Computed tomography (CT) scanning

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14 Computed tomography (CT) scanning

Definition of computed tomography scanning

  Is the process of producing a cross-sectional image of the body by using a collimated beam of radiation that rotates round the patient. Some of the radiation is absorbed and scattered by the body and some is transmitted through the patient and is collected by a number of detectors which are linked to photomultipliers. A signal is then sent to a computer which calculates the amount of radiation absorbed by the patient and reconstructs an image which can then be viewed on a television monitor

Terminology

Dynamic CT When a number of scans are performed in rapid succession, e.g. to demonstrate blood flow
Enhanced CT The use of a contrast agent to improve the appearance of vessels or organs that are similar in density to the surrounding tissues
Field of View The part of the scanned plane which may be included in the final image
Helical Spiral
Image Acquisition The collection of data in order to produce an image
Image Format The process of storing an image, on computer disk, magnetic tape, film or on the World Wide Web
Image Manipulation To digitally change the appearance of the acquired image in order to improve it
Image Reconstruction The process of generating an image from raw data or a set of unprocessed measurements
Isotropic Having the same properties in all directions, e.g. density
Matrix The columns and rows that form a digital image
Mean Window Level The average range of pixel values in an image
Noise Anything that distracts from the information required on an image
Nutating Detector Ring When the detectors vibrate in such a way as to keep the detectors nearest the tube out of the way of the X-ray beam
Pitch The table movement during one complete rotation of 360° divided by the column width (or slice thickness)
Example
If the table moves 10 mm during one complete rotation and a beam width of 5 mm was used, the pitch value = 2
Pixel A two dimensional ‘picture cell’ or ‘dot’ that makes up the image on a digital display screen
Profile Line of data
Slice A section through the patient which is recorded when the X-ray tube and detector make one complete rotation
Slice Interval The distance between reconstructed slices
Spatial Resolution The smallest part of an image that can be seen
Translate Movement in a horizontal direction
Voxel A three dimensional pixel

Window The range of colour (or grey) scale values displayed on a digital image
Window Width The range of pixel values displayed in the digital image

Hardware

Gantry A circular device for holding the detectors
X-ray Tube A method of producing X-rays which are collimated so that they are aligned to a specific number of detectors
Detectors
• A solid state device containing caesium iodide crystals which collects the amount of radiation transmitted through the patient
• Or UFCTM (Ultrafast Ceramic) LSO (Lutetium Oxyorthosilicate) crystal technology
Photomultipliers
• A device for increasing the number of electrons
• The photons from the detector hit a cathode which produces electrons which bombard plates and produce more electrons
• Used in earlier scanners – replaced by photodiodes
Photodiode
• Simply, a light controlled variable resistance
• When dark there is very little current flow
• But when the PN junction is exposed to light, current flows in direct proportion to the quantity of light it is exposed to
• Solid state devices coated with a fluorescent rare earth phosphor, smaller, more stable and more sensitive and a third of the size of photomultipliers
Housing
• A ‘doughnut’ shaped structure which contains the X-ray tube and the detectors
• With a patient opening (aperture) in the order of 70 cm diameter
Movement
• Originally high tension cables wound round a drum
• Disadvantages that the cables had to be ‘unwound’ after each cycle
• Replaced with slip ring and brushes
• Gives a constant electrical supply
• Therefore allows continual movement during rotations
Table For the patient to lie on and can move forward at a predetermined distance or at a constant speed to enable the next ‘slice’ to be taken
Operator Console Where the operator can determine the settings for the scan
Display Station For the viewing, analysis, networking and storage of the final image

Principles of Image Recording

Tomographic Principle
• The X-ray tube and detector move together
• They rotate round a set point
• Structures above and below that point are ‘blurred’ due to the movement of the tube and detector
• The point remains in focus and therefore is sharp
image

Fig. 14.1 The tomographic principle.
Tissue Attenuation As different parts of the body have different densities they absorb radiation to different degrees
Attenuation Coefficient
• Is the measure of the absorption of an X-ray beam along a specific path through a body or substance
• Is dependent upon the density and atomic number of the substance
• Attenuation coefficient = μ
CT Number Calculated as:image

