CT scanning uses ionising radiation to produce cross sectional images through the body. The basic concept of computed tomography involves an x-ray tube that rotates round the subject. The 1-dimensional data are collected at the opposite end and are processed to produce a 2-dimensional image. Sequential axial slices, typically done with the x-ray tybe rotating in a spiral fashion allows acquisition of a volume of data. Digital reconstruction allows reconstruction in planes other than transverse plane.
In breast radiology, digital breast tomosynthesis is a technique that uses multiple low dose projections in an arc. The acquired information allows reconstruction of slices, a useful feature when dense breast could obscure an abnormality on conventional mammography.
Developments in CT
CT is a fairly recent concept, and even during fairly junior Radiologists' careers, the advances within CT imaging have been significant.
In the early days of CT scanning, limitations in engineering meant that the x-ray tube had to return to its starting position for each slice. Consequently, older scanners took sequential images through the area of interest, requiring a single breath hold per slice, with the whole scan taking up to 30 minutes for an abdomen and pelvis scan.
With the advent of spiral (or sometimes helical) CT scanners, the x-ray tube can continue to rotate beyond 360˚. Continued whole areas of the body can now be imaged in a single breath-hold, so much so that the rate-limiting factor is now getting the patient on and off the scanner!
Because the scans can now be performed in a single breath hold, intravenous contrast can be used much more usefully. Previously, on a sequential scanner, by the time the scan reached the pelvis, all the contrast would be in the bladder. Now it is possible to obtain scans in the arterial, portal venous and venous phases of contrast enhancement (should this be required!), revealing far more information than before. Oral contrast may also be required if bowel delineation is felt necessary.
In addition, the advent of multi-slice scanners, means that whole blocks of tissue are more easily scanned, and consequently, superb 3D reconstructions can be obtained. Sophisticated processing of CT data can construct virtual fly-throughs of the internal surface of the gut (see CT colonography).
Tissues can be characterised on CT by their Hounsfield Units or H.U. Each tissue has it's own range of densities measured in Hounsfield units, and the scan can be manipulated by altering the window level and / or width of densities viewed to give more information (the CT machine can capture more dynamic range than can be displayed in gray scale and perceived by the human eye). Water is defined as zero, air as -1000 and substances greater than 0 are scored according to the attenuation coefficient (μ) of the material relative to water:
The large density contrast between bone (containing calcium) or metallic implants and other soft tissues can cause artifact and resolution issues, which limits the usefulness of CT in central nervous system imaging. This is interesting mainly as an historical footnote, as the first CT scanners revolutionised scanning of the brain, but were not able to accommodate the adult body.
The downside of CT is of course the amount of radiation involved. A standard unenhanced CT brain gives at least 100 times the radiation does of a chest x-ray, and an abdomen and pelvis CT will be the equivalent of over 400 chest x-rays - not something to be underestimated, particularly in people having repeated scans!
see also Radiation exposure