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Diagnostic Study - Description & Definition

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

Normal Study Findings - Images (For abnormal findings images, click on Diagnoses below)
CT system includes 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.
Soft tissue window (left) axial images and bone window (right) axial images
 Soft tissue window (left) coronal images and  bone window (right) axial Images
Soft tissue window (Top) and bone window (Bottom)
3D reformatted coronal image of the hand
Normal Study Findings - Video
Diagnoses Where These Studies May Be Used In Work-Up (with abnormal findings images)
Comments and Pearls
  • The clinical applications of CT are vast, and it can be used to aid in diagnosing and staging numerous pathologies.
  • Patient selection is crucial.  The radiation dose, although a lot lower when using utilizing modern scanners, the dose can still potentially have long term sequel, such as slightly increasing an individual’s lifetime cancer risk. A risk to benefit assessment must be made by the clinician, especially when deciding to scan pregnant or pediatric patients.
  • CT is especially useful in trauma. It can provide surgeons with information about internal organ injury or ongoing internal hemorrhage in unconscious patients, or in patients with non-localizing symptoms.
  • CT can accurately determine the extent of a fracture, the planes of the fracture, the number of fracture fragments and helps determine if a fracture extends into a joint. It can also help identify subtle fractures missed by routine radiographs.
  • CT can be especially useful in assessing for carpal bones fractures. Carpal bone fractures can be very subtle or non-displaced which can make them radiographically occult.
References
  1. G. Donald Frey: Basic CT Parameters, American Journal of Roentgenology 2014 203:2, W126-W127
  2. Ma CB, Steinbach LS: Musculoskeletal Imaging, in Boyer MI, ed. AAOS Comprehensive Orthopaedic Review, Rosemont, Il. Academy of Orthopaedic Surgeons, 2014, pp 159-165.
  3. National Research Council (US) and Institute of Medicine (US) Committee on the Mathematics and Physics of Emerging Dynamic Biomedical Imaging. Mathematics and Physics of Emerging Biomedical Imaging. Washington (DC): National Academies Press (US); 1996. Chapter 3, X-Ray Computed Tomography. Available from: https://www.ncbi.nlm.nih.gov/books/NBK232484/
  4. Cynthia H. McCollough, Shuai Leng, Lifeng Yu, and Joel G. Fletcher: Dual- and Multi-Energy CT: Principles, Technical Approaches, and Clinical Applications, Radiology 2015 276:3, 637-653
  5. Paul I. Mallinson, Tyler M. Coupal, Patrick D. McLaughlin, Savvas Nicolaou, Peter L. Munk, and Hugue A. Ouellette: Dual-Energy CT for the Musculoskeletal System, Radiology 2016 281:3, 690-707
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