• A CT number with a density greater than water is positive
• A CT number with a density less than water is negative
Hounsfield Unit (HU)
• A standardised unit for reporting and displaying reconstructed CT values
• Water has a nominal value of 0 HU
• Air has a value of −1000 HU
• Other structures have values relative to water; examples could include Fat −0, Tissue +70, Bone +400, titanium +1000
• A change of 1 HU corresponds to 0.1% of the attenuation coefficient between water and air
Voxel Display
• The voxels in the image are displayed showing the relative density of the subject
• Density is determined by the mean attenuation of the tissues in the beam
• Scale of −1024 to +3071 Hounsfield units are used
• Giving 4096 possible values to be allocated to each voxel
Window
• The range of grey scale values shown in a digital image
• Or, the range of colour scale values shown in a digital image
Windowing
• Process of using the Hounsfield units to make an image
• 256 shades of grey can be assigned
• But the human eye can only determine about 100 shades of grey
• The shades of grey can be distributed over a wide or a narrow range of Hounsfield units
Narrow Window
• The grey is distributed over a narrow range of units
• The central unit being the average HU for the structure of interest
• If the average HU was 50 and a narrow window of 170 was selected then HUs of 85 (half 170) above and below 50 would be used
• Therefore the grey scale would extend from −35 to +135
• Any readings below −35 would be pure black
• Any readings above +135 would be pure white
Wide Window
• The grey is distributed over a wide range of units
• The central unit being the average HU for the structure of interest
• If the average HU was 400 and a wide window of 2000 was selected then HUs of 1000 (half 2000) above and below 400 would be used
• Therefore the grey scale would extend from −600 to +1400
• Any readings below −600 would be pure black
• Any readings above +1400 would be pure white
Scanning
• The process when data is produced by moving the tube and detectors round the patient
• Is dependent on the amount of radiation absorbed by the body leaving a quantity of radiation to be detected by the detectors
• The operator can set the average HU and the window width so that the signal from the detectors to the voxels (or pixels in earlier units) assigns the appropriate shade of grey to each voxel
Detector Action
• During the rotation of the X-ray tube the detectors record many profiles (or snapshots)
• In the order of 1000 profiles per 360° rotation are taken
• Each profile is subdivided into sections and fed into 700 individual channels
• Each profile is back projected (reconstructed backwards) by computer to show the two dimensional slice through the body that was scanned
Image Reconstruction Diagram A

• Taking a snapshot of one point in the scan
• The graph shows the total CT numbers per profile as seen by the detectors
• A back projection of this data results in a number of lines of different densities

image

Fig. 14.2 (A) Image reconstruction (simplified).
  Diagram B

• If a second snapshot is taken at 90° to the first
• A back projection of this data still results in a number of lines of different densities
image

Fig. 14.2 (B) Scan through 90°.
  Diagram C

• If the two images are superimposed
• The original image begins to appear
If this process is repeated many hundreds of times, a cross-section through the body can be produced. A minimum display would be a matrix of 512 × 512 = 262 144 pixels

image

Fig. 14.2 (C) Superimpose images.
Multiplanar Reformatted Imaging
• A three dimensional image is produced but it is often viewed as a two dimensional image
• The advantage is that the imaged can be sliced horizontally or vertically to give a coronal, sagittal or axial view of the patient

image

Fig. 14.3 Multiplanar reformatted image.

Data manipulation

Segmentation
• Selection of the tissue of interest
• Elimination of data from other organs
• Can be done manually or automatically with an appropriate software package
Rendering
• Surface of an organ is assigned a specific shade or colour
• Gives the appearance of depth and shade
Surface rendering

• Contours of a specific organ are added together
• The surfaces can be highlighted to give an impression of depth
• Demonstrates the relationship with adjacent or overlapping structures
Volume rendering

• All the data within an organ or area is assigned a specific shade or colour
• Data is manipulated to demonstrate the volume of tissue

Development

First Generation Scanner
• A narrow X-ray beam
• A single detector
• The beam translates slowly across the width of the patient and then stops
• The tube and beam were rotated through one degree and the process repeated until an arc of 180° had been completed
• Power was supplied to the X-ray tube using high voltage cables wrapped round rotating drums
• Reconstruction time was 20 minutes (later 4 minutes)
• The image was made up of ‘slices’ through the body
image

