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Overview and Description

CT (computed tomography) and MRI (magnetic resonance imaging) are sophisticated, noninvasive imaging modalities utilized for targeted evaluations of the body. The information obtained from these tests often help to establish a diagnosis and guide treatment. Given their clinical relevance and potential cost, it is important to understand the appropriate indications for each exam prior to ordering these imaging techniques. A research study at an academic primary care clinic indicated that the most common imaging studies ordered were head CTs for chronic headaches, lumbar spine MRIs for acute back pain, and MRIs of the knee or shoulder due to osteoarthritis; however, only 74% of these diagnostic tests were found to be appropriately ordered.1 A European study showed similar data in which spinal CT scans were the most inappropriately ordered study, at an appropriateness rate of only 28%.2 The choice of CT or MRI is dependent on multiple factors, such as diagnostic impression, anatomic location of suspected abnormality, safety (radiation), cost, and level of urgency. Physicians must weigh each of these factors to maximize diagnostic utility while minimizing any harm to the patient and reducing medical waste. Thus, the appropriate selection of imaging studies is important for the quality of patient care, as well as safety. General guidelines for choosing the appropriate imaging study for a given clinical scenario is available at American College of Radiology Appropriateness Criteria (ACR AC; http://www.acr.org/Clinical-Resources/ACR-Appropriateness-Criteria).3 The ACR Appropriateness Criteria was established by the consensus of an ACR panel after a comprehensive literature review.4 These criteria provide an easily accessible clinical resource to guide clinicians when ordering imaging studies. The need for these guidelines was highlighted by Bautista et al. who found that residents and attendings often resorted to UpToDate searches, radiology consultations, or web searches in order to determine imaging appropriateness.4

The table below shows the ACR imaging criteria for low back pain as an example.

*Red flag symptoms: history of cancer, unexplained weight loss, immunosuppression, urinary tract infection, intravenous drug use, prolonged corticosteroid use, low back pain not improved with conservative treatment, infection.

Relevance to Clinical Practice

CT vs MRI

CT imaging uses multiple x-ray beams to produce cross-sectional images of the body in a relatively short time, thereby exposing the patient to ionizing radiation.

MRI imaging is a more complex process than CT. The technique consists of the application of a magnetic field which alters the alignment of hydrogen protons in various human tissues containing water. The MRI machine then measures the time it takes for the protons to revert back to their original alignment, which produces a signal that is processed to form an image. The table below highlights the primary differences between CT and MRI.

There are many different types of MRI groups (e.g. Diffusion Weighted, Perfusion Weighted, Spin Echo, Magnetic Resonance Angiography, etc.) and sequences (T1, T2, FLAIR, etc.). A selection of these is discussed below.

T1 vs T2 Imaging Sequences5

T1- and T2-weighted images are derived from net magnetic vectors of hydrogen nuclei in various human tissues. In normal human tissue, increased concentrations of hydrogen nuclei are generally found in water and fat. When the patient is outside of a magnetic resonance (MR) machine, these hydrogen nuclei point at random directions, effectively “canceling each other out”.  When placed in a MR machine, this tissue is first exposed to a static magnetic field that aligns all of the hydrogen nuclei in a net baseline vector. Another magnet then rotates all of the hydrogen nuclei perpendicularly to the baseline static magnetic field, a process known as excitation of transverse magnetization.  The second magnet is then turned off and the hydrogen nuclei return to their baseline magnetic field in a process known as relaxation (decay) of transverse magnetization. This transverse magnetization is equivalent to the signal (brightness of tissue) on MR image.

It should be noted that excitation and decay are not necessarily inversely proportional due to multiple factors outside of this topic’s scope. The rate at which the hydrogen nuclei return to the baseline varies from tissue to tissue depending on properties such as tissue-specific density of hydrogen nuclei (proton density).  Tissues on MR imaging are distinguished by their contrast levels (i.e., the amounts of signal) produced by each tissue type.

T1 and T2 refer to time constants during the relaxation phase. T1 signals are based on the rate of recovery of the baseline (longitudinal) magnetization during relaxation, while T2 signals are based on the rate of decay of the perpendicular/transverse magnetic force during relaxation. The rate of decay of the perpendicular magnetization is longer than the rate of recovery of the baseline magnetization. Therefore, it takes a longer time for T2 images to develop than T1.

When observing the MR T1-weighted images, the signal of fatty tissue is enhanced, while the signal of water is suppressed. Conversely, T2-weighted images enhance the signal of water.

Each weighted image has specific characteristics (see Table 3). These sequences on MRI are helpful in determining pathology on spinal imaging. For example, Modic type 1 changes show low T1 signal and high T2 signal indicative of marrow inflammation and edema. Modic type 2 changes show high T1 and T2 signals indicative of fatty replacement of marrow. Modic type 3 changes show low signals on both T1 and T2, suggestive of trabecular microfractures and sclerosis.

