MRI's Role in Emergency Diagnoses Examined

As technologies evolve and images become easier to obtain, the use of MRI in the ED is increasing.

This testing modality is often thought to have been underutilized due to its limited availability.

Recent data suggest that the US has the highest utilization of MRI in the world and the second highest number of MRI scanners per capita, after Japan.

With the expected continued improvement of MRI technology, it is essential to understand when its use within the ED is appropriate.

> Stroke/transient ischemic attack

When choosing an imaging modality for suspected transient ischemic attack or ischemic stroke (CIA), the old adage “time is brain” should determine which imaging modality is selected.

The mean sensitivity of MRI to detect acute ischemic injury is 93% while the sensitivity of computed tomography (CT) within 12 hours of onset is 73-87%. Non-contrast head CT has a sensitivity of 12% for stroke of less than 3 hours duration.

CT angiography has a sensitivity of 83.2% and a specificity of 95% in the evaluation of acute stroke. Regarding acute stroke, CT perfusion has a sensitivity of 82% and no significant difference between patients who were thus studied within 6 hours of the onset of symptoms compared to those who were studied after 6 hours. from the beginning. This makes it an invaluable tool in the study of stroke. When deciding between these imaging modalities in a time-sensitive setting, many factors should guide decision making, including ease of access and the time it takes to obtain imaging results.

In most cases, MRI for a stroke patient is likely to be obtained during the patient’s hospitalization and should not delay management and intervention in the ED.

> Cerebral venous sinus thrombosis (CVST)

CVST is caused by a thrombus that obstructs blood flow to the cerebral veins and/or the dural sinus. This lack of blood flow can lead to a stroke. Dural sinus occlusion can lead to decreased absorption of cerebrospinal fluid and therefore increased intracranial pressure.

When CVST raises concerns, the combined use of MRI and MR venography is the preferred evaluation method that has replaced more invasive endovascular arteriography and digital subtraction angiography.

venous. Although venous MRI is the imaging modality of choice, it is not universally available. CT venography is more available and provides comparable results, with a sensitivity of 95-100%. However, it should be noted that the data are based on studies with small samples. The sensitivity of venous RNNM with T2-weighted images is 90%.

The choice of imaging modality for CVST will likely depend on the availability of both. Diagnostic and weather equipment.

> Non-infectious spinal cord compression

Spinal cord compression can occur in various forms, from traumatic injuries with compression of bone fragments to compression by tumors or hematomas. When neoplastic compression of the spine is suspected, MRI can identify spinal metastases in patients in whom radiographs, CT scans, and bone scans have been unrevealing.

Symptoms that may indicate the need for MRI include acute-onset back pain or worsening chronic back pain combined with warning signs, such as extremity weakness, Lhermitte’s sign, hyperreflexia, and bowel or bladder incontinence.

The gold standard image for the diagnosis of spondylotic myelopathy and metastatic spinal cord compression is gadolinium-enhanced MRI, which has a sensitivity of 93% and a specificity of 97%.

The initial study of choice when there is suspicion of bone lesions in the spine is CT without contrast. Clinical decision rules, such as the Canadian C-spine Rule or the National Emergency X-Radiography Utilization Study, can help inform the decision to image the cervical spine. If imaging in the setting of trauma is clinically indicated, non-contrast CT is the first-line modality as it is highly sensitive in identifying bony lesions.

Cervical spine CT has a sensitivity of 90.9% and a specificity of 100%. For the thoracolumbar spine, CT has an overall sensitivity of 98% and a specificity of 97%.

MRI is less sensitive and specific for bone injuries from acute trauma, with a sensitivity of 76% for fracture lines. The time it takes to obtain MRI also precludes its use in acute traumatic situations. Its usefulness in spinal trauma lies in the evaluation of soft tissue injuries that are not well evaluated with CT images.

MRI can be useful in visualizing soft tissue changes, such as ligamentous injuries, epidural hematomas, and disc bulges. When evaluating soft tissue injuries, contrast-enhanced MRI has a sensitivity of 100% compared to the 33% sensitivity of non-contrast MRI. This is significantly more sensitive than CT, which has a sensitivity of 32% and a specificity of 100% for ligamentous injuries of the cervical spine.

MRI is also a valuable tool for patients with neurological deficits that are not explained by plain radiograph or CT findings. Spinal cord injury without radiographic abnormality refers to patients with clinical signs (pain, numbness, paresthesia) of a post-traumatic spinal cord injury, without radiographic evidence of fracture or misalignment on radiography or CT.

Spinal cord injury without radiographic abnormalities is more common in pediatric patients due to ligament laxity, but is also seen in adults. Involvement of the cervical spine is most common, but the thoracolumbar spine may also be affected. In patients with suspected traumatic spinal injury who have negative radiographs and CT scans, MRI is a valuable tool for further evaluation.

SCE occurs due to compression of the sacral and lumbar nerve roots within the lower vertebral canal, causing ischemia or infarction of the nervous tissue. This condition frequently presents with bowel or bladder dysfunction and saddle anesthesia. It is a clinical diagnosis but an emergency MRI with contrast must be performed, which is the diagnostic test of choice to evaluate the possible underlying causes of the syndrome, since it is a surgical emergency.

