What is a CT scan?
The technique of CT scanning was developed in 1973 by Hounsfield. A thin fan beam of X-rays generated by a conventional X-ray tube passes through a single 'slice' of a patient through to a bank of X-ray detectors. The detectors can be manufactured as a 'solid state' component or may be composed of the inert gas xenon. The detectors produce a voltage proportional to the intensity of incident X-rays. The voltage is read and digitized for subsequent processing in a computer. By taking thousands of readings from multiple angles around the patient, a two dimensional image can be reconstructed. 'CT' stands for Computerized Tomography, tomography referring to a cross sectional image in a single plane. Modern CT scanners acquire images rapidly with each slice taking as little as a fraction of a second to obtain. Multiple slices are obtained in sequence. By stacking the data from multiple slices together, a three dimensional image can be reconstructed. The processed images are transferred onto special laser film with between 6 to 24 images per film.
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The radiographers responsible for the examination undertake special training in CT scanning. The images are interpreted by a Radiologist who issues a diagnostic report. CT scanning is now commonly performed particularly for imaging the head, sinuses, spine, chest, abdomen, pelvis and limbs. At QDI CT cases are checked by a radiologist prior to the patient leaving. This ensures that all the required information is obtained. The images are read by a radiologist immediately following the scan. The written report is usually available within half to one hour of the completion of the scan. The radiographers and radiologist are happy to answer any questions you may have. |
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The CT scan room consists of the CT X-ray generator, CT Gantry and CT table. The gantry contains the X-ray tube which spins around the patient and has a large central opening called the aperture. The gantry can tilt to different angles. Patients lie on the table top and are progressively moved into the opening as a series of slices are obtained.
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Spiral CT
| In the 1990's spiral CT scanners were developed. A Spiral CT has a slip ring which enables the tube to spin continuously at one revolution or more per second. Data is obtained in a continuous spiral or helix rather than in individual slices. The advantages of spiral CT include faster scanning with reduced examination time, greater accuracy especially for small lesions in the liver or lungs, better intravenous contrast usage and the ability to obtain seamless 3D images. Reduced examination times are important for seriously ill patients, children, restlessness or claustrophobia. | |
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The accompanying animations show the working components of a spiral CT scanner with the cover of the gantry removed. |
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Multislice Spiral CT
A major development has been the introduction of multi-slice CT. The latest multi-slice scanners have 64 detector rows allowing the acquisition of 64 slices of information each tube rotation. The advantage is improved volume coverage and longitudinal spatial resolution. New applications have evolved including cardiac scanning, virtual endoscopy and CT angiography. Please refer to our Imaging Updates for more information. |
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Advantages of Spiral CT: Faster scan times. Conventional CT may miss small lesions in the liver and lung. The data set is three dimensional allowing multiplanar and 3D reconstructions. CT angiograms can be performed where 3D pictures of arteries are reconstructed from a spiral data set obtained during rapid injection of contrast into an arm vein. Less IV contrast is needed.
Indications for Spiral CT: Used routinely for imaging the thorax and abdomen. CT cholangiography to delineate the bile ducts particularly following cholecystectomy.
Indications for Spiral CT Angiograms: Imaging of pulmonary emboli (Spiral CT pulmonary angiography). Evaluation of aneurysms e.g. abdominal aorta, Circle of Willis, etc. Evaluation of vascular stenoses e.g. carotids, renal arteries, coeliac axis, superior mesenteric artery etc.
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| Click here for examples of |
• Cat Scan images |
• 3D CT images |
• CT Angiography images |
| Link to frequently asked questions in |
• CT |
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Preparation
An injection of intravenous contrast may be required and a history of asthma or allergy is therefore obtained . It is preferable to refrain from eating for 2-3 hours prior to the exam to ensure an empty stomach. Clear fluids and usual medications are allowed during this time. For abdominal and pelvic scans oral contrast is required to outline the intestine. The oral contrast is composed of dilute barium and is administered during the one hour period prior to the scan. The staff should be alerted in the case of pregnancy or potential early pregnancy.
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