Fluid Attenuated Inversion Recovery (FLAIR)6

Fluid attenuated inversion recovery (FLAIR) is a specific MR imaging sequence that selectively nulls the signal for certain tissues (e.g. fat or fluid). Its purpose is to remove signal from the cerebrospinal fluid in the resulting images. This can be especially useful for imaging the brain, as brain tissue will appear similar to T2-weighted images (grey matter will have a brighter signal than white matter while CSF appears dark). FLAIR is especially helpful in identifying lacunar infarction, meningitis, multiple sclerosis (MS) plaques, and subarachnoid hemorrhage.

Diffusion Weighted Imaging7

Diffusion Weighted Imaging (DWI) is an MRI group that measures Brownian motion of water molecules and displays a high signal within minutes of a cerebral infarction. DWI MRI is superior to noncontrast CT when diagnosing acute ischemic stroke in the first 12 hours of symptom onset.8 DWI is able to detect infarction as early as 3 – 30 minutes from onset, at which point signals on conventional MRI and CT images would still appear to be normal.9-11 Additionally, studies have compared non-contrast CT, DWI, and fluid-attenuated inversion recovery (FLAIR), which have shown that abnormal DWI is both sensitive and specific for ischemic stroke in patients presenting with symptoms in the first six hours of symptom onset.12-17 However, it is important to note that some patients with deficits related to acute ischemic may have normal diffusion weighted imaging. 

Perfusion Weighted Imaging18

Perfusion Weighted Imaging is a MRI group that measures blood flow. During a cerebral infarction, the infarcted core and penumbra have decreased perfusion with delayed contrast arrival.

Magnetic Resonance Angiography

Magnetic resonance angiography (MRA) and venography is a specialized MRI technique designed to analyze blood vessels and can assist in diagnosing arterial and vascular conditions such as aneurysms, dissections, or stenoses.19 Multiple techniques encompass MRA, such as Phase Contrast, Time-of-Flight (TOF), and Contrast-Enhanced (CE-MRA)—each approach provides slightly different information about the vasculature. Phase contrast imaging accesses velocity within the vasculature, TOF can enhance flow visualization within vessels, and CE-MRA is used to evaluate vessel structure.20-22 

Contrast Agents

Utilizing MRI when imaging anatomical structures or blood flow do not require contrast agents. This is because tissues or blood have properties that provide natural contrasts. However, in certain studies (indicated under “Specific Imaging Considerations” below), it is feasible to utilize contrast agents. The most common agents are gadolinium-based, which is paramagnetic.23 In general, these agents have proved safer than the iodinated contrast agents used in X-ray radiography or CT, especially with regards to nephrotoxicity.24

Specific Imaging Considerations for MSK and Cerebrovascular Pathology (per ACR Appropriateness Criteria)

