Terminology and Technology
- 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
Types of CT scanners
- 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
- 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
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)
|Substance||Hounsfield units (HU)|
|Air||–1000||The darkest (most hypodense)|
The brightest (most hyperdense)
|Fat||–100 to –50|
Contrast agents may be used to enhance visualization of targeted tissues.
- Used for defining the bowel on abdominal and pelvic CT scans
- Barium sulfate (most common)
- Isovue (an iodine-based solution)
- Gastrografin (water-soluble, used to evaluate for bowel perforation)
- Does not affect the kidneys
- 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)
- Used for suspected penetrating colonic injury
- Given as an enema
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
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.
- Severe head trauma
- Intracranial bleeding (appears hyperdense when acute)
- Enhances neoplasms and infections
- The angiographic phase is used to look for:
- Large-vessel occlusion
- Arteriovenous malformation
- Lung parenchymal diseases:
- Interstitial lung disease
- Lung cavitations
- Lung cancer
- Chest trauma:
- Rib fractures
- Pulmonary contusion
- Diaphragmatic rupture
- Laceration of the aorta or great vessels
- Pleural disease:
- Loculated effusions
- Mediastinal pathology:
- Cardiac tumors
- Pericardial effusion
- Vascular disease:
- Pulmonary embolism
- Aortic aneurysm or dissection
Abdominal and pelvic CT
Evaluation for abdominal and pelvic pathology:
- Blunt or penetrating abdominal injury
- Intra-abdominal infections
- Renal, ureteral, and bladder calculi
- 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
Special Considerations and Contradictions
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.
- 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
|Mechanism of acquisition||Ionizing radiation||Ionizing radiation||Acoustic energy||Ferromagnetic pulses|
|Relative cost||Inexpensive||Expensive||Inexpensive||Very expensive|
|Length of exam||Seconds||< 1 minute||Seconds||Approximately 1 hour|
|Contrast||No||May be needed||May be needed||May 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.
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