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.

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