
-The dual-source CT scanner has temporal resolution of a
quarter of the gantry rotation time
-Dual-source CT permits a temporal resolution as short as
83 ms.
-Data from only one cardiac cycle are used, and temporal resolution is
independent of heart rate.
-Dual-source CT can perform
multisegment reconstruction, further improving temporal resolution.