Computed Tomography; Chapter. 18

Example of oversampling pitch:

.5:1 ratio. (would increase patient dose)

Advantages of multislice CT scanners over single slice:

1. Shorter Acquisition Time 2. Decreased Amount of contrast medium 3. Improved Spatial resolution 4. Improved Image Quality

Example of undersampling pitch:

2:1 (would increase risk of missing pathology)

Second generation scanner

30 or more detectors; 40 slices in 10 minutes

Third generation scanner

Bank of up to 960 detectors; detectors and x-ray tube rotate 360 degrees.

Appearance and HU unit (CT number) of Various Tissues:

Cortical Bone; +1000; White White Matter; +45; Light Gray Blood; +20; Gray Water; 0 (baseline) Fat; -100; Dark gray Air; -1000; Black

Window Level:

Determines CT number that will be. center of window width; controls image brightness.

First Generation scanner

One or two detectors; 4 1/2 minute. scan

Patient Dose Factors:

Pitch ratio (oversampling=higher dose) Thinner slices=higher dose Increased kV=lower dose. CT shields reduce dose to radiosensitive tissues. Pediatric protocol (Image Gently)

Window Width:

Range of CT numbers used to map signals into shades of gray; controls. displayed image contrast.

What is pitch in CT:

Ratio between. couch speed and slice thickness; calculates. amount of anatomy examined during particular multislice scan.Pitch=couch movement (mm/sec) per 360 degree rotation divided by collimation; a 1:1 pitch means that tablespeed and slice thickness are equal.

Volume (spiral scanning)

Slip-ring technology; continuous 360 degree rotation; multiplanar reconstruction options; multiple slices obtained. simultaneously.

Fourth generation scanner

Up to 4800 or more detectors on a fixed ring; only x-ray tube rotates; shorter scan times

CT provides sectional anatomic images in the_________________________planes.

axial, sagittal, and coronal