Computed Tomography (CT)

Computed tomography (CT) is one of the most commonly used imaging methods because it is widely available, fast, and reliable. CT scans deploy X-rays to obtain cross-sectional images of the body. A CT scanner consists of a tube that rotates around the patient and emits an X-ray beam and a detector that uses specialized software to receive and convert the beam to an image. The ability to create multiple views (axial, sagittal, coronal) and use contrast (intravenous, oral, rectal) allows for enhanced diagnostic yield. Patients are exposed to radiation, and special consideration should be given to patients with a history of iodine allergy, renal disease, or thyroid disease or patients who are pregnant.

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Terminology and Technology

General component

  • Rotating X-ray tube: 
    • Spins around the patient
    • High-energy electrodes in the tube emit radiation beams. 
    • Radiation passes through the patient’s body.
  • Radiation detectors: 
    • On the opposite side of the patient’s body from the X-ray tube
    • Absorbs and measures the remaining radiation (in the form of varying density) after it has passed through tissues
    • Computer software detects the tomography generated by the detector and reconstructs an image.
  • Motorized table: advances the patient through the scanner

CT scan components:
Arrangement of the X-ray tube and detectors in the CT scanner

Image by Lecturio.

Types of CT scanners

Helical (“spiral”):

  • Most common due to its speed
  • As the patient is moved through the CT, the rotating beam and X-ray detector spin. This creates a helical path.
  • Results in a 3-dimensional data set
  • Minimizes errors due to patient movement or breathing

Sequential (step-and-shoot):

  • Was the conventional method before helical CT
  • The patient is moved through the CT with short pauses to capture images at each position.
  • Results in an ↑ radiation dose
  • Used for high-resolution scanning of the lungs and coronary arteries and for coronary CT angiography

Differences between sequential and helical CT scanners:
Sequential CT takes discrete imaging slices as the patient moves through the scanner, whereas the continuous movement of the helical CT results in a spiral path.

Image by Lecturio.

Image processing

Digital images are created with a matrix of voxels (3-dimensional pixels), which are measured in Hounsfield units (HU).

  • An index used to universally quantify the radiodensity of imaged findings on CT
  • Based on the amount of radiation that a material absorbs:
    • Dense material will appear bright.
    • Less dense material will appear dark.
  • The index is constructed on assigned values to water (0 HU) and air (–1000 HU).

Postprocessing can be used to accentuate tissues of different densities.

  • Lung window
  • Bone window
  • Soft tissue window

Images can also be produced in different viewing planes.

  • Axial (looking from the feet up to the head)
  • Sagittal (looking from the side)
  • Coronal (looking from the front)
Table: Density of different material images on CT
SubstanceHounsfield units (HU)
Air–1000The darkest (most hypodense)












The brightest (most hyperdense)
Fat–100 to –50
Water0
Soft tissue20–300
Bone250‒700
This table shows the density of different materials that would be imaged on CT. As you can see, materials with a higher density than water will have a positive value (and are brighter), while negative values are assigned to those with lower densities (and are darker).

CT image viewing planes:
The slices through this model demonstrate how axial, coronal, and sagittal images correlate with the patient’s anatomy.

Image by Lecturio.

Contrast enhancement

Contrast

Contrast agents may be used to enhance visualization of targeted tissues.

  • Oral:
    • Used for defining the bowel on abdominal and pelvic CT scans
    • Agents: 
      • Barium sulfate (most common)
      • Isovue (an iodine-based solution)
      • Gastrografin (water-soluble, used to evaluate for bowel perforation)
    • Does not affect the kidneys
  • IV:
    • Used for enhancement and differentiation of vascular structures and solid organs
    • Agent: iodinated, low ionic, low osmolar solution
    • X-rays are absorbed by the contrast → ↑ attenuation
      • Contrast becomes more dilute as it moves from the arteries → tissues → veins
      • Image appearance changes over time. 
    • Enhancement is based on:
      • The amount of blood flow
      • Timing of the image after contrast administration
    • Excreted by kidneys (should be used judiciously in acute or chronic renal failure)
  • Rectal:
    • Used for suspected penetrating colonic injury
    • Agents: 
      • Isovue
      • Gastrografin
    • Given as an enema

Phases

Multiphase CT can identify structures at various intervals after IV contrast administration.