  • Cervical Spine25
    • New or increasing nontraumatic cervical or neck pain without red flag symptoms*
      • Initial imaging study: Radiographs of the cervical spine are usually appropriate for initial evaluation
      • There is insufficient literature to conclude appropriateness of MRI or CT cervical spine without IV contrast in this patient population. Experts did not agree on this modality in this population, but it may be appropriate
    • New or increasing nontraumatic cervical radiculopathy without “red flags”
      • Initial imaging study: MRI cervical spine without IV contrast usually appropriate
      • CT cervical spine without IV contrast may be appropriate
      • Use of radiographs was controversial in this patient population, as there is disagreement on appropriateness. Therefore, it is considered “may be appropriate”
    • Prior cervical spine surgery with new or increasing nontraumatic cervical or neck pain or radiculopathy 
      • Initial imaging study: Use of cervical spine radiography and CT cervical spine without IV contrast is usually appropriate
      • MRI cervical spine with AND without IV contrast, MRI cervical spine without IV contrast both may be appropriate, but there are disagreements without consensus
      • CT myelography cervical spine may be appropriate
    • Suspicion for infection with new or increasing nontraumatic cervical or neck pain or radiculopathy
      • Initial imaging study: MRI cervical spine without AND with IV contrast is usually appropriate
      • CT cervical spine with IV contrast and MRI cervical spine WITHOUT IV contrast may be appropriate
      • CT cervical spine WITHOUT IV contrast, MRI cervical spine with IV contrast, Radiography of the cervical spine may be appropriate, but there is disagreement
    • Known malignancy with new or increasing nontraumatic cervical or neck pain or radiculopathy.
      • Initial imaging study: MRI cervical spine with AND without IV contrast, or MRI cervical spine without IV contrast alone are usually appropriate
      • Radiography of the cervical spine may be appropriate
      • Bone scan of the whole body with SPECT or SPECT/CT neck may be appropriate
      • CT cervical spine with OR without IV contrast, or MRI cervical spine with IV contrast alone may be appropriate but there is disagreement
    • Cervicogenic headache and new or increasing nontraumatic cervical or neck pain WITHOUT neurological deficit
      • Initial imaging study: Radiography of the cervical spine, CT cervical spine without IV contrast, or facet injection/medial branch block of the cervical spine may be appropriate
      • MRI cervical spine without IV contrast may be appropriate, although there is disagreement
    • Chronic cervical or neck pain
      • Initial imaging study: Radiography of the cervical spine is usually appropriate
      • MRI of the cervical spine without IV contrast may be appropriate, although there is disagreement
    • Chronic cervical or neck pain without neurologic findings but with radiographs that show degenerative changes
      • Next imaging study after radiographs: MRI cervical spine without IV contrast is usually appropriate
      • CT cervical spine without IV contrast or CT myelography of the cervical spine may be appropriate
    • Chronic cervical or neck pain with or without radiculopathy, but radiographs show ossification in the posterior longitudinal ligament
      • Next imaging study after radiographs: CT cervical spine without IV contrast is usually appropriate
      • CT myelography of the cervical spine or MRI cervical spine without IV contrast may be appropriate, although there is disagreement
  • Shoulder
    • Traumatic shoulder pain26
      • Traumatic shoulder pain of any etiology
        • Initial Imaging Study: Radiography of the shoulder is usually appropriate
      • Traumatic shoulder pain (nonlocalized) with negative radiographs
        • Next Imaging Study: MRI shoulder without IV contrast
        • CT arthrography or MR arthrography of the shoulder may be appropriate
        • US of the shoulder may appropriate, although there is disagreement
      • Traumatic shoulder pain. Radiographs show humeral head or neck fracture.
        • Next Imaging Study: CT shoulder without IV contrast is usually appropriate
      • Traumatic shoulder pain with radiographs showing scapular fracture
        • Next Imaging Study: CT shoulder without IV contrast is usually appropriate
      • Traumatic shoulder pain. Radiographs show Bankart or Hill-Sachs lesion.
        • Next Imaging Study: MR arthrography of the shoulder or MRI of the shoulder without IV contrast is usually appropriate
        • CT arthrography of the shoulder or CT shoulder without IV contrast may be appropriate
      • Traumatic shoulder pain with normal radiographs. Physical examination and history consistent with dislocation event or instability.
        • Next Imaging Study: MR arthrography of the shoulder or MRI of the shoulder without IV contrast is usually appropriate
        • CT arthrography of the shoulder or CT shoulder without IV contrast may be appropriate
      • Traumatic shoulder pain with normal radiographs. Physical examination findings consistent with labral tear.
        • Next Imaging Study: MR arthrography of the shoulder, CT arthrography of the shoulder, or MRI of the shoulder without IV contrast is usually appropriate
      • Traumatic shoulder pain with normal radiographs. Physical examination findings consistent with rotator cuff tear
        • Next Imaging Study: MRI of the shoulder without IV contrast, MR arthrography of the shoulder, or ultrasound of the shoulder is usually appropriate
        • CT arthrography of the shoulder may be appropriate
