MRI and CT Scanning

Author(s): Chong Tae Kim, MD

Originally published:09/15/2019

Last updated:09/15/2019

1. Overview and Description:

CT (computerized axial tomography) and MRI (magnetic resonance imaging) are common  noninvasive diagnostic tools used for imaging of the physiologic processes of the body in both health and disease.  Physiatrists utilize CT and MRI imaging to help guide diagnosis and treatment plan. It is often challenging for non-radiologist to choose which modality is appropriate. One study at an academic primary care clinic showed about 26% of imaging study orders were inappropriately chosen. The most common imaging study orders were head CT for chronic headache, lumbar spine MRI for acute back pain, and knee or shoulder MRI for osteoarthritis 1. The choice of CT or MRI is dependent on multiple factors, such as diagnostic impression, anatomic location of suspected abnormality, safety (radiation), cost, or level of urgency. Appropriate selection of imaging studies is very important for quality of patient care as well as safety. General guidelines for choosing the appropriate imaging study is available at American College of Radiology Appropriateness Criteria (ACR AC; http://www.acr.org/Clinical-Resources/ACR-Appropriateness-Criteria) 2 (Table 1).  This section is written to help physiatrists better understand CT and MRI studies.

Table 1. Appropriateness Criteria for low back pain.

Clinical features Procedures Appropriateness Category
Acute subacute, or chronic uncomplicated low back pain or radiculopathy. No red flags. No prior management Lumbar spine X-ray Usually not appropriate
Lumbar spine CT Usually not appropriate
Lumbar spine MRI Usually not appropriate
Lumbar spine CT myelography Usually not appropriate
Acute subacute, or chronic uncomplicated low back pain or radiculopathy. One or more of the following: low velocity trauma, osteoporosis, elderly individual, or chronic steroid use. Lumbar spine X-ray Usually appropriate
Lumbar spine CT Usually appropriate
Lumbar spine MRI Usually appropriate
Lumbar spine CT myelography Usually not appropriate
Acute subacute, or chronic uncomplicated low back pain or radiculopathy. Surgery or intervention candidate with persistent or progressive symptoms during or following 6 weeks of conservative treatment. Lumbar spine X-ray May be appropriate
Lumbar spine CT May be appropriate
Lumbar spine MRI Usually appropriate
Lumbar spine CT myelography May be appropriate

2. Relevance to Clinical Practice:

CT vs MRI

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

MRI imaging is a more complex process than CT. The technique includes alignments of hydrogen protons in human tissue containing water molecules which are spun in a magnetic field and computed to create a signal that is processed to form an image of the body.

Table 1. Comparison CT and MRI

CT MRI
Images made X-ray emission Magnet and radio waves
 

Radiation exposure

 

Higher than plain X-ray

 

None

 

Time taken for images

 

Short (minutes)

 

Long (minutes-hours)

 

Comfort level

 

Comfortable

 

Claustrophobia; noisy

 

Limited application

 

Extremely large body size

 

Patients with cardiac pacemakers, implanted electronic implants, and metal implants are contraindicated due to possible injury to patient or image distortion (artifact);

Cost  Cheaper than MRI More expensive than CT
 

Clinical Application

 

Bone injuries, lung and chest imaging, cancer detection; acute intracranial hemorrhage

 

Soft tissue evaluation, e.g., ligaments, tendons, muscles, brain and spinal cord

Intravenous contrast precaution Allergic reaction is rare, but more common than MRI contrast; high risk of contrast induced nephropathy in renal insufficiency Allergic reaction is very rare, but high risk in kidney or liver disorders
 

Advantages

 

Good imaging for bone structures

Available for implanted metals or pacemakers

The time taken for total testing is shorter than taken by MRI

 

Better imaging for soft tissues

Different contrast settings will highlight different types of tissue

Able to change the imaging plane without moving the patient

T1 weight (T1) vs T2 weight (T2) vs Fluid Attenuated Inversion Recovery (FLAIR) MRI

The difference between T1 and T2 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 positon, 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 2). FLAIR images are obtained similar to T2 images, but take much longer to process. This process results in lesion sites remaining bright, with normal CSF fluid appearing attenuated (dark). FLAIR images are very sensitive to pathology, which is useful for highlighting differentiation between CSF and abnormality (see Table 2).

