Background
Computer Tomography is an imaging modality that generates images of the internal anatomy of any body part without superimposition of adjacent structures. Since the development of CT, multiple different generations of CT scanners have been produced drastically decreasing scan time, increasing resolution, and decreasing radiation dose. This brief introduction of this complex and continuously evolving topic will not discuss the various generations of CT scanner, but only focus on a brief conceptual understanding.
Historical Overview
Computed tomography (CT) was founded based on developments in two fields: x-ray imaging and computing. X-rays were discovered in 1895 and quickly became an established medical tool. Tomography was being developed in the 1930s, enabling the visualization of sections though the body. By the 1960s, several independent researchers had worked on cross-sectional imaging, which culminated in Hounsfield's development of a CT scanner. Image data were acquired from multiple x-ray transmissions through the object under investigation, and the computer used the data to reconstruct the image.1
The first clinical CT scan was performed in October 1971 at Atkinson Morley's Hospital in London. The patient, a woman with a suspected frontal lobe tumor, was scanned with a prototype scanner, developed by Godfrey Hounsfield and his team at EMI Central Research Laboratories in Hayes, west London.1 Since then, CT has revolutionized diagnostic decision making.
Description
The essential components of a CT system are a circular scanning gantry housing the x-ray tube and detector which are opposite to one another, a moving table on which the patient is placed, an x-ray generator, and a computerized data processing unit. The patient lies on the table and is placed inside the gantry.
CT essentially uses X-rays to construct cross-sectional images. A source emits photons which pass through a given volume of tissue and are then detected by a detector. The source and detector are diametrically opposite to one another.3They scan while rotating around the patient, always emitting photons in a straight line. The advantage of having a system that rotates around a patient is it can provide a more accurate measurement of a unit of tissue regardless of the density in front of it or behind it. The photons will simply pass through the volume of tissue at a different incidence angle and measure the density.
For simplicity, the detector essentially subtracts the initial photon energy from the received photon energy after a photon has passed through a given volume of tissue. This information is then sent to the data processing unit. The data processing unit converts the x-ray beam attenuation of the tissue into a Hounsfield units (HU). HU is an artificially generated density number by scanning pure water and designating it the number 0. The processing unit then compares the density of pure water to all other densities and labels it with a number. Some densities attenuate more than water and have HU of greater than 0 for example muscle HU +40 to +80. Others may attenuate less than water, and are labelled with HU less than 0, for example fat HU -60 to -100.
Once every pixel is given a density, these pixels are combined, and an image is formed. These images are then stacked together and can be viewed in multiple different planes to help the interpreter form a 3D view into the human body.
The table below provides a list of attenuation ranges by different bodily tissue types:
|
Tissue Type
|
Hounsfield units (HU)
|
|
Air
|
−400 to −1000
|
|
Fat
|
−60 to −100
|
|
Pure Water
|
0
|
|
Body fluid
|
+20 to +30
|
|
Muscle
|
+40 to +80
|
|
Trabecular bone
|
+100 to +10000
|
|
Cortical bone
|
+1000
|
Dual energy CT is a technique which has been gaining favor of late. It is a technique of obtaining two separate CT using different energies, hence the name dual energy. Meaning the source will emit photons at different energies.4 The principle this relies on the fact that different compounds attenuate photons of different energies differently. Dual energy will not only help in detecting and characterizing lesions in the abdomen and pelvis but may have utility in helping detect musculoskeletal pathologies such as bone marrow edema, metal reduction artifact, and other utilities.5