      • Traumatic shoulder pain with radiographs already performed. Physical examination consistent with vascular compromise
        • Next Imaging Study: CTA of the shoulder with IV contrast or arteriography of the shoulder is usually appropriate
      • Traumatic shoulder pain with radiographs already performed. Patient presents with neuropathic syndrome (excluding plexopathy)
        • Next Imaging Study: MRI shoulder without IV contrast is usually appropriate 
        • Bone Scan of the shoulder may be appropriate
    • Chronic Shoulder Pain27
      • No other signs/symptoms
        • Initial Imaging Study: Radiography of the shoulder
        • Ultrasound of the shoulder may be appropriate
      • Chronic shoulder pain. You suspect rotator cuff disorders or subacromial subdeltoid bursitis (no prior surgery). Initial radiographs are normal or inconclusive
        • Next Imaging Study: Ultrasound of the shoulder, MR arthrography of the shoulder, or MRI of the shoulder without IV contrast is usually appropriate
        • Image-guided anesthetic with or without corticosteroid injection of the shoulder or surrounding structures, or CT arthrography of the shoulder may be appropriate
      • Chronic shoulder pain with radiographs that demonstrate calcific tendinopathy or calcific bursitis
        • Next Imaging Study: Image-guided anesthetic with or without corticosteroid injection of the shoulder or surrounding structures is usually appropriate
        • Ultrasound of the shoulder, MR arthrography of the shoulder, MRI of the shoulder without IV contrast may be appropriate
      • Chronic shoulder pain. You suspect labral pathology or shoulder instability. Initial radiographs are normal or inconclusive
        • Next Imaging Study: MR arthrography of the shoulder or MRI shoulder without IV contrast are usually appropriate
        • CT shoulder without IV contrast or CT arthrography of the shoulder may be appropriate
      • Chronic shoulder pain. You suspect adhesive capsulitis. Initial radiographs normal or inconclusive.
        • Next Imaging Study: Image-guided anesthetic with or without corticosteroid injection of the shoulder or surrounding structures or MRI shoulder without IV contrast is usually appropriate
        • MR arthrography of the shoulder may be appropriate
        • Ultrasound of the shoulder may be appropriate, although there is disagreement
      • Chronic shoulder pain. You suspect biceps tendon abnormality. Initial radiographs are normal or inconclusive
        • Next Imaging Study: Ultrasound of the shoulder, image-guided anesthetic with or without corticosteroid injection of the shoulder or surrounding structures, MR arthrography of the shoulder, or MRI shoulder without IV contrast are usually appropriate
        • CT arthrography of the shoulder may be appropriate
      • Chronic shoulder pain. Initial radiographs demonstrate osteoarthritis
        • Next Imaging Study: MRI shoulder without IV contrast is usually appropriate
        • Image-guided anesthetic with or without corticosteroid injection of the shoulder or surrounding structures, MR arthrography of the shoulder, Ct of the shoulder without IV contrast, or CT arthrography of the shoulder may be appropriate
      • Chronic shoulder pain. Patient has a history of prior rotator cuff repair. You suspect rotator cuff disorders or subacromial subdeltoid bursitis. Initial radiographs are normal or inconclusive
        • Next Imaging Study: Ultrasound of the shoulder, MR arthrography of the shoulder, MRI shoulder without IV contrast, or CT arthrography of the shoulder are usually appropriate
        • Image-guided anesthetic with or without corticosteroid injection of the shoulder or surrounding structures may be appropriate
    • Imaging after Shoulder Arthroplasty28
      • Routine follow-up of the asymptomatic patient with a primary shoulder arthroplasty
        • Initial Imaging Study: Radiography of the shoulder
      • Symptomatic patient with a primary shoulder arthroplasty
        • Initial Imaging Study: Radiography of the shoulder
      • Symptomatic patient with a primary shoulder arthroplasty, infection not excluded
        • Next Imaging Study after radiographs: Imaging-guided aspiration of the shoulder is usually appropriate
        • Ultrasound of the shoulder, MRI of the shoulder without and with IV contrast, 3-phase bone scan with WBC scan and sulfur colloid scan of the shoulder, with or without SPECT or SPECT/CT of the shoulder may be appropriate
        • MRI of the shoulder without IV contrast or WBC scan and sulfur colloid scan of the shoulder alone may be appropriate, although there is disagreement
      • Symptomatic patient with a primary shoulder arthroplasty, infection excluded. You suspect loosening of hardware
        • Next Imaging Study after radiographs: MRI shoulder without IV contrast or CT shoulder without IV contrast are usually appropriate
        • 3-phase bone scan with SEPCT or SPECT/CT of the shoulder may be appropriate
        • Ultrasound of the shoulder may be appropriate, although there is disagreement
      • Symptomatic patient with a primary shoulder arthroplasty, infection excluded. Suspected rotator cuff tear or other soft-tissue abnormality.
        • Next Imaging Study after radiographs: Ultrasound of the shoulder, MRI of the shoulder without IV contrast, or CT arthrography of the shoulder is usually appropriate
  • Plexopathy29
    • MRI is the mainstay for any brachial or lumbosacral plexus injury
    • Potential mass → MRI with and without contrast helps determine if the mass is intrinsic or extrinsic to the plexus 
    • Cancer patient → MRI with and without IV contrast helps determine tumor involvement/recurrence vs radiation injury 