Table 2. Tissues’ Images Dependent on Different MRI technique

Tissue T1 T2 Flair
CSF Dark Bright Dark
White matter Light Dark gray Dark gray
Cortex Gray Light gray Light gray
Fat Bright Light Light
Inflammation/infection Dark Bright Bright

3. Cutting edge/emerging and unique concepts and practice

Diffusion-weighted MRI (DWI)

DWI generates images from the differences in micromobility of water molecules. Water molecules in tissues move in all directions freely (isotrophic diffusion) or restrictively (anisotrophic diffusion). For example, when an ischemic lesion develops in the brain, the microbility of the water molecules in the lesion is restricted (becoming anisotrohic) compared to healthy neighboring brain tissue. This difference in a water molecule’s micromobility enhances MR images in acute ischemic areas, which appear bright regions (abnormal) on DWI. DWI has a significant advantage in the early identification of ischemic stroke (even before T1 or T2 detects the lesion)3, 4, differentiation of acute from chronic strokes, assessment of active demyelination, and extent of diffuse axonal injury. However, when the lesions become chronic, MRI sensitivity decreases 5. In addition, DWI is known to be a useful neuroimaging tool to aid in the diagnosis of severity, functional progress, and prognosis following traumatic brain injury 6 – 9. It is also valuable in the evaluation of other brain lesions (tumor, infection, and demyelination) as well as of other solid tumors 10-12.

Diffusion tensor imaging (DTI)

DTI is a kind of DWI and is specifically utilized to investigate the white matter tract of the central nervous system (so called tactography).  Water molecule micromobility is faster in the normal tract (such as in normal nerve conduction) than in diseased tract (such as in slow nerve conduction). The difference in the speed of water micromobility (fractional anisotropy) is computed and transformed into a different tract image. DTI is applied to evaluate long tracts in the nervous system: 1) intracerebral language pathway abnormalities in schizophrenic patients 13; 2) pyramidal track abnormality in patients with amyotrophic lateral sclerosis 14; 3) extrapyramidal tract changes in stroke recovery 15;and 4) peripheral neuropathy 16. It is further reported to prove the mechanism of motor function recovery after rehabilitation in patients with stroke 15, 17 and cerebral palsy 18.

Functional MRI (fMRI)

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 the 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 regular MRI. It has advantages in real time mapping of brain areas with corresponding functions. Its usefulness decreases in cases of tumors or lesions, because it can change blood flow by altering blood supply, not by brain tissue activity.

fMRI is further classified into resting-state functional MIR (RS-fMRI) and task-based functional MRI (TS-fMRI). The former provides clinical information by evaluation of low frequency fluctuations in BOLD signal while the 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, so it is quite useful for target-oriented real time evaluation. fMRI is useful to monitor brain functional reorganization of motor and visuomotor network function after stroke and traumatic brain injury 19-22. 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 23. Clinical application of fMRI has also expanded for the evaluation of neuropsychology and mental health disorders such as schizophrenia, attention deficit hyperactivity, and autism. 24. Recently, it has also been used in sports medicine for skeletal muscle training 25, 26.

4. 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 performed safely for patients with cardiac implanted devices; however, a radiologist’s consultation is strongly recommended prior to ordering an MRI for patients with cardiac device implantation 19.

SynchroMed II ®(Medtronic), a drug infusion pump, is well known to physiatrists for treatment of spasticity. This model is has a conditional 1.5 tesla and 3-tesla compatibility for full body scans in a horizontal closed bore system 20. Its performance has not yet been established in an open-sided or standing MRI. It is recommended to review the pump model and to discuss with the radiologist about pre and post-MRI. The pump will temporarily stop an infusion 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 study 21.

The following items may cause a health hazard or other complications during an MRI 30:

  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 cochlea 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 exact 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 30:

  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)

References

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  2. American College of Radiology Appropriateness Criteria (ACR AC; http://www.acr.org/Clinical-Resources/ACR-Appropriateness-Criteria).
  3. Lansberg MG, Albers GW, Beaulieu C, et al. Comparison of diffusion weighted MRI and CT in acute stroke. Neurol 2000; 54:557-1561.
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Author Disclosure

Chong Tae Kim, MD
Nothing to Disclose

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