CT was invented in 1972 by British Engineer, Sir Godfrey Newbold Hounsfield in Hayes, United Kingdom at EMI Central Research Laboratories using X-rays. EMI Laboratories is best known today for its music and recording business. About the same time South Africa-born American Physicist, Allan McLeod Cormack of Tufts University in Massachusetts independently invented a similar process, and both shared the 1979 Nobel Prize.
The first clinical CT scan was installed in 1974. The initial systems were dedicated only to head scanning due to small gantry, but soon this was overcome and whole body CT systems with larger gantry became available in 1976. Basic principle is to obtain a tomogram having thickness in millimeters of the region of interest using pencil beam x-radiation. The radiation transmitted through the patient is counted by scintillation detector. This information when fed in the computer is analyzed by mathematical algorithms and reconstructed as a tomographic image by the computer so as to provide an insight into the structure being studied.
DEVELOPMENTS IN CT TECHNOLOGY
Conventional Axial CT
Table 1.1 gives various generations of CT scan.
Spiral/Helical CT
Spiral CT uses the conventional technology in conjunction with slip ring technology, which simultaneously provides high voltage for X-ray tube, low voltage for control unit and transmits digital data from detector array. Slip ring is a circular instrument with sliding bushes that enables the gantry to rotate continuously while the patient table moves into the gantry simultaneously, thus three dimensional volume rendered image can be obtained. The advantages over the conventional scanner are the reduced scan time, reduced radiation exposure and reduced contrast requirement with superior information.
Electron Beam CT (EBCT)
In EBCT both the X-ray source and the detectors are stationary. High energy focused electron beam is magnetically steered on the tungsten target to emit X-rays 5which pass through the subject on to the detectors and image is acquired. EBCT is particularly used for faster imaging in cardiac studies.
Multislice/Multidetector CT (MDCT)
Spiral CT uses single row of detectors, resulting in a single slice per gantry rotation. Multislice CT, multiple detector arrays are used resulting in multiple slices per gantry rotation. In addition, fan beam geometry of spiral CT is replaced by cone beam geometry.
The major advantages over spiral CT are improved spatial and temporal resolution, reduced image noise, faster and longer anatomic coverage and increased concentration of intravenous contrast.
Dual Source CT
The dual energy technology of the new Flash CT provides higher contrast between normal and abnormal tissues making it easier to see abnormalities while reducing radiation. With its two rotating X-ray tubes, enhanced speed and power allows children to be screened more effectively. It turns off the radiation when it comes close to sensitive tissue areas of the body like thyroid, breasts, or eye lens. Pediatric patients benefit because they do not need to hold breath or lay completely still during the examination and they do not have to be sedated.
Hounsfield Units
CT numbers recognized by the computer are from (-) 1000 to (+) 1000, i.e. a range of 2000 Hounsfield units which are present in the image as 2000 shades of gray, but our eye cannot precisely discriminate between these 2000 different shades.
Hounsfield scale assigns attenuation value of water as zero (HU 0). And other media attenuation value as compared to water is listed in Table 1.2:
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Window level (WL) and window width (WW)
To permit the viewer to understand the image, only a restricted number of HU are put on view and this is accomplished by setting the WL and WW on the console to an suitable range of Hounsfield units, depending on the tissue, for interpreting the image. The expression WL represents the central Hounsfield value of all the Hounsfield numbers within the WW. Tissues with CT numbers outside this array are shown as either black of white. Both the WL and WW can be set on the displayed image as desired by the viewer. On CT examination of the chest, a WW of 300 to 350 and WL of 35 to 45 are chosen to image the mediastinum (soft tissue window where as WW of 1500 and WL of 0 is used to assess the lung window.
Image Reconstruction
The acquisition of volumetric data using spiral CT means that the images can be postprocessed in ways appropriate to the clinical situation.
Multiplanar reformatting (MPR) is by taking standard axial images and subject to the three-dimensional array of CT numbers obtained with a series of contiguous slices; and can be viewed sagittal, coronal, oblique and paraxial planes (Figs 1.1A to C).
Figs 1.1A to C: Bilateral renal cysts seen in axial section (A) are reformatted into sagittal (B) and coronal (C) planes
Three-dimensional Imaging
Many fractures like fracture of the mandible associated frontal bone with or without walls of sinuses can be reconstructed into a three-dimensional image (Figs 1.2A to D).
CT Angiography
CT angiography (CTA) sequence is created subsequent to intravenous contrast, images are acquired in the arterial phase and then reconstructed and exhibited in 2D or 3D format. This performance is used for imaging the aorta, renal, cerebral, coronary and peripheral arteries (Figs 1.3 to 1.5).
CT is readily available in most hospitals and stand alone CT centers. It is fast imaging modality and provides with cross sectional high resolution images. Data acquired on axial scans can be used for multiplanar and 3D reconstructions. It detects subtle differences between body tissues. However it uses X-rays radiation which has radiation hazards, CT need contrast media for enhanced soft tissue contrast. Contrast is contraindicated in asthma, cardiac disease, renal and certain thyroid conditions.