    • Local trauma → MRI to determine if injury is preganglionic (intraforaminal) vs postganglionic (plexus) 
    • Contraindication to MRI → CT8 
    • Brachial plexopathy, acute or chronic, nontraumatic. No known malignancy.
      • Initial Imaging Study: MRI brachial plexus without IV contrast, or MRI brachial plexus with AND without IV contrast is usually appropriate
      • MRI cervical spine with and without IV contrast, MRI cervical spine without IV contrast alone, or CT neck with IV contrast may be appropriate
    • Brachial plexopathy, traumatic (not perinatal)
      • Initial Imaging Study: MRI brachial plexus without IV contrast, or MRI brachial plexus with AND without IV contrast is usually appropriate
      • MRI cervical spine with and without IV contrast, MRI cervical spine without IV contrast alone, or CT cervical spine without IV contrast, or CT myelography of the cervical spine may be appropriate
    • Brachial plexopathy, known malignancy or post-treatment syndrome
      • Initial Imaging Study: MRI brachial plexus without IV contrast, or MRI brachial plexus with AND without IV contrast is usually appropriate
      • The following may be appropriate: MRI cervical spine with and without IV contrast, MRI cervical spine without IV contrast, CT neck with or without IV contrast, FDG-PET/CT whole body
    • Lumbosacral plexopathy, acute or chronic, nontraumatic. No known malignancy
      • Initial Imaging Study: MRI lumbosacral plexus with and without IV contrast, or MRI lumbosacral plexus without IV contrast is usually appropriate
      • MRI lumbar spine with and without IV contrast, MRI without IV contrast alone, or CT abdomen and pelvis with IV contrast may be appropriate
    • Lumbosacral plexopathy, traumatic
      • Initial Imaging Study: MRI lumbosacral plexus with and without IV contrast, or MRI lumbosacral plexus without IV contrast is usually appropriate
      • The following may be appropriate: MRI lumbar spine without and with IV contrast, MRI lumbar spine without IV contrast alone, MRI pelvis with and without IV contrast, MRI pelvis without IV contrast alone, CT abdomen and pelvis with or without IV contrast, CT lumbar spine without IV contrast, or CT myelography of the lumbar spine
    • Lumbosacral plexopathy, known malignancy or post-treatment syndrome
      • Initial Imaging Study: MRI lumbosacral plexus with and without IV contrast, or MRI lumbosacral plexus without IV contrast is usually appropriate
      • The following may be appropriate: MRI lumbar spine with and without IV contrast, MRI lumbar spine without IV contrast alone, MRI pelvis with and without IV contrast, MRI pelvis without IV contrast alone, CT abdomen and pelvis with IV contrast
    • Recurrent low back pain and/or surgical intervention → Radiography spine, MRI lumbar spine with and without contrast
      • Spinal hardware (e.g., fusion) may limit visualization of structures
  • Lumbar Spine30
    • Acute low back pain with or without radiculopathy. No red flags
      • No imaging is usually appropriate
    • Subacute or chronic low back pain with or without radiculopathy. No red flags or prior management
      • No imaging is usually appropriate
    • Subacute or chronic low back pain with or without radiculopathy. Patient is a surgery or intervention candidate with persistent or progressive symptoms during or following 6 weeks of optimal medical management:
      • Initial Imaging Study: MRI lumbar spine without IV contrast
      • Radiography of the lumbar spine, MRI lumbar spine with AND without IV contrast, Bone scan of the whole body with SPECT or SPECT/CT complete spine, CT lumbar spine without IV contrast, or CT myelography of the lumbar may be appropriate
    • Low back pain with suspected cauda equina syndrome
      • Initial Imaging Study: MRI lumbar spine with and without IV contrast, or MRI lumbar spine without IV contrast alone are usually appropriate
      • CT lumbar spine without IV contrast or CT myelography of the lumbar spine may be appropriate
    • Low back pain with history of prior lumbar surgery and with or without radiculopathy. Patient shows new or progressing symptoms or clinical findings
      • Initial Imaging Study: Radiography of the lumbar spine, MRI lumbar spine without and with IV contrast, or MRI lumbar spine without IV contrast alone are usually appropriate
      • CT lumbar spine without IV contrast, or CT myelography of the lumbar spine may be appropriate
    • Low back pain with or without radiculopathy. The patient displays one of the following: low-velocity trauma, osteoporosis, elderly individual, or chronic steroid use:
      • Initial Imaging Study: Radiography of the lumbar spine, MRI of the lumbar spine without IV contrast, or CT lumbar spine without IV contrast are usually appropriate
      • MRI lumbar spine with AND without IV contrast, or Ct myelogram of the lumbar spine may be appropriate
    • Low back pain with or without radiculopathy. The patient displays one or more of the following: suspicion of cancer, infection, or immunosuppression
      • Initial Imaging Study: MRI lumbar spine with AND without IV contrast, or MRI lumbar spine without IV contrast alone are usually appropriate
      • CT lumbar spine with OR without IV contrast, or CT myelography of lumbar spine may be appropriate
      • Radiography of the lumbar spine may be appropriate, although there is disagreement among experts
  • Hip
    • Acute Hip Pain31
      • Acute hip pain after fall or minor trauma, with fracture suspected.
        • Initial Imaging Study: Radiography of the hip and/or pelvis are usually appropriate
      • Acute hip pain with fall or minor trauma with fracture suspected but negative radiographs
