CT THE SCANNER

CT cooling system

maintains stable temperature of gantry components.

Cooling mechanisms

include blowers, filters, and oil-to-air heat exchangers.

Cooling importance

prevents damage from temperature fluctuations.

CT x-ray tube

requires high power capacity for imaging.

Rotating anode

used in most CT tubes for heat dissipation.

Anode heat capacity

measured in MHU or Mega Heat Units.

2.1 MHU

most CT scanner anode heating capacity

4-5 MHU

advanced CT scanner

about 0.5 MHU or 500 KHU.

Minimum anode heat capacity

8 MHU

Maximum anode heat capacity

X-ray tube failure

main cause of CT scanner malfunction and principal limitation on sequential scanning frequency

Small focal spot

used to achieve high spatial resolution.

Collimation

controls slice thickness by shaping the x-ray beam.

Source collimator

located before the patient to shape the primary beam.

Source collimator function

controls patient dose and beam width entering the patient.

Predetector collimator

located after the patient before detectors.

Predetector function

reduces scatter and ensures proper beam width.

Detector array

collection of detectors arranged in arc or ring.measures the intensity of transmitted x-ray radiation along a beam projected from the x-ray source to that particular detector element.

Reference detectors

used for calibration and artifact reduction.

SFOV

determines fan beam size and number of detectors.

Detector characteristics

include high efficiency, low afterglow, and stability.

Detector assembly

measures the radiation exiting the patient and feeds back the information (primary data) to the host computer.

CT detector requirements

high efficiency, fast response, wide dynamic range.

Capture efficiency

ability to receive photons from the patient.

Absorption efficiency

ability to absorb incoming x-ray photons.

Conversion efficiency

ability to convert photons into digital signals.

Xenon gas detector

uses pressurized gas to detect x-rays.

Xenon efficiency

absorbs about 60–87% of photons.

Xenon advantage

stable and less expensive.

Xenon disadvantage

requires pressurized aluminum casing and has lower efficiency.

Solid-state detector

uses scintillation crystals to detect x-rays.

Solid-state efficiency

absorbs nearly 100% of photons.

Solid-state advantage

high absorption efficiency.

Solid-state limitation

sensitive to temperature and moisture.

CT computer

processes raw data and reconstructs images.

Reconstruction time

depends on processor and memory speed.

Array processor

instead of a microprocessor because this allows image reconstruction in less than 1 second.

Data acquisition

process of scanning the patient.

Operating console

controls scan parameters and data entry.

CT technique factors

include kVp, mA, scan time, and slice thickness.

Typical scan settings

around 120 kVp, 100 mA, and 1–5 seconds.

Normal Slice thickness

ranges from 1 to 10 mm.

0.5 mm thickness

for high resolution

Scan monitor

used for scanning and entering patient data.

Display monitor

used for viewing and post-processing images.

CT spatial resolution

ability to define small objects distinctly.

CT contrast resolution

ability to differentiate, on the image, objects with similar densities

CT temporal resolution

speed of data acquisition.

Ray sum

The total x-ray transmission measured by each detector is the result of the sum of the attenuation by all the tissues the beam has passed through

Image reconstruction

uses algorithms to form cross-sectional images.

Hounsfield unit

measures tissue attenuation values.

Interpolation

estimation of missing data between measured points.

Extrapolation

estimation of data outside measured range.

CT principle of operation

A mathematical algorithm takes the multiple projection data (raw data) and reconstructs the cross-sectional CT image

Filtered back projection

main reconstruction algorithm in CT.

FBP function

reduces blurring from simple back projection.

Filter function

adjusts balance between resolution and noise.

Soft tissue filters

provide some smoothing, which decreases image noise but also decreases spatial resolution.

Parallel beam geometry

all of the rays in a projection are parallel to each other.

Fan beam geometry

the rays at a given projection angle diverge and have the appearance of a fan.

Cone beam geometry

uses cone-shaped beams for volumetric imaging.

Step-and-shoot scanning

acquires one slice at a time with gantry stopping.

Axial scanning, conventional scanning, or serial scanning

another term for step-and-shoot scanning.

Helical scanning

continuous rotation with simultaneous table movement.

Helical path

spiral trajectory of tube around patient.

Multidetector CT

uses multiple detector rows for faster imaging.

MDCT advantage

acquires multiple slices per rotation.