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Computed Tomography

  • CT is a noninvasive diagnostic technique that uses a rotational radiographic source to generate cross-sectional images.
  • CT is particularly advantageous for musculoskeletal imaging when used with multiplanar, volume-rendered reconstruction techniques.
  • Chronology of development :
    • 1972: Introduction
    • 1974-1976: 1st clinical scanners installed
    • 1980: Became widely available
    • 1985-1986: Introduction of modern applications, such as dynamic imaging, multiplanar reformatting, and 3D CT with volume rendering and shaded surface display
    • 1989: Beginning of routine spiral scanning
  • Advantages:
    • Rapid image acquisition, particularly useful in pediatric, trauma, and very ill patients, with reduced need for sedation
    • Clear evaluation of anatomically complex areas not always well evaluated by plain radiographs, such as the axial skeleton and small joints (ankle and wrist)
    • Multiplanar reformatted and 3D capabilities: For 16-slice multidetector CT and beyond, acquisition in only 1 plane is required because the data set may be reconstructed into different planes and perspectives.
    • May virtually eliminate streak artifact secondary to metal hardware through volume rendering of a multidetector CT axial database .
      • MRI evaluation in such patients often is extremely limited.
    • Can safely image patients with contraindications to MRI, such as aneurysm clips, pacemaker, or orbital metallic fragments
    • Cost-effective modality for a wide range of clinical problems
    • Widely available
  • Disadvantages:
    • Inferior to MRI for bone marrow and soft-tissue details
    • Requires ionizing radiation exposure
    • More expensive than plain radiography
    • If contrast is necessary, a risk of allergic reaction or contrast nephropathy exists.
  • Skeletal pathology:
    • Typically imaged with thin-section collimation (0.75 mm for a 16-slice multidetector CT).
    • Postprocessing of data into multiplanar reformatted images and 3D reconstructions is performed on a workstation.
  • Soft tissues:
    • Thin-section imaging is not as crucial as for the evaluation of the skeleton.
    • Reconstructed slice thickness typically is set at 2-3 mm.
    • Studies performed to evaluate a soft-tissue mass, potential abscess, or vascular injury typically require the administration of intravenous contrast material, requiring injection rates of 3 mL/sec.
      • Intravenous contrast should be used with caution in patients with renal insufficiency.
      • Patients with potential allergies to intravenous contrast should be identified, premedicated, or not injected.
Differential Diagnosis
  • Postoperative indications:
    • Identification of complications of hardware implantation, such as osteomyelitis or fracture
    • Identification of potential tumor recurrence in the presence of hardware
    • Detection of retained foreign body
  • Oncology indications:
    • Detection of calcification within a lesion: Distinction of myositis ossificans and neoplasm by detecting pattern of mineralization
    • Characterization of cortical and periosteal changes for distinguishing benign and malignant processes
    • Assessment of bone destruction and fracture risk
    • Definitive treatment of osteoid osteomas with CT-directed radiofrequency ablation of the nidus
    • Detection of compartmental and neurovascular involvement, although typically more commonly assessed by MRI
  • Trauma indications:
    • Definition or exclusion of a fracture that is equivocal on plain radiograph: 3D CT and multiplanar reconstructions are particularly useful for detecting fractures oriented in the axial plane.
    • Determination of extent of fracture, including physeal and intra-articular involvement
    • Identification of intra-articular fracture fragments
    • Identification of fracture nonunion
    • Detailed cervical spine evaluation in moderate- and high-risk trauma patients
  • Evaluation of anatomically complex areas, such as pelvis, scapula, wrist, ankle, and spine.
  • Infection indications:
    • Determination of compartments of tissue involvement (bone, muscle, fascia, subcutaneous tissue) necessary for patient triage as medical or surgical candidates
    • Assessment of response to antibiotic therapy
  • Pediatric indications :
    • Skeletal dysplasias:
      • Useful for applications, such as dysplasias, that require imaging of a large field of view to define the anatomy and evaluate the skeleton postoperatively
      • Such cases often are difficult to image completely by radiography or MRI.
    • DDH:
      • Diagnosis usually is made by physical examination, plain radiographs, and ultrasound.
      • CT may be used in difficult cases or, with low-dose scanning, as an imaging alternative.
      • CT more commonly is used to define success of reduction after cast placement.
    • SCFE:
      • Detection of contralateral involvement with coronal and sagittal display
      • Exclusion of other causes of hip pain, such as osteoid osteoma or septic joint
    • Legg-Calve-Perthes disease:
      • Presurgical: Planning for identification of severity of disease
      • Postoperative: Assessment for determination of success of intervention or evaluation of new symptoms
    • Pectus deformities:
      • Surgical approach and anatomic definition, especially if initial repair was unsuccessful
    • Tarsal coalition:
      • CT reconstruction in multiple planes to define the different osseous and nonosseous coalitions
Treatment Alert
Pediatric Considerations
  • Children are more sensitive to radiation than are adults and are more likely to develop radiation-induced neoplasm over a lifetime.
  • CT exposure parameters should be adjusted, and only necessary examinations performed.
  • Multiphase imaging (both with and without contrast) should be avoided.
  • Multidetector technology with volume visualization and postprocessing minimizes radiation exposure.
Pregnancy Considerations
  • Scan volume should be limited to necessary anatomy.
  • Multiphase imaging should be avoided.
  • Establish protocols that appropriately use radiation and are tested regularly by departmental medical physicists.
  • The magnitude of leukemogenic fetal risk is uncertain.
  • No long-term effects of intravenous contrast on fetus are known, but the usual dose should be reduced by 59% to 0.5 mL/kg .
  • Consider nonionizing alternative modalities, such as MRI or ultrasound, if possible.
  • Lead shield the abdomen and pelvis, if possible.
  • Nursing mothers should wait 24 hours after intravenous contrast administration to resume breast-feeding.
  • Contrast allergy:
    • Risk factors include history of asthma and previous reactions.
    • Option to premedicate at-risk patients with steroids and diphenhydramine
  • Contrast nephropathy:
    • Risk factors include pre-existing renal disease, multiple myeloma, diabetes, and dehydration
    • Aggressive hydration and use of low-osmolar agents
Patient Teaching
  • The patient should expect to be in the CT scanner, motionless, for up to several minutes during the acquisition of images, but with modern-day scanners, a typical CT examination may require <30 seconds.
  • Intravenous contrast may be administered, depending on the indication, and informed consent should be obtained.
Q: How much time does a CT scan require?
A: Depending on the type of scanner available, as short as 10 seconds (64-slice multidetector CT) and usually <1 minute.

Q: Can I order a CT scan on a pregnant patient?
A: Yes, if clinically necessary. However, the protocol is adjusted to reduce radiation exposure.
Q: Can I order a CT scan for a patient with a pacemaker, aneurysm clips, or other metal hardware?
A: Yes. Metal is not a contraindication for CT.
Q: How does metal hardware affect CT imaging?
A: Metal creates streak artifact. This artifact can be reduced or eliminated with volume-rendered 3D CT.
Q: How do I order a 3D CT?
A: 3D CT is most effective with advanced CT technology (16-slice multidetector CT and beyond). A discussion of imaging equipment and techniques with the radiologist is the 1st step.

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