        • Next Imaging Study: MRI pelvis and affected hip without IV contrast, or CT pelvis and hips without IV contrast are usually appropriate
    • Chronic Hip Pain32
      • Initial Imaging Study: Radiographs of the pelvis and hip
      • Chronic hip pain with suspected noninfectious extra-articular abnormality, such as tendonitis or bursitis. Radiographs are negative or non-diagnostic
        • Next Imaging Study: Ultrasound of the hip or MRI of the hip without IV contrast are usually appropriate
        • Image-guided anesthetic with or without corticosteroid injection of the hip joint or surrounding structures may be appropriate
      • Chronic hip pain with suspected impingement or dysplasia with negative or non-diagnostic radiographs
        • Next Imaging Study: MR arthrography of the hip or MRI hip without IV contrast are usually appropriate
        • Radiography of the hip with additional views, CT arthrography of the hip, CT hip without IV contrast, or image-guided anesthetic with or without corticosteroid injection of the hip joint or surrounding structures may be appropriate
      • Chronic hip pain with suspected labral tear and negative/nondiagnostic radiographs
        • Next Imaging Study: MR arthrography of the hip, or MRI of the hip without IV contrast are usually appropriate
        • CT arthrography of the hip, or image-guided anesthetic with or without corticosteroid injection of the hip joint or surrounding structures may be appropriate
      • Chronic hip pain with equivocal radiographs or radiographs positive for mild osteoarthritis.
        • Next Imaging Study to evaluate for articular cartilage integrity: MR arthrography of the hip, or MRI of the hip without IV contrast are usually appropriate
        • CT arthrography of the hip may be appropriate
      • Chronic hip pain with radiographs suspicious for intra-articular synovial hyperplasia or neoplasia (including nodular synovitis, diffuse tenosynovial giant cell tumor, osteochondromatosis, other synovial neoplasm)
        • Next Imaging Study: MRI hip with and without IV contrast, or MRI hip without IV contrast alone are usually appropriate
        • CT arthrography of the hip may be appropriate
      • Chronic hip pain with low back or knee pathology or pain. Radiographs show hip osteoarthritis
        • Next Imaging Study to quantify amount of pain related to the hip: Image-guided anesthetic with or without corticosteroid injection of the hip joint or surrounding structures are usually appropriate
    • Imaging after total hip arthroplasty33
      • Routine follow-up of asymptomatic patient after hip arthroplasty
        • Initial Imaging Study: Hip radiography is usually appropriate
      • Symptomatic patient with hip prosthesis
        • Initial Imaging Study: Hip radiography is usually appropriate
      • Symptomatic hip arthroplasty patient with history of acute injury
        • Next Imaging Study:  CT hip without IV contrast is usually appropriate
        • MRI hip without IV contrast may be appropriate
      • Symptomatic hip arthroplasty patient, infection not excluded.
        • Next Imaging Study:  Image-guided aspiration of the hip, or MRI hip without IV contrast, or WBC scan and sulfur colloid scan of the hip are usually appropriate
        • MRI hip with AND without IV contrast, Ultrasound of the hip, CT hip with OR without IV contrast may be appropriate
      • Symptomatic hip arthroplasty patient with infection excluded
        • Next Imaging Study: MRI of the hip without IV contrast or CT hip without IV contrast are usually appropriate
        • Image-guided anesthetic injection of the hip, or bone scan with SPECT or SPECT/CT hip may be appropriate
      • Evaluation of symptomatic hip arthroplasty patient with metal-on-metal prosthesis or findings suggestion trunnionosis.
        • Next Imaging Study if there is a question of adverse reaction to the metal debris: MRI hip without IV contrast is usually appropriate
        • Ultrasound of the hip or CT of the hip without IV contrast may be appropriate
      • Hip arthroplasty patient with trochanteric pain
        • Next Imaging Study if you suspect abductor injury, trochanteric bursitis, or other soft tissue abnormalities: Ultrasound of the hip, or MRI of the hip without IV contrast
        • Image-guided anesthetic with or without corticosteroid injection of the hip joint or surrounding structures may be appropriate
  • Knee
    • Acute knee pain34
      • Adult or child 5 years of age or older after a fall or acute twisting trauma to the knee. Patient has no focal tenderness, no effusion, and can walk
        • Initial Imaging Study: Radiography of the knee may be appropriate
      • Adult or child 5 years of age or older after a fall or acute twisting trauma to the knee with one or more of the following: focal tenderness, effusion, inability to bear weight
        • Initial Imaging Study: Radiography of the knee is usually appropriate
      • Adult or skeletally mature child after a fall or acute twisting trauma to the knee. No fracture seen on radiographs.
        • Next Imaging Study if you suspect occult fracture or internal derangement: MRI of the knee without IV contrast is usually appropriate
        • CT of the knee without IV contrast may be appropriate
      • Skeletally immature child after a fall or acute twisting trauma to the knee. No fracture seen on radiographs.
        • Next Imaging Study if you suspect occult fracture or internal derangement: MRI of the knee without IV contrast
        • CT of the knee without IV contrast may be appropriate
      • Adult or child 5 years of age or older after a fall or acute twisting trauma to the knee. Radiographs show tibial plateau fracture
