22. CT (Computerised Tomography) Scan
- While magnetic resonance imaging is about the magnetization of protons, X-rays (including computerized tomography) can be considered a play of electrons
- A heated wire is known to produce radiation. A common example is a poker held in a fire which first glows red and then white, the radiation being greater when it is white. Thompson studied this radiation in what is famously known as the cathode ray tube in the Cavendish laboratory established by Maxwell, the well known Scottish Physicist in 1870 in Cambridge, England. The experiment consisted of a tube which was evacuated of all air and the surroundings of which had been neutralized of its electro-magnetism. In this tube a filament was heated with electricity and the radiation thus produced was recorded at the other end of the tube and this radiation was deduced to be electrically charged particles because only particles could travel in a straight line in that field neutralized of its electro-magnetism. These particles were called electrons which normally form the shell of an atom and are negatively charged and are easily displaced because they are light as compared to the central nucleus and move when energy is imparted to them. By tradition any source which gives out electrons is called a cathode and the opposite is called an anode.
- In 1895 while experimenting with such a cathode ray tube Wilhelm Roentgen in Germany accidentally found that these cathode rays (electrons), when they collided with a material object must have produced a secondary radiation because they had fogged photographic plates left nearby by mere chance. He called them X-rays because the letter X is usually used as the unknown quantity in a mathematical equation. These rays were found to be capable of passing through many materials, are also a form of electro-magnetic waves and like everything else in the atomic world, also exhibit particle like behavior. These particles are discrete packets of energy and were named photons by Einstein. X-rays when passing through the body produce different types of images on a film. The bone appears white because it offers great resistance to the rays, the lung appears grey while the air in a pneumothorax offers least resistance and appears black.
- In a conventional x-ray film only a single exposure is performed in a single plane and it therefore lacks precision because of overlapping tissues. However when an x-ray emitter (tube) is used which is capable of continuous exposure and the tube itself rotates around an object, multiple different views of the object can be obtained in different planes. These observations reveal details of a tissue in parts or sections of various thickness (from the Greek tomos=section). That is the genesis of the words ‘Computerised Tomographs’.
- A CT-scanner has a cathode which emits electrons which when they collide against an anode produce ‘X’ radiation. All this happens in a heavily shielded tube in which the voltage difference between the cathode and the anode is high. This mechanism is fitted in one part of a circular apparatus. Placed exactly opposite in the circular ring is a bank of detectors. The patient is placed in the centre of this circular apparatus which can be compared to the hole of a donut. When in operation the circular apparatus rotates around the patient through 360° during which multiple x-ray beams are sent through the patient to the sensors in the detectors placed opposite and are then picked up.
- In order to prevent scatter of x-rays a process called collimation is used. This is done at two places. One, prior to the x-rays penetrating the body (pre-patient) and later before they are picked up by the signal detector receiving array (post-patient). This improves focus. The collimators can create beams which can either run parallel or also can create a fan shaped dispersal from a single focal point. Collimation can be so manipulated that a desired thickness or a slice of a part of the body of the choice of the clinician can be viewed for a more accurate localized diagnosis.
- The detector array consists of solid state detectors. X-rays which are a part of the non-visible part of the light spectrum (the visible parts are the seven colours of the rainbow that we see) can be converted into electrical signals by these solid state detectors. The strength and intensity of the electrical signals produced depends upon the density and thickness of the body part traversed by the X ray beam. The whole process from the beginning to the end is therefore electrical current → heat → stream of electrons → x-rays → collimation → penetration of the body → recollimation → incident upon the detectors → electrical signals → the final image. Philosophically this is a very intelligent manipulation of the basic entity in the universe called ‘energy fields’.
- In a standard computerized tomogram a specific slice is viewed and recorded and several slices of adjacent parts are then sequentially recorded each time the patient is told to hold his/her breath. In a major advance now the patient is moved in a linear manner through the ring which houses both the x-ray tube (emitter) and the detectors (sometimes more than one or numerous) creating a spiral effect where the interrupted sequence mentioned above is now carried out continuously. In a single breath hold several hundred images with rapid exposures are created in a continuous link of a major portion of the body. These data can be digitally reconstructed in any desired plane to give sagittal, coronal, oblique or curved images. Also the data can be depicted in a 3D format. Selected portions of the images can be digitally subtracted e.g. the skin and the muscle, to depict particular tissues better. In what is known as a virtual computerized tomographic imaging hollow viscera such as the colon or bronchi can be viewed as in an actual endoscopy including reproduction of natural colours and this has now become a very important non-invasive diagnostic tool. The basis for all the above are some extremely ingenious and painstakingly created softwares and computers.
- Because the rate of dispersal of a given intravenous contrast medium is known and the radiologist knows about the speed of the linear movement of the body through the ring the contrast can be imaged within the vessels in CT Angiography. This data can be digitally reconstructed to give 3D views and endoluminal views.
- In view of its versatility in imaging for a variety of tissues within the body, computerized tomography is probably one of the most commonly used radiological investigations for an accurate anatomical and physiological diagnosis.
Ravi Ramakantan, the former professor and head of the department of Radiology at the KEM hospital in Mumbai has reviewed these articles and sent the following comments:
- In view of the large number of cases of trauma that a general plastic surgeon is asked to treat, a CT scan will be a far more frequently used diagnostic tool than a MRI. The chapter on CT scan therefore should have preceded the one on MRI. Over a period of time for a better understanding of the subject I hope several representative illustrations of CT scans will be added as the blog grows in size.
- Having said that the role of MRI cannot be underestimated for e.g. only an MRI will properly show the intra-cranial extension of a maxillofacial lesion and the various planes it has invaded in order to help surgery at the base of the skull.
- The article on CT scan omits any reference to radiation hazards. That hazard needs to be stressed and a mention should have been made that pregnant women and children will need special protection when undergoing the investigations.
- However the modern CT scan machines even when they take multiple sub-millimeter slices are quite safe for adults and the hazard for e.g. in a fully grown adult with trauma or tumors are only theoretical.
- 3-D reconstruction is after all only a reconstruction of the original slices or images. The better the resolution, the sharper is the 3-D reconstruction. Unnecessary movement is a great hindrance to resolution and if this is avoided a reasonable 3-D reconstruction can result from an average scan.
Two representative patients with very little obvious deformity are shown here in whom 3D CT scans reveal a variety of fractures which may not have been diagnosed by physical examination alone. Arrows to indicate the lines of fractures are not required because all the fractures are delineated very clearly and might be recognised even by a lay person. Photographs courtesy: Dept. of Plastic Surgery, BYL Nair Hospital, courtesy Amresh Baliarsingh, HOD