  • Noncontrast phase: prior to contrast injection
  • Vascular (arterial) or bolus phase:
    • 15–20 seconds after injection
    • Contrast diffuses into the vasculature.
    • Opacifies the aorta and its branches
    • Allows differentiation of renal cortex and medulla
  • Redistribution (venous) phase:
    • 1–3 minutes after injection
    • Contrast diffuses from intravascular to extravascular compartment.
    • Opacifies the inferior vena cava, large veins, and solid organ parenchyma
  • Equilibrium (delayed) phase:
    • 6–10 minutes after injection
    • Contrast reaches dynamic equilibrium in the intravascular and extravascular compartments.
    • Opacifies the renal collecting system, ureters, and urinary bladder

Axial multiphase CT images of the abdomen:
a: Unenhanced image shows an infiltrative tumor-like hepatic mass.
b: Postcontrast arterial phase shows a nonenhancing, hypoattenuating lesion.
c: Postcontrast portal-venous phase shows a faint enhancement of fibroinflammatory components surrounding the parasitic pseudocyst.

Image: “Hepatic alveolar echinococcosis imaging” by Wenya Liu et al. License: CC BY 4.0

Interpretation

The best approach is a systematic approach.

  • Check the patient’s demographics and name and the date.
  • Note the reason for the study.
  • Determine the part of the body that was imaged.
  • Look for prior imaging to allow for comparison.
  • Determine the image viewing plane and orientation.
  • Use multiple window levels and scroll through multiple times to ensure that all sections are covered. 
  • Viewing windows can be changed to optimize imaging of the desired organ system.
  • Evaluate one organ at a time.

Indications

Head CT

Noncontrast:

  • Severe head trauma
  • Stroke
  • Intracranial bleeding (appears hyperdense when acute)

Contrast:

  • Enhances neoplasms and infections
  • The angiographic phase is used to look for: 
    • Large-vessel occlusion
    • Arteriovenous malformation
    • Aneurysm

Chest CT

  • Lung parenchymal diseases:
    • Interstitial lung disease
    • Pneumonia
    • Lung cavitations
    • Abscess
    • Lung cancer
  • Chest trauma:
    • Rib fractures
    • Pulmonary contusion
    • Diaphragmatic rupture
    • Laceration of the aorta or great vessels
  • Pleural disease:
    • Loculated effusions
    • Empyema
    • Hemothorax
  • Mediastinal pathology:
    • Cardiac tumors
    • Pericardial effusion
    • Pneumomediastinum
  • Vascular disease:
    • Pulmonary embolism
    • Aortic aneurysm or dissection

Abdominal and pelvic CT

Evaluation for abdominal and pelvic pathology:

  • Blunt or penetrating abdominal injury
  • Appendicitis
  • Diverticulitis
  • Pancreatitis
  • Intra-abdominal infections
  • Renal, ureteral, and bladder calculi 
  • Pneumoperitoneum
  • Bowel obstruction
  • Aortic aneurysm and dissection
  • Retroperitoneal hemorrhage

Evaluation of visceral malignancies:

  • Characterization of abdominal and pelvic masses
  • Staging evaluation of known malignancy
  • Monitoring with treatment

CT scan of the middle abdomen showing a subcapsular hepatic hematoma on the surface of the right lobe of the liver (hypodense area indicated by the arrows) with a source of active bleeding (circle)

Image: “CT scan of middle abdomen showing a subcapsular hepatic hematoma” by From the Department of General Surgery, C.S. General and Emergency Surgery, Azienda Ospedaliera – IRCCS Arcispedale Santa Maria Nuova, Reggio Emilia, Italy. License: CC BY 4.0

Special Considerations and Contradictions

Radiation exposure

Like other imaging methods, CT scans expose patients to radiation.

  • Ionizing radiation is additive.
  • Number of scans should be limited whenever possible.
  • Radiation dose:
    • Chest and abdominal CT: 10 mSv
    • Pelvic CT: 7 mSv
    • Head CT: 2 mSv
  • In context:
    • Chest X-ray: 0.013 mSv
    • Low risk of fetal malformation: < 50 mSv
    • Substantial fetal damage: > 500 mSv
  • Effects of radiation:
    • Molecular damage
    • Free radical formation
    • Disruption of cellular metabolic function
    • Cell death after a certain threshold
    • Carcinogenic risk increases with exposure.