        • Next Imaging Study if you suspect additional bone or soft-tissue injury: MRI of the knee without IV contrast or CT of the knee without IV contrast
      • Adult or child 5 years of age or older after cute trauma to the knee due to unknown mechanism. Patient presents with focal patellar tenderness, effusion, able to walk
        • Initial Imaging Study: Radiograph of the knee
      • Adult or child 5 years of age or older after significant trauma to the knee (e.g., motor vehicle accident, knee dislocation)
        • Initial Imaging Study: Radiograph of the knee or CTA lower extremity with IV contrast are usually appropriate
        • Arteriography of the lower extremity, CT of the knee with OR without IV contrast, MRA of the knee with AND without IV contrast, MRA knee without IV contrast only, or MRI knee without IV contrast only may be appropriate
    • Chronic knee pain35
      • Adult or child greater than or equal to 5 years of age with chronic knee pain.
        • Radiography of the knee is usually appropriate
      • Adult or child greater than or equal to 5 years of age with chronic knee pain. Initial knee radiograph is negative or demonstrates joint effusion
        • Next Imaging Study: MRI knee without IV contrast is usually appropriate
      • Adult or child greater than or equal to 5 years of age with chronic knee pain. Initial knee radiograph shows osteochondritis dissecans (OCD), loose bodies, or history of cartilage or meniscal repair.
        • Next Imaging Study: MRI knee without IV contrast is usually appropriate
        • CT arthrography of the knee or CT knee without IV contrast may be appropriate
        • MR arthrography of the knee may be appropriate, although there is disagreement
      • Adult or child greater than or equal to 5 years of age with chronic knee pain. Initial knee radiograph shows degenerative changes or chondrocalcinosis
        • Next Imaging Study: MRI knee without IV contrast or CT knee without IV contrast may be appropriate
        • Image-guided aspiration of the knee may be appropriate, although there is disagreement
      • Adult or child greater than or equal to 5 years of age with chronic knee pain. Initial knee radiograph demonstrates signs of prior osseous injury (i.e., Segond fracture, tibial spine avulsion, etc.)
        • MRI k nee without IV contrast is usually appropriate
        • CT arthrography of the knee or CT knee without IV contrast may be appropriate
    • Imaging after Total Knee Arthroplasty36
      • Follow-up of symptomatic or asymptomatic patients with a total knee arthroplasty
        • Initial Imaging Study: Radiography of the knee
      • Suspected infection after total knee arthroplasty.
        • Next Imaging Study following radiographs: image-guided knee aspiration
        • Ultrasound of the knee, MRI knee with and without IV contrast, or MRI knee without IV contrast alone, CT knee with IV contrast, 3-phase bone scan of the knee, or WBC scan and sulfur colloid scan of the knee may be appropriate
      • Pain after total knee arthroplasty. Infection excluded. Suspicion of aseptic loosening or osteolysis or instability
        • Next Imaging Study following radiographs: MRI knee without IV contrast or CT knee without IV contrast are usually appropriate
        • 3-phase bone scan of the knee may be appropriate
      • Pain after total knee arthroplasty. Suspicion of periprosthetic or hardware fracture
        • CT of the knee without IV contrast is usually appropriate
        • MRI of the knee without IV contrast or 3-phase bone scan of the knee may be appropriate
      • Pain after total knee arthroplasty. To measure component rotation
        • Next Imaging Study following radiographs: CT knee without IV contrast
        • MRI of the knee without IV contrast may be appropriate
      • Pain after total knee arthroplasty. If suspecting periprosthetic soft-tissue abnormality unrelated to infection, including quadriceps or patellar tendinopathy (quadriceps or patellar tendon tears, postoperative arthrofibrosis, patellar clunk syndrome, or impingement of nerves or other soft tissues)
        • Next Imaging Study following radiographs: Knee ultrasound or MRI of the knee without IV contrast are usually appropriate
  • Cerebrovascular
    • ACR appropriateness criteria for general cerebrovascular disease can be found at the following URL: https://acsearch.acr.org/docs/69478/Narrative
    • Available criteria variants include:
      • “Asymptomatic. Structural lesion on physical examination (cervical bruit) and/or risk factors.”
      • “Carotid territory or vertebrobasilar TIA, initial screening survey.”
      • “New focal neurologic defect, fixed or worsening. Less than 6 hours. Suspected stroke.”
      • “New focal neurologic defect, fixed or worsening. Longer than 6 hours. Suspected stroke.”
      • “Proven parenchymal hemorrhage (hematoma).”
      • “Suspected dural venous sinus thrombosis.”
  • ACR appropriateness criteria for Cerebrovascular Diseases-Aneurysm, Vascular Malformation, and Subarachnoid Hemorrhage can be found at the following URL: https://acsearch.acr.org/docs/3149013/Narrative/
    • Available criteria variants include:
      • “Known acute subarachnoid hemorrhage (SAH) on CT. Next imaging study.”
      • “Suspected cerebral vasospasm. Initial imaging.”
      • “Known cerebral aneurysm; untreated. Surveillance monitoring.”
      • “Known cerebral aneurysm; previously treated. Surveillance monitoring.”
      • “High-risk cerebral aneurysm screening.”
      • “Known high-flow vascular malformation (AVM/AVF). Surveillance monitoring.”
      • “Suspected central nervous system (CNS) vasculitis. Initial imaging.”

