Computers and Computed Tomography An Overview

X-ray films:

1: Reveal the total attenuation along the line of sight. The x-ray image can only show a projection of all of the organs lying between the source of x-rays and the film. It does not provide any 3-D or depth information.

2: Allow contrasts of the order of 2% to be seen. Details of heart anatomy or blood vessels cannot be seenWithout using a liquid contrast agent.

3: Provide high resolution images

Computer Tomography:

1: Provides better contrast between tissues with slightly varying attenuation coeficients as well as depth information. 2: Generates cross‑sectional images in the axial or transverse plane with a spatial resolution of about 1mm  3: Maps the relative linear attenuation values of tissues with density resolution of about 1%  CT x‑ray techniques use a high kVp (120 to 140) with heavy filtration and so can penetrate bone. The dominant interaction with tissue is due to Compton scattering which depends on the electron density of the material.

Imaging The image produced by the CT is a two dimensional slice through the transverse plane of the body. • Field of view (FOV) is the diameter of the area

being imaged (e.g., 25 cm for a head). • CT pixel size is determined by dividing the FOV

by the matrix size. The matrix size used for CT is generally 512 x 512 (0.5 k).  • For example, pixel sizes are 0.5 mm for a 2S‑cm

diameter FOV head scan (25 cm divided by 512) and 0.7 mm for a 35‑cm FOV body scan

(35 cm divided by 512).

Posterior

Anterior

The section is divided into Voxels (volume element)

The Voxel volume is the product of the pixel area and slice thickness

depends on the spatial resolution of the CT.

Voxels

The Relative Attenuation Coefficient ()

The attenuation of the tissue in each projection is normally converted to Hounsfield units (HU) or CT numbers. HUx = 1000 x (x ‑ water)/ water

         The 1000 in this equation determines the contrast scale.        By definition HU value for water is 0, and HU value for air is ‑1000.        x and water are dependent on photon energy (keV) therefore HU values depend on the kVp and filtration.        HU values generated by a CT scanner are approximate and only valid for the effective kVp used to generate the image.

Hounsfield Unit for Representative Materials     Material

Density (e/cm3)

Electron Density(e/cm3) x 1023

Approx HU Value

Air < 0.01 <.01 ‑1000

Lung 0.25 0.83 ‑300

Fat 0.92 3.07 ‑90

Water 1.00 3.33 0

White matter

.03 3.42 30

Gray matter

.04 3.43 40

Muscle .06 3.44 50

Cortical bone

1.8 5.59 1000+

An overview of Scanner DesignSince the introduction of the first CT their design has improved to allow 1.      faster scans to decrease the effect of patient movement2.      lower x-ray doses3.      3-D imaging using table motion and spital scanning4.      improved x-ray tube reliability

First‑ generation scanners

The following procedures occur:        An x‑ray beam is scanned across the patient        The measured intensities are used to produce x‑ray transmission values or projections.        The gantry is rotated by a few degrees The tangential scan repeated.        CT images are derived by mathematical analysis of multiple projections.

 1970: EMI scanners used a pencil beam and sodium iodide (NaI) detectors that moved across the patient ( and generated approximately 160 data points per projection

Second‑generation scanners

• larger rotational increments• faster scans• with a single section being generated in approximately 1 minute.

• translate‑rotate technology • multiple detectors• fan‑shaped beam

Modern Computed Tomography ScannersThird‑ and Fourth‑generation scanners

 Third‑generation scanners rotating fan beam and detectors (rotate‑rotate system)

Fourth‑generation scanners

 Rotating tube and fixed ring of detectors (up to 4800) the gantry (rotate‑fixed system)         Single section in l or 2 seconds.      Use of slip ring technology Voltage and Data supplied to the tube through contact rings With slip ring scanners, the x‑ray tube can rotate in one direction continuously.

Fifth‑generation scanners

Use an electron gun that deflects and focuses electron beam along a 210‑degree arc of a large diameter tungsten target ring

  Multiple detector rings permit the simultaneous acquisition of multiple image sections.        No moving parts, images obtained 50 to 100 ms,    Used in cardiac imaging

Helical computed tomography

Continuous motion of table Achieving a continuous spiral of helical scan. This mode of scanning requires the gantry rotates continuously.       Slip ring CT scanners may be used in a helical (spiral) mode.    The x‑ray beam central ray entering the patient follows a helical path dur ing the CT scan.        The relation between patient and tube motion is called pitch table movement (mm ) / collimation width (mm).    Example 5‑mm section thickness, patient may move 10 mm during the 1 second for 1 revolution Pitch would thus be equal to 2.        

CT Construction GantrySupports X-ray tube HT transformers ( Tanks) Detectors Computer controls

X‑ray tubes and collimators • CT tubes have focal spots of approximately 1 mm.• Heat loading on CT x‑ray tubes is generally high, requiring high anode heat capacities.• Modern x‑ray tube capacities of more than 2 megajoules.• High performance CT x‑ray tubes may cost $50,000 or more.• The beam is collimated as it exits the tube and again before it strikes the detectors.• Collimation defines the section thickness and reduces scatter.• Adjustable collimators allow section thickness to range between l and l0 mm.• The heavy filtration used with CT scanners produces a beam with an aluminium half‑value layer (HVL) of approximately 10 mm.

Detectors and data acquisition

Detectors and data acquisition

1: ScintillatorsScintillation crystals produce light when x‑ray photons are absorbed.Coupled to a light detector (photomultiplier tube or photo diode), Common material in solid‑state detectors is cadmium tungstenate (CdWO4), which is an efficient x‑ray detector.

Large single crystals separated by tungsen collimator plates.Plates reduce the scattered radiation falling on the detector.

Detectors and data acquisition

1:Gas detectorsOlder Ct scanners use Xenon gas ionisation detectors  •Incident x‑ray photons ionise the gas producing electron‑ ion pairs.

•The current produced is proportional to the intensity of incident radiation.

• Gas detectors are usually maintained at a high pressure (25 atm) to increase x‑ray detection efficiency.• Gas detectors are more stable than solid‑state detectors.

Detectors and data acquisition