Ultimately, if obtaining MRI would result in a delay in care, it is prudent to discuss the neurosurgical treatment plan while awaiting imaging results.

When evaluating spinal stenosis, which is a possible cause of SCE, the sensitivity of MRI is 81-97% and the sensitivity of myelography is 67-78%. If the patient cannot undergo MRI (due to intolerance, implanted metal devices, etc.), an alternative is CT myelography. If MRI myelography or CT can cause a delay in care, CT can be used (although its sensitivity and specificity are limited in identifying the causes of CSS).

To identify herniated disc, CT has a sensitivity of 77% and a specificity of 75%. For spinal canal stenosis, the sensitivity of CT is 87% with a specificity of 75%. Although not the ideal modality, a rapid CT image can provide the consultant neurosurgeon with vital information while waiting for a more definitive modality.

CT myelography is usually performed by a radiologist and involves injecting contrast material into the spinal canal during imaging. It is likely to be performed in hospitalized patients, since it is an examination that requires coordination between specialized personnel and the radiologist. If imaging may cause a delay in care, evaluation by the spine surgeon should be sought.

X-ray images have low sensitivity and specificity, 43-75% and 75-83%, respectively. During the first 2 weeks after the onset of infection, up to 80% of patients with osteomyelitis have normal x-rays. The sensitivity and specificity of contrast-enhanced CT for osteomyelitis are 67% and 50%, respectively.

Although MRI is more expensive than CT, a recent study found that the use of rapid MRI protocols for acute stroke in the ED resulted in an 18.7% reduction in total direct cost and a reduction in 17% in the duration of hospital stay.

Logistical considerations in determining whether an MRI should be performed in the ED include the time required to obtain the study and the fact that the MRI may require the patient to leave the department, which may not be advisable if the patient is unstable, and may accentuate the limitations of ED staff. The patient’s ability to tolerate the study should also be considered.

Claustrophobia can increase your anxiety, making it difficult to obtain images. While patients undergo MRI they may require sedation, analgesia, anxiolytics and even anesthesia. This is often not practical in the ED setting. Another consideration is the patient’s body build, as the size of the MRI opening typically ranges from 60 to 80 cm wide and the maximum patient weight for the MRI table typically ranges from 250 to 300 kg.

Implants, such as artificial joints, insulin pumps, or neurostimulators, may prevent the use of MRI. It is noted that implants manufactured in the last 3 decades are made with non-ferromagnetic material, and are generally labeled as "MRI safe" or "MRI conditional." This means that to obtain the image, specific examination conditions must be met.

Most often it is due to the presence of a weakly ferromagnetic implant component. These scenarios require coordination with the radiology department, consultants, and manufacturers. Despite safety or conditional labels for MRI, heating metal during MRI can still cause injuries, including burns and pain.

In patients with an altered level of consciousness or who are unresponsive, it is recommended that a plain radiograph be obtained before performing MRI to rule out the presence of metallic foreign bodies, implants or other devices.

Current practice guidelines for non-contrast MRI do not recommend special considerations during pregnancy, as no current literature has demonstrated negative effects on fetal development.

However, it is recommended that during pregnancy it only be obtained in the following situations: the information expected from MMR; cannot be obtained by ultrasound; The results of the study may affect the care of the patient or the fetus, and the referring physician does not consider it appropriate to wait to perform the study until after pregnancy.

MRI with gadolinium contrast during pregnancy has been associated with an increased risk of skin, rheumatological, inflammatory or infiltrative conditions in the newborn, as well as stillbirth and neonatal death. In comparison, CT during pregnancy results in increased risk of cancer for the fetus due to the teratogenic effects of ionizing radiation.

Obtaining images in pregnant women with suspected life-threatening pathology should not be delayed or suspended due to their pregnancy. MRI is the most sensitive test modality for the identification of metallic foreign bodies. However, depending on the material, MRI runs the risk of heating and displacing the material during the scan.

MRI should not be performed if there is confirmation or suspicion of a metallic foreign body, especially if its composition is unknown.

This is very important if the foreign body is near critical structures, including the eyes, brain, spinal cord, nerves, or large vessels. If the risk of heating and migration during scanning is considered low, the clinical benefit may outweigh the risk.

In the specific evaluation of a foreign body, MRI should not be the initial examination. Conventional x-ray images are widely available and provide high sensitivity, showing up to 80% of all foreign bodies and up to 98% of radiopaque bodies, making radiography an excellent first-line evaluation.

In case of negative radiographs and high clinical suspicion of a foreign body, CT or ultrasound should be considered. CT is 15 times more sensitive than X-rays and is the best modality for visualizing plastic, glass and stone. However, CT is less sensitive than ultrasound for evaluating small superficial foreign bodies. Ultrasound has an overall sensitivity of 72% and a specificity of 92% for foreign bodies and is the best modality for visualizing wooden foreign bodies, with a sensitivity of 90% and a specificity of 96.7%.