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Overview and Description:

CT (computerized tomography) and MRI (magnetic resonance imaging) are noninvasive, advanced imaging modalities used to evaluate a certain area of the body. The information from these tests often helps to establish a diagnosis and guide treatment. Given their clinical relevance and potential cost, it is important to understand when to order each imaging modality. One study at an academic primary care clinic showed only 74% of imaging studies were appropriately ordered. The most common studies were head CT for chronic headache, lumbar spine MRI for acute back pain, and knee or shoulder MRI for osteoarthritis 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.

Table 1. ACR Appropriateness Criteria for Low Back Pain

*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 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 of CT and MRI.

Table 2. Comparison CT and MRI

T1 vs T2 Imaging

The difference between T1- and T2-weighted images is derived from the net magnetic vectors of different tissues. When human tissue is exposed to a magnetic field, hydrogen protons spin and then return to their original position which produce vectors. The spinning directions and returning time are different from tissue to tissue depending on the components of hydrogen protons.  The difference of T1 and T2 images of the same tissue depends on the time it takes for protons to realign in the tissues. It takes a longer time for T2 images to develop than T1 because the magnetism needs to decay. Each weighted image has its 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.

Table 3. Signal Intensity of Tissues on MRI Sequences

Specific Imaging Considerations for MSK Pathology

  1. Cervical Spine
    1. Neck pain without red flag symptoms* → Radiographs usually appropriate for initial evaluation
      1. There is insufficient literature to conclude appropriateness of MRI in this patient population. Experts did not agree on this modality in this population, but it may be appropriate
    2. Cervical Radiculopathy → MRI
      1. Use of radiographs were controversial in this patient population
    3. Neck pain and/or radiculopathy with prior cervical spine surgery → Radiographs or CT
      1. Use of MRI with or without contrast was controversial in this patient population 5
  2. Shoulder
    1. Traumatic
      1. Radiographs for initial evaluation
      2. Non-localized pain and negative X-Ray →  MRI, consider US
        1. Fx of humeral head/neck or scapula →  CT to characterize fracture planes (especially if displaced)
        2. Bankart or Hill-Sachs lesion →  MRI or MR arthrogram
        3. Hx of dislocation and/or instability on exam → MRI or MR arthrogram
      3. Exam suggestive of labral tear → MR arthrogram, MRI, or CT arthrogram appropriate
      4. Exam suggestive of rotator cuff tear → MR arthrogram, MRI, or US appropriate
      5. Possible vascular compromise → CTA with IV contrast and arteriography 6
    2. Atraumatic
      1. Adhesive capsulitis suspected → MRI, consider US
      2. Biceps tendon pathology suspected → MRI or US
      3. Shoulder pain s/p rotator cuff repair → MR arthrogram, MRI, US
      4. Neurogenic shoulder pain → MRI or US 7
  3. Plexopathy
    1. MRI is the mainstay for any brachial or lumbosacral plexus injury
    2. Potential mass → MRI with and without contrast helps determine if the mass is intrinsic or extrinsic to the plexus
    3. Cancer patient → MRI with and without IV contrast helps determine tumor involvement/recurrence vs radiation injury
    4. Local trauma → MRI to determine if injury is preganglionic (intraforaminal) vs postganglionic (plexus)
    5. Contraindication to MRI → CT 8
  4. Lumbar Spine
    1. Uncomplicated low back pain with or without radiculopathy → No imaging
      1. Treat conservatively with physical therapy and/or medication
    2. Low back pain with red flag symptoms* → MRI to evaluate for cauda equina syndrome, infection, malignancy
    3. Progressive symptoms during therapy or persistent symptoms after 6 weeks of conservative treatment → MRI
      1. MRI ordered to guide an intervention or surgery
    4. Recurrent low back pain and/or surgical intervention → MRI 9
      1. Spinal hardware (e.g. fusion) may limit visualization of structures
  5. Hip
    1. Radiographs for initial evaluation
    2. Chronic hip pain with suspected extra-articular abnormality (e.g. tendonitis) → MRI or US
    3. Suspected femoroacetabular impingement → MRI, MR arthrogram, or CT arthrogram
    4. Evaluation of articular cartilage → MRI, MR arthrogram, or CT arthrogram
    5. Suspected labral tear → MR arthrogram or CT arthrogram 10
  6. Knee
    1. Traumatic
      1. Initial presentation → use Ottawa knee rules to determine if radiographs necessary
      2. Suspected ligamentous, meniscal, or chondral lesion → MRI
      3. Tibial plateau fracture on X-ray → CT or MRI for complete injury assessment and/or surgical planning
      4. Significant trauma with suspected dislocation → MR angiography to concomitantly evaluate for potential vascular and ligamentous injury**
        1. MR angiography has been shown to be 100% as sensitive and specific as traditional arteriography 11
    2. Atraumatic
      1. Radiographs for initial evaluation
      2. Chronic knee pain with negative radiographs plus joint effusion, loose bodies, or suspicion of osteochondral injury → MRI
      3. Prior osseous injury (e.g. segond fracture, tibial spine avulsion) on radiograph → MRI 12

* 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 a true medical emergency. 30% involve vascular/neurologic injury. Untreated vascular injury can result in limb amputation 11.

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 13.

Magnetic Resonance Fingerprinting

Magnetic resonance fingerprinting (MRF) differs from conventional MRI as it can provide images from several quantitative tissue relationships simultaneously 13. 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.


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 14.

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 15-18. 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 19. Clinical application of fMRI has also expanded for the evaluation of neuropsychology and mental health disorders such as schizophrenia, attention deficit hyperactivity, and autism 20. Recently, it has also been used in sports medicine for skeletal muscle training 21, 22.

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 MRI 15.

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 16. 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 17.

The following items may cause a health hazard or other complications during an MRI exam 24:

  1. Certain cardiac pacemakers or implantable cardioverter defibrillators (ICDs)*
  2. Ferromagnetic metallic vascular clips for intracranial aneurysms
  3. Some implanted or external medication pumps*
  4. Certain cochlear implants
  5. Certain neurostimulation systems*
  6. Catheters that have metallic components
  7. A metallic foreign body within or near the eye
  8. 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 specific type that the patient has 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 include 24:

  1. Metallic spinal rod
  2. Plates, pins, screws, or metal mesh used to repair a bone or joint
  3. Joint replacement or prosthesis
  4. Metallic jewelry including those used for body piercing or body modification
  5. 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)
  6. Makeup, nail polish, or other cosmetics that contain metal
  7. 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

Author Disclosure

Jared Aida, DO
Nothing to Disclose

Kevin Cipriano, MD
Nothing to Disclose