The above CT scanner is used in Northwick Park Hospital
Tomography is the method of imaging a single plane, or slice, of an object resulting in a tomogram. There are two principal methods of obtaining such images, conventional and computer assisted tomography. Conventional tomography uses mechanical means to record an image directly onto X-ray film, while in computer assisted tomography, a computer processes information fed to it from detectors then constructs a virtual image which can be stored in digital format and can be displayed on a screen, or printed on paper or film.
In computer-assisted tomography, a computer processes data received from radiation detectors and computationally constructs an image of the structures being scanned. Imaging techniques using this method are far superior to conventional tomography as they can readily image both soft and hard tissues (while conventional tomography is quite poor at imaging soft tissues). The following techniques exist:
X-ray computed tomography (CT), or Computed Axial Tomography (CAT) scan, is a helical tomography technique (latest generation), which traditionally produces a 2D image of the structures in a thin section of the body. It uses computer-processed X-rays to produce tomographic images (virtual ‘slices’) of specific areas of a scanned object, allowing the user to see inside the object without cutting. In CT, a beam of X-rays spins around an object being examined and is picked up by sensitive radiation detectors after having penetrated the object from multiple angles. A computer then analyses the information received from the scanner’s detectors and constructs a detailed image of the object and its contents using the mathematical principles laid out in the Radon transform. It has a greater ionizing radiation dose burden than projection radiography; repeated scans must be limited to avoid health effects. CT is based on the same principles as X-Ray projections but in this case, the patient is enclosed in a surrounding ring of detectors assigned with 500-1000 scintillation detectors (fourth-generation X-Ray CT scanner geometry). Previously in older generation scanners, the X-Ray beam was paired by a translating source and detector.
Digital geometry processing is used to generate a three-dimensional image of the inside of the object from a large series of two-dimensional radiographic images taken around a single axis of rotation. Medical imaging is the most common application of X-ray CT. Its cross-sectional images are used for diagnostic and therapeutic purposes in various medical disciplines.
Positron emission tomography (PET) is also used in conjunction with computed tomography, PET-CT, and magnetic resonance imaging PET-MRI.
Magnetic resonance imaging (MRI) commonly produces tomographic images of cross-sections of the body.
In a CT scan the patient lies on a table and is moved though a doughnut-shaped machine. It creates images that are slices through the patient.
It does this by moving the x-ray tube and detector in a circle taking x-ray images of the slice from all angles around the body.
A computer then processes these images to produce a cross sectional image (a picture of a slice through the body).
CT scans are useful as they can show a range of very different tissue types clearly: lung tissue, bone, soft tissue and blood vessels.
Sketch of a CT scanner
Above left is a CT image of the pelvis which has been processed so that only the bones are shown (hips, backbone and the top of the legs).
By adding together CT slices, 3-D images can be generated.
They are often used to plan radiotherapy treatments.
CT is useful for diagnosing internal injuries in trauma victims. Because a scan takes only a couple of minutes it can find problems quickly and save their lives.
One problem with x-ray CT is the radiation dose to the patient. A scan of the abdomen gives a dose of 10mSv, which is equivalent to the natural background radiation exposure over 4 years. This is about 100 times more than a standard chest x-ray.
CT (computed tomography) takes images that are slices through the body. They can be reconstructed to make a 3D image of the body.
Below is a CT image (3D x-ray) which has been processed by computer so that only the bones and the muscles are shown. The second metatarsal bone (the bone that David Beckham broke in 2002) is shown.