• Clinical science

Computed tomography

Summary

Computed tomography (CT) is an imaging technique that employs a rotating x-ray generator and multiple detectors to produce a large number of cross-sectional images on several planes. Like traditional radiography, CT creates images by projecting x-ray beams at an object and registering the amount of radiation that passes through. The resulting images visualize the inside of the object according to the amount of x-rays the constituent materials (e.g., different types of tissue) absorb or allow to pass through. The denser the material, the higher the attenuation and the brighter it will appear on the image. However, unlike traditional radiography, which produces projectional images of structures superimposed on each other, CT visualizes slices of the patient only a few millimeters thick, eliminating the problem of superimposition. The resulting images can be viewed individually or they can be digitally reconstructed into a 3D image. Furthermore, CT assigns each voxel, the basic unit of CT-image calculation, a specific value (Hounsfield unit; HU) according to the density of the material scanned. Postprocessing of the images allows the radiologist to map a range of voxel values to specific HU ranges (windowing), allowing for greater differentiation of densities than with traditional radiography. A CT scan can be performed with or without contrast to better visualize certain tissues. CT angiography employs a contrast medium to visualize blood vessels. The advantages of CT scan besides allowing for image manipulation and 3D reconstruction include shorter study time and lower cost than MRI and higher resolution than projectional radiography, while disadvantages include lower resolution than MRI and, especially, exposure to ionizing radiation. For example, radiation exposure from a chest CT scan is approximately equivalent to natural radiation exposure within a 4-year period and is ∼ 80 times higher than two-phase x-ray imaging. Accordingly, as with all imaging modalities that employ ionizing radiation, the radiation dose should be maintained as low as reasonably possible (ALARA principle) and appropriate safety measures should be observed.

Indications

As with any radiographic study, performing a CT scan should always be weighed against the risks associated with radiation exposure. CT scan has a very wide range of indications, including:

The American College of Radiology offers ACR Appropriateness Criteria®, which are evidence-based guidelines intended to help healthcare providers in making clinical decisions regarding imaging for a wide variety of diagnostic and interventional topics. They can be found at https://acsearch.acr.org/list. [1]

Technical background

Image generation

  • CT scan employs a rotating x-ray generator (x-ray tube) and multiple detectors located opposite to the generator.
  • The detectors are arranged in rows, each one capturing only a thin slice of the object scanned.
    • The use of multiple rows of detectors allows for higher spatial resolution.
    • Rotation of the generator and detectors permits the visualization of any structure from different angles.
    • A CT scan produces hundreds of cross-sectional images. Digital reconstruction allows for generating a 3D image from the cross-sectional images.
    • Earlier CT machines scanned the body incrementally (∼ 5–15 mm per layer) by gradually advancing the table. Today, the patient is scanned by rotating continuously.

Contrast CT

  • Description: CT scan using a radiocontrast agent administered orally or intravenously
  • Types
    • Oral or rectal (contrast enema) radiocontrast
      • Mainly barium sulfate suspension, but also water-soluble
      • Used to enhance visualization of the gastrointestinal tract
    • Intravenous (IV) radiocontrast
      • Mainly iodinated contrast
      • Used to enhance the visualization of specific organs or vessels. The time interval between contrast administration and image acquisition (phase) is predetermined depending on the tissue examined. For example:
        • Arterial phase: The contrast agent reaches the large arteries approx. 20 s after IV injection.
        • Venous phase: The contrast agent reaches the veins approx. 70 s after injection.
    • Negative contrast agent: Since air has the lowest density and appears black on x-ray, it can be used as a negative contrast agent (e.g., to enhance visualization of the gastrointestinal tract).
    • Inhaled contrast:
      • Xenon or krypton
      • Used in dual-energy CT pulmonary ventilation imaging

Image processing

  • Hounsfield scale: An index used to quantify the radiodensity of materials imaged by CT.
    • A CT scan produces a greyscale image, with high-density material appearing bright (e.g., bone) and low-density material appearing dark (e.g., fat).
    • The shades of gray are described in terms of Hounsfield units (HU), also referred to as CT numbers, on a scale with water as the reference point (0 HU). Higher-density materials have values above 0 (e.g., bone: 1000–1500 HU), while lower-density materials have a value below 0 (e.g., air: -1000 HU).
  • Voxel: a discrete element of a 3D image mapped on a regular grid and basic unit of CT-image calculation
    • The smaller the voxel, the sharper the image.
    • Tissue with high x-ray absorption (high attenuation) generates bright voxels.
    • Tissue with low x-ray absorption (low attenuation) generates dark voxels.
  • Windowing: the manipulation of a CT image's grayscale to highlight specific structures.
    • Involves mapping a range of voxel values to specific HU ranges.
    • Windows are defined by the “window level” (the midpoint of an image's HU range) and the “window width” (the range of HU in an image, sets contrast).
    • Voxels with an HU above or below the preset range will appear as entirely white or entirely black respectively.
  • 3D reconstruction: the digital assembly of the cross-sectional images obtained through a CT scan or select structures within those images into a 3D image.
    • Especially helpful for deciding the most appropriate management (e.g., surgical vs. nonsurgical) and planning for surgery

Preparation

  • All patients planned for CT should first be evaluated for:
  • The risk of complications should be weighed against the benefits of contrast CT scan for all affected patients.

All patients planning to undergo a CT scan should first be evaluated for complications, including past allergic reactions, contrast-induced nephropathy, and iodine-induced hyperthyroidism (thyroid storm). See “Complications” below for more details.

Interpretation/findings

Basics of CT interpretation

  • Planes:
  • Interpretation
    • Hypodensity: an area of tissue with lower density and, accordingly, lower x-ray attenuation than normal or surrounding tissue
      • Hypodense tissue appears darker than normal or surrounding tissue, as decreased x-ray absorption by the tissue increases the amount of x-rays that pass through the tissue and expose the detector.
    • Hyperdensity: an area of tissue with higher density and, accordingly, higher x-ray attenuation than normal or surrounding tissue
      • Hyperdense tissue appears brighter than normal or surrounding tissue, as increased x-ray absorption by the tissue decreases the amount of x-rays that pass the tissue and expose the detector.
    • Isodensity: an area of tissue with the same density and x-ray attenuation as surrounding tissue
      • Isodense tissue appears with the same brightness as the reference structure

CT scan examples

CT images of the CNS

CT images of abdominal organs

CT images of thoracic organs

CT images of bones and joints

Complications

  • Contrast-induced nephropathy (dose-dependent side effect)
    • Measure renal function parameters (creatinine, GFR) to assess risk.
    • Provide IV hydration, acetylcysteine for prophylaxis.
  • Iodine-induced hyperthyroidism (thyroid storm)
    • Measure thyroid function to assess risk.
    • Prophylaxis with perchlorate is indicated patients with latent hyperthyroidism.
    • In patients with clinically significant hyperthyroidism, iodinated radiocontrast should only be used if contrast CT is necessitated by a life-threatening emergency. In this case, an antithyroid agent (thiamazole ) should be given.
  • Allergic reactions
    • In patients with a history of allergic reaction to radiocontrast or iodine, contrast CT should only be performed if necessitated by a life-threatening emergency.
    • If a CT is necessary, premedication with prednisone and diphenhydramine should be given.

Iodinated radiocontrast can trigger thyroid storm in patients with latent hyperthyroidism! Further complications include acute kidney failure, permanent reduction of kidney function, and allergic reactions.

We list the most important complications. The selection is not exhaustive.

last updated 08/10/2020
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