* Red flags symptoms include history of cancer, unexplained weight loss, immunosuppression, urinary tract infection, intravenous drug use, prolonged corticosteroid use, low back pain not improved with conservative treatment, infection.

** Knee dislocations make up 0.1% of orthopedic injuries but is considered a true medical emergency. 30% involve vascular/neurologic injury. Untreated vascular injury can result in limb amputation.34

Cutting Edge/Emerging and Unique Concepts and Practice

Imaging modalities were initially invented to evaluate and gather anatomical information. More recently, imaging has been able to evaluate anatomy while gathering functional, metabolic, and/or biochemical data concomitantly.37

Magnetic Resonance Fingerprinting

Magnetic resonance fingerprinting (MRF) differs from conventional MRI as it can provide images from several quantitative tissue relationships simultaneously.37 MRF studies T1/T2 relaxometry parameters which are relaxation times of tissue. Possible musculoskeletal applications involve T1 mapping of bone marrow which can show early changes in tissue properties indicative of neoplastic disease.

PET/MRI

Using PET scanning in conjunction with MRI allows for anatomical evaluation as well as early metabolic changes in tissue at a molecular level. One musculoskeletal imaging advantage that has been demonstrated in preliminary research is the ability to detect subchondral bone change before structural changes are evident on traditional MRI. PET/MRI is also helpful in evaluating for spondylodiscitis which can increase sensitivity from 50% to 100% and specificity from 71% to 80% when compared to traditional MRI.38

Functional MRI

Functional MRI (fMRI) is obtained by computation of blood-oxygen-level-dependent (BOLD) contrast. When brain tissue is in a metabolically active state, the tissue demands more glucose and oxygen than when it is in a resting state. The blood flow increases to the active site to match the glucose and oxygen demand. However, since the supply always exceeds the demand, the oxygen level of blood at the active site is much higher than less active sites. This difference is computed to produce images. The activity level of the brain is differentiated by the blood oxygen level. This type of study requires a longer imaging time than a traditional MRI. One tremendous advantage of fMRI is its ability to evaluate and localize brain function in real-time. However, its usefulness is limited in the case of tumors due to their well-vascularized nature.

fMRI is further classified into resting-state functional MRI (RS-fMRI) and task-based functional MRI (TS-fMRI). The former provides clinical information by evaluating low frequency fluctuations in BOLD signal while a subject is at rest. It is particularly beneficial when the subject is unable to cooperate with tasks. The latter is scanned while the subject performs a task making it quite useful for target-oriented real-time evaluation. fMRI is useful to monitor brain functional reorganization of motor and visuomotor network functions after stroke or traumatic brain injury.39-42 A negative correlation between outcome and the degree of TS-fMRI was seen in parts of both contralateral and ipsilateral primary motor cortex of stroke patients.43 Clinical application of fMRI has also expanded for the evaluation of neuropsychology and mental health disorders such as schizophrenia, attention deficit hyperactivity, and autism.44 Recently, it has also been used in sports medicine for skeletal muscle training.45

Gaps in Knowledge/Evidence Base

The first MRI-conditional pacemaker received FDA approval for use in the United States in February 2011. This first-generation device has important limitations (compatible with 1.5 Tesla MRI only). MRI can be safely performed for patients with implanted cardiac devices; however, a radiologist’s consultation is strongly recommended prior to ordering an MRI46.

SynchroMed™ II (Medtronic), a drug infusion pump, is well known to physiatrists for treatment of spasticity. This model has a conditional 1.5 Tesla and 3 Tesla compatibility for full body scans in a horizontal closed bore system.47 Its performance has not yet been established in an open-sided or standing MRI. It is recommended to review the pump model’s MRI-compatibility and to discuss pre- and post-MRI planning with the radiology department. The pump will temporarily stop an infusion while in the magnetic field of the MRI scanner but automatically return to prior function after the scan is completed. However, interrogation by a clinician is recommended to confirm the pump is functioning appropriately after the MRI scan.48

The following items may cause a health hazard or other complications during an MRI exam48

  • Certain cardiac pacemakers or implantable cardioverter defibrillators (ICDs)*
  • Ferromagnetic metallic vascular clips for intracranial aneurysms
  • Some implanted or external medication pumps*
  • Certain cochlear implants
  • Certain neurostimulation systems*
  • Catheters that have metallic components
  • A metallic foreign body within or near the eye
  • A bullet, shrapnel or other type of metallic fragment

* Some items, including certain cardiac pacemakers, neurostimulation systems, and medication pumps are acceptable for MRI. However, the MRI technologist and radiologist must know the patient’s specific item type in order to follow special procedures to ensure the patient’s safety.

Objects that may interfere with image quality if close to the area being scanned include48

  • Metallic spinal rod
  • Plates, pins, screws, or metal mesh used to repair a bone or joint
  • Joint replacement or prosthesis
  • Metallic jewelry including those used for body piercing or body modification
  • Some tattoos or tattooed eyeliner (these alter MR images, and there is a chance of skin irrigation or swelling; black and blue pigments are the most troublesome)
  • Makeup, nail polish, or other cosmetics that contain metal
  • Dental fillings (while usually unaffected by the magnetic field, these may distort images of the facial area or brain; the same is true for orthodontic braces and retainers)

Overall, the choice of an imaging modality and interpretation of its results should be based on the patient’s history, physical exam, lab values, and pertinent evidence-based algorithms. However, there are some clinical scenarios where there is insufficient data to provide criteria to guide imaging selection. In these instances, clinicians may consult the radiology department and use clinical judgment to select the most appropriate imaging modality.

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Original Version of the Topic

Chong Tae Kim, MD. MRI and CT Scanning. 9/15/2019

Previous Revision(s) of the Topic

Jared Aida, DO, Kevin Cipriano, MD. MRI and CT Scanning. 8/29/2020

Author Disclosure

Erika Trovato, DO
Nothing to Disclose

Hameer Thatte, DO, MPH, MS
Nothing to Disclose

Eli Dayon, DO
Nothing to Disclose

Ashley Covatto, DO
Nothing to Disclose