Contraindications

  • Allergic reactions to contrast agents:
    • May manifest as hives or anaphylaxis
    • Patients can be premedicated with steroids and antihistamines if a contrast CT is necessary.
  • Pregnancy: Iodinated contrast can cross the placenta.
  • Thyroid disease: 
    • Iodinated contrast will reduce uptake of radioactive iodine → treatment is less effective
    • Hyperthyroidism patients may develop thyroid storm from contrast.
  • Chronic or acutely worsening renal disease: Iodinated contrast can cause acute tubular necrosis.

Other Imaging Methods

Comparison of imaging methods

Table: Comparison of imaging methods
RadiographyCTUltrasoundMRI
Mechanism of acquisitionIonizing radiationIonizing radiationAcoustic energyFerromagnetic pulses
Relative costInexpensiveExpensiveInexpensiveVery expensive
PortableYesNoYesNo
Length of examSeconds< 1 minuteSecondsApproximately 1 hour
ContrastNoMay be neededMay be neededMay be needed

Imaging method options by system

  • Imaging of the CNS (brain, spinal cord, and vertebral column): 
    • Radiography is often used to evaluate for fractures of the vertebral column. 
    • CT is a good choice for head trauma and to exclude intracranial hemorrhage. 
    • MRI provides more detailed images of the brain and spinal cord, allowing identification of infarction, tumors, disc herniation, and demyelinating disease.
  • Pulmonary radiology and imaging of the mediastinum: 
    • Radiography is the preferred initial imaging study for viewing lung pathology. 
    • CT scan provides more detailed views of the lung parenchyma, mediastinal structures, and vasculature. 
    • MRI is not often used, but may be employed for evaluating malignancies and cardiac disease. 
    • Ultrasonography can be used for a rapid bedside trauma assessment and for guiding procedures (thoracentesis).
  • Breast imaging: 
    • Mammography is often the initial choice for breast cancer screening. 
    • MRI may be used to further evaluate and stage breast cancer. 
    • Ultrasonography is helpful for evaluating lymph nodes and to guide biopsy.
  • Imaging of the abdomen and renal imaging: 
    • Radiography is often used to evaluate for kidney stones, bowel obstruction, and pneumoperitoneum. In addition, barium may be used to assess swallowing and bowel function. 
    • CT and MRI provide more detailed assessments of the abdominal viscera and vasculature. 
    • Nuclear medicine can be used to assess gallbladder function and gastric emptying and for GI bleeding.
  • Imaging of the uterus and ovaries: 
    • Ultrasonography is the most commonly used method to evaluate the ovaries and uterus, including assessing pregnancies and the causes of abnormal uterine bleeding. 
    • CT and MRI provide more detailed views and are often useful in assessing cysts, malignancies, and benign masses.
  • Imaging of the musculoskeletal system: 
    • Radiography is often used to exclude fractures. 
    • CT is more sensitive to bone pathology, including osteomyelitis. 
    • MRI is preferred for a soft tissue evaluation, such as assessing for malignancy and myositis. 
    • Bone scanning can be useful in finding occult fractures, osteomyelitis, and metabolic bone disease.

References

  1. Kocak, M. (2019). Computed tomography. MSD Manual Professional Version. Retrieved December 2, 2020, from https://www.msdmanuals.com/professional/special-subjects/principles-of-radiologic-imaging/computed-tomography
  2. Stark, P. (2020). Principles of computed tomography of the chest. In Finlay, G. (Ed.), UpToDate. Retrieved December 2, 2020, from https://www.uptodate.com/contents/principles-of-computed-tomography-of-the-chest
  3. Rawson, J. V., Pelletier, A. L. (2013). When to order a contrast-enhanced CT. American Family Physician 88(5):312–316.
  4. Knipe, H., and Nadrljanksi, M.M. (2019). Computed tomography. Radiopedia. Retrieved December 2, 2020, from https://radiopaedia.org/articles/computed-tomography
  5. Fertikh, D. (2015). Head computed tomography scanning. In Taylor, C.R. (Ed.), Medscape. Retrieved December 2, 2020, from https://emedicine.medscape.com/article/2110836-overview
  6. Taylor, C. R., Abramovici, G. (2017). Abdominal computed tomography scanning. In Mathur, M. (Ed.), Medscape. Retrieved December 2, 2020, from https://emedicine.medscape.com/article/2114236-overview
  7. Rogers, D. C., and Tadi, P. (2020). Intravenous contrast. StatPearls. Retrieved January 19, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK557794/

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