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