Fig. 14.4 First generation scanner.
Second Generation Scanner
• A fan shaped X-ray beam
• A number of detectors arranged in a row
• Each detector collected information for a different angle
• The movement was identical to the first generation scanners but through a 30° arc
• Reconstruction time 90 seconds
• The image is made up of ‘slices’ through the body
image

Fig. 14.5 Second generation scanner.
Third Generation Scanner
• A wide, fan shaped X-ray beam which extended beyond the edge of the patient
• A number of detectors, in two rows, arranged in an arc
• The tube and detectors rotated together
• The construction was simpler as there was no translational movement
• Reconstruction time 10 seconds
• The image is made up of ‘slices’ through the body
• Basis for modern scanners
image

Fig. 14.6 Third generation scanner.
Fourth Generation Scanner
• A wide, fan shaped X-ray beam which extended beyond the edge of the patient
• A number of detectors, in two rows, arranged in a complete circle
• Only the tube rotates
• Originally, the tube was inside the ring of detectors but this resulted in poor geometry and higher patient dose
• Solved by using a nutating detector ring
• Reconstruction time 1 second
• The image is made up of ‘slices’ through the body
• More expensive than third generation scanners
• More sensitive to artefacts as the relationship between the tube and detectors was not fixed
image

Fig. 14.7 Fourth generation scanner.
Helical Scanner
• Central ray of the X-ray beam follows a spiral round the patient’s body
• Collimation of the beam can be varied between 2 mm and 5 mm
• The tube and two rows of detectors rotate together at a speed of 60 rotations per minute
• Slip rings and brushes maintain constant electrical contact during the scan
• The table and patient move slowly and evenly into the gantry until area of interest has been scanned
• The pitch can be set between 1.0 mm and 2.0 mm
• Reconstruction time is several seconds
• Data is collected for the total volume scanned rather than individual slices
• It is possible to overlap image construction by manipulating the raw data to reduce artefacts and increase the detection of small lesions
image

Fig. 14.8 Helical scanner.
Multi-slice Scanners
• Central ray of the X-ray beam follows a spiral round the patient’s body at a speed of 120 rotations per minute
• The tube and several rows of up to 64 detectors (arranged in the direction of the table travel) rotate together
• Slip rings and brushes maintain constant electrical contact during the scan
• The table and patient move slowly and evenly into the gantry until area of interest has been scanned
• The equivalent volume of 4 slices are acquired at the same time
• The pitch is set between 1.0 mm and 1.5 mm
• Reconstruction time is several seconds
• Data is collected for the total volume scanned rather than individual slices
image

Fig. 14.9 Multi-slice scanners.
Multi-detector-row CT
• 64 detector rows
• 16 slices in one acquisition
• Faster scanning time, e.g. head scan, using 1 mm spiral technique takes less than 10 seconds
• Possible to acquire 32+ slices per second
• Slice width of less than 0.4 mm is available
• High isotropic spatial resolution
• Reconstruction time can be less than 1 second
Dual Source CT
• 2 X-ray sources
• 2 detector arrays
• X-ray source and detectors are at 90° to each other
• Gantry aperture of 78 cm
• Rotation time 0.33 seconds
• Scan range 200 cm
• 4 cm volume in one rotation
• Acquisition time about 0.1 seconds
• Used for cardiac work
Fluoroscopic CT
• Works like a video camera
• Allows the acquisition and immediate display of 9 images per second
• Images are constantly updated
• Uses the same kVp but lower mA than other scanners

Sources

  Connor S E J, Guest P 1999 Spiral computed tomography pulmonary angiography. Synergy, April
Harvey D 2004 CT evolution. Radiology Today 5(18):August
Hogg P 2005 An overview of PET/CT and its place in today’s UK healthcare system. Synergy, December
Patel H, Clarkson L M 2006 MDCT: Practical implementation of a new technology. Synergy, March
Robinson L 1999 Evaluating the colon with CT. Synergy, August
Scally A, Webster M 2001 The evolution of CT technology and its applications. Synergy, March
Smyth J, Hutchinson J 2004 Cardiac CT: A radiographer’s perspective. Synergy, October

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