Cardiac CT is rapidly making a name for itself as a viable alternative to traditional diagnostic angiograms, and with technology advancing so quickly, we believe it won’t be too long before you see a CT in your department.
Some cardiologists in the USA are retraining to become qualified to perform CT’s, and speak to the majority of cardiac registrars today and they will tell you cardiac CT (and MRI) is the way of the future.
Computed Tomography (CT), previously known as Computed Axial Tomography (CAT), uses x-rays to image the body in ‘slices’ or cross section. CT was invented in 1972 and was first used in clinical practice in 1974. Early scanners were dedicated to brain imaging and took hours to produce one image. CT scanners have advanced significantly since the 1970s and are used today to produce fast, high resolution images of just about any part of the body.
CT scanners consist of an x-ray tube and a row (array) of detectors which are fixed opposite each other on a ring that rotates inside the gantry. The gantry is the main hardware component of the CT and is often described as a big donut. The patient table sits within the opening of the gantry and is able to move in both directions through it. A computer is another necessary component of a CT and is used to control the scanner, store acquired data and reconstruct images.
The basic principle of CT is the same as that of radiographs. An x-ray beam that passes through a body will be absorbed or attenuated according to the density of structures it passes through. The resulting beam can be described as a profile of the differing tissues within the body through which it has passed.
Unlike radiographs which use a single stationery x-ray beam, CT requires the x-ray tube to rotate around the patient. X-rays are directed through the body onto the detectors for an entire 360° rotation. The detectors record hundreds of samples of the x-ray beam during this rotation. These samples are back projected (reconstructed) by the computer into a 2D image or one slice.
The CT image is made up of a grid or matrix of pixels. Each pixel is given a CT number or Hounsfield unit (HU) according to the attenuation of the x-ray beam at each point in the body on that slice. These numbers are assigned relative to the attenuation of water which has a HU of 0. Air has a HU of -1000 and bone +1000. On the CT image Hounsfield units are represented on a grey scale which ranges from black (-1000 HU) up to white (+1000 HU). The human eye cannot differentiate between 2000 shades of grey so the operator can select a range and level on the grey scale (windowing) to best demonstrate certain structures.
Conventionally, CT scans were performed in a rotate-step manner. The x-ray tube would rotate once around the patient, then rotate once the other way to unwind the high voltage cables attached. The patient table moved to the position of the next slice to be scanned while the tube reversed. This was a very slow process, especially for imaging large areas of the body.
A major development in CT scanning occurred in the late 1980s when the slip ring was introduced, eliminating the need for cables. The x-ray tube was able to continuously rotate around the patient as they moved through the gantry in a manner called spiral or helical CT. Scanning became much quicker and whole areas of the body could be imaged at once. Instead of imaging the body in slices, in spiral CT a volume of data is acquired from which slices are reconstructed. This volume data set can also be reconstructed into slices in other planes and 3D images of bones and blood vessels.
One of the latest advances in CT technology is multi-slice CT in which there are multiple rows of detectors so several slices can be acquired at once. A greater area of the body is covered during one rotation of the x-ray tube so scan times are even quicker and a larger area of the body can be imaged at once. Very thin slices are able to be scanned without lengthening scan time so the reconstructed images have more detail (resolution). This has led to new applications for CT such as imaging of the coronary arteries and CT colongraphy. The first multi-slice CT scanners had two detector rows, followed by four eight and sixteen rows. The most advanced CT scanner in clinical use today has 64 detector rows. Manufacturers are trialling detectors with even more slices, improving reconstruction methods and developing software all the time.
Who knows what CT will be able to do in the future, but it is certain there will be many more exciting developments to come.
