Emergent causes for Magnetic Resonance Imaging (MRI) in the Emergency department (ED) are conventionally limited to critical spinal cord injuries and more recently in ischemic strokes. Generally, MRI imaging is limited in the ED and the few number of ED MR studies partially stems from only 66% of EDs having MRI machines onsite and only 13% having the needed personnel present 24/7. [1] Even though EDs do not always have the capabilities to complete MR imaging it is common for EM physicians to evaluate imaging they did not order. Frequently, this is from patients being transferred from an outside facility or through patients in ED observation who undergo MR imaging as a part of their protocolized care. For these reasons it is important for Emergency Medicine Physicians to have a general grasp of MR imaging to ensure patients receive the care they need in a timely manner and to appropriately discuss the patient’s case with the involved consultants. Additionally, over the course of residency, most trainees will rotate with other services that necessitate a basic fundamental understanding of MR imaging. in this post we will discuss the physics requisite to interpret the images, what the series you will look at entail and a brief guide to the ED reading of an MRI of the brain.

Indications and Contraindications

Within the ED emergent conditions requiring brain MRI are generally limited to ischemic strokes with an unknown time of onset. This development has come as part of the rapidly changing care delivered to stroke patients in the emergency department. Now MRIs capable of identifying lesions amenable to thrombolysis who conventionally would be outside of the treatment windows.[2,3] Less emergent conditions such as dural venous thrombosis, multiple sclerosis flairs, metastatic progression or infectious processes also can benefit from MR imaging in the emergency department. [4-7] If MR imaging is going to be undertaken in the ED it is important to consider the contraindications unique to MRIs. Because of the necessary magnetic fields any implanted magnetic alloys are an absolute contraindication and indwelling items (pacemaker, stimulators, clips etc) need to be thoroughly vetted with the manufacturer prior to imaging. Consideration also needs to be given to cutaneous metals included in topical patches or devices. Patients with renal dysfunction must also be considered special populations when contrast is considered as there is a 2.4% lifetime risk of developing Nephrogenic Systemic Fibrosis with gadolinium-based contrasts. [8]

Indications

Acute Ischemic Stroke with unknown time of onset
Dural venous thrombosis (MR Venography)
Cavernous Sinus Thrombosis (MR Venography)
Consider for Meningoencephalitis (especially if concerned for viral etiology)
Consider for Carotid Artery Dissection (MR arteriography)

Contraindications [9]

Any retained or implanted metal objects (i.e. AICDs and some Aneurysm clips)
Topical metal objects (i.e. continuous glucose monitors and some medication patches with foil in them)
Allergic to gadolinium (if using contrast)
Renal disease (if using contrast)
Inability to tolerate laying flat
Fear of confined areas

In understanding MR imaging it is important to briefly start at the basic level of the physics behind imagine creation. MR imaging at its core is based on protons and primarily the proton of hydrogen atoms. Though many other types of molecules contribute to MR images, the hydrogen atom is specifically useful because it has, one proton, one neutron, and a tiny intrinsic magnetic field associated with it. The proton’s magnetic field can be aligned in parallel or in antiparallel to a magnetic field. MR imaging uses a magnet to align all protons to the field. Once aligned a radio frequency wave tuned to the protons resonate frequency is used to “flip” the protons into their reverse alignment with the magnetic field. The protons “flip” their alignment because they absorb energy from the radio wave. As the protons relax back from the flipped to unflipped stages they release energy that is registered by a radio antenna coil. Smaller local coils can be used closer to the body to give higher fidelity images through improved signal detection.[10] All of this can be broken down into the 1-2-3 of MRI 1. Align 2. Flip 3.Relax.

MRI Sequences and Image Description

Due to the way MRI images are created (remember the 1-2-3 of MRIs) different kinds of images can be created through looking at the different ways protons relax and different combinations/repetitions of steps 2 and 3. These different combinations are called MRI sequences. The two main sequences are T1 and T2 sequences primarily because they look at the proton’s relaxation in the most fundamental ways. T1 looks at the particle’s ability to realign with the magnetic field and T2 looks at the particles’ ability to release energy to the surrounding structures. Additionally, there are Diffusion-weighted images (DWI), Fluid Attenuated Inversion Recovery (FLAIR), and Susceptibility weighted imaging (SWI). Each of these processes the MR information a little differently to give specific information for things such as ischemia, blood products, calcium, or parenchymal lesions.

Since MRIs are based on measuring released energy from particles in the body their resulting images are described with “intensities”. This is analogous to CT scans being described with densities. Bright (white) areas on MRI are considered high-intensity signals and dark (black) areas have low-intensity signals. Below is an explanation of each sequence and a representative image from a case of left occipital parenchymal hemorrhage.