MRI is an imaging modality that uses non-ionizing radiation to create diagnostic images. The concept of MRI was first described by Felix Bloch and Edward Purcell in 1946 and was initially called Nuclear Magnetic Resonance Imaging after its early use for chemical analysis. It was not until 1971 when the potential medical uses of this technology were realized.
A MRI scanner consists of a large and powerful magnet. A radio wave antenna is used to send signals to the body and returning signals are converted into images by a computer.
MRI images are created based on the absorption and emis- sion of radiofrequency energy, without using ionizing radia- tion. MRI scanning involves the use of primary and secondary magnetic fields. The use of 1.5 or 3 Tesla (T) terminology refers to the strength of the magnetic field. The current FDA approval in terms of upper limit of field strength for adults is 8T and 4T for adults and children, respectively [4]. Most scan- ners in current clinical use are “electromagnets” or super con- ducting magnets, which implies that the static magnetic field is always turned “on” even when a patient is not being scanned.
Safety precautions are to be strictly followed at all times.
“Quenching,” the process of turning the scanner off, is rarely performed. “Open” MRI are useful in claustrophobic patients, although the upper field strength is usually limited to 1T.
For all practical purposes, if a patient develops a medical emergency while being scanned, it is best to get the patient out of the scanner before starting resuscitation. All equipment inside the scanner room must be MR compatible (non- ferromagnetic).
In addition to the main magnetic field, secondary fields are created using radio frequency pulses (RFP) and gradient pulses emitted from the scanner to spatially encode the signal in the x-, y- and z-axis. These secondary gradients cause the loud metallic banging noise inside the scanner. Image density depends on several contrast parameters intrinsic to the tissue being scanned (T1 recovery time, T2 decay time, Proton den- sity, Flow and Apparent diffusion coefficient), as well as extrin- sic parameters that are varied by the radiologist to change image quality (TR, TE, Flip angle, TI or inversion time, Turbo factor/Echo train length).
MRI Definitions:
• Radiofrequency (RF) pulse – MRI technique where short electromagnetic signals oscillate to change the direction of the magnetic field. They are cycled at a rate of pulses per second.
• Relaxation time – The time it takes for protons to regain their equilibrium state during MRI imaging. T1 and T2 are two types of relaxation phases.
• T1- T1 is considered the “anatomic image”. Clues to recog- nizing T1: CSF is black, subcutaneous fat is white and gray matter (cortex) is hypointense relative to white matter (Fig. 3.2). The time it takes for 63% of longitudinal (parallel to the magnetic field) relaxation of protons to occur. Not all tissues get back to equilibrium at the same rate, and a tissue’s T1 reflects the amount of time its protons’ spins realign with the main magnetic field. Fat quickly realigns its longitudinal
magnetization, and therefore appears bright on a T1 weighted image. Conversely, water has much slower longitudinal mag- netization realignment after a RF pulse. Thus, water has low signal and appears dark on T1 weighted imaging.
• T2 – T2 is considered the “pathologic image”. Most patholo- gies increase the fluid content of tissues and show up as high signal (bright) including surrounding edema. This is the time it takes for 63% of transverse (perpendicular) relaxation of protons during MRI image acquisition. T2 is a “fluid –sensi- tive” sequence and free fluid appears hyperintense/white on T2 imaging.
• T2- FLAIR (fluid attenuated inversion recovery) – This sequence is basically a T2 image without the CSF brightness.
The CSF signal is nulled out (appears gray/black). This tech- nique attempts to minimize distraction of the CSF brightness while highlighting underlying pathology (Fig. 3.3). This is especially important for detecting small lesions in periven- tricular and subcortical white matter, and FLAIR is the sequence of choice for evaluation of patients with Multiple Sclerosis. CSF should always be dark/low signal on FLAIR- presence of increased signal indicated presence of subarach- noid hemorrhage (SAH) or increased protein in cases of meningitis. FLAIR is at least as sensitive as CT for detection of acute SAH.
• Diffusion Weighted Imaging (DWI) – Follows the changes in the movement of water through tissues and uses these changes as a contrast medium. Free diffusion of protons occurs only when cell membrane integrity is lost. DWI sequence is sensi- tive to abnormal water motion and diffusion through the tis- sues. DWI is a manipulated T2 image and therefore high signal areas can be caused by true restricted diffusion or be T2 shine through. DWI is used in acute stroke identification as well as to differentiate abscess versus necrosis versus cys- tic brain lesions [5, 6]. DWI is considered the gold standard for infarct core estimation and is very sensitive early in the evolution of ischemia (Fig. 3.6).
• Apparent Diffusion Coefficient (ADC) – A measure of the magnitude of diffusion (of water molecules) within tissue, and is commonly clinically calculated using MRI with diffu- sion weighted imaging (DWI). ADC values are calculated automatically by the software and then displayed as a para- metric map that reflects the degree of diffusion of water mol- ecules through different tissues. The impedance of water molecule diffusion can be quantitatively assessed using the apparent diffusion coefficient (ADC) value. ADC maps are devoid of T2 effects that may mimic or obscure lesions on DWI [7].
• Susceptibility Weighted Imaging (SWI)- An echo MRI sequence which is particularly sensitive to compounds which distort the local magnetic field and as such make it useful in detecting blood products, calcium etc. Blood is dark in this sequence and shows the “blooming artifact” where it is much darker than any other sequence (Fig. 3.7).
MRI is rarely the initial imaging study in acute clinical scenarios. It follows, most often, an initial CT screening. In contrast, MRI is the preferred method of evaluation for many
a b
Fig. 3.6 Axial DWI (a) and ADC (b) maps demonstrating right MCA acute infarct. Involvement of the cortex helps distinguish cytotoxic from vasogenic edema, which is typically confined to white matter
non-acute pathologies. MRI is also more sensitive for visualization of posterior brain structures, regardless of the tim- ing of presentation.
Typical indications for MRI:
• Acute stroke, following head CT (Fig. 3.7)
• Mass lesion characterization (Fig. 3.8)
• Posterior fossa evaluation
• Traumatic diffuse axonal injury
• Demyelinating disease
• Diplopia
• Cranial nerve dysfunction
• Seizures
• Ataxia
• Suspicion of neurodegenerative disease
• Developmental delay
• Neuroendocrine dysfunction
• Encephalitis (after a head CT)
a b
Fig. 3.7 Parenchymal bleed due to Cavernoma. GRE/SWI (a) is very sen- sitive to parenchymal blood and shows “blooming” or marked low signal relative to T2 WI (b)
• Drug toxicity
• Cortical dysplasia, and migration anomalies or other mor- phologic brain abnormalities
The potential advantages and disadvantages of MRI imaging are summarized below:
a
c d
b
Fig. 3.8 Axial CT (a) density showing fluid density (0–20 HU) in the left frontotemporal region. Axial T2 (b), Axial T1 (c), and FLAIR (d) images confirm fluid nature, consistent with an arachnoid cyst
Advantages
• The ability to image without the use of ionizing radiation (unlike x-ray and CT scanning)
• Superior soft tissue contrast over CT scans and plain films, making it the ideal examination of the brain, spine, joints and other soft tissue body parts
• The ability to visualize posterior structures of the brain better than CT imaging
• Angiographic images can be obtained without the use of con- trast material, unlike CT or conventional angiography
• Advanced techniques, such as diffusion and perfusion, allow for specific tissue characterization rather than merely ‘macro- scopic’ imaging
• Functional MRI allows the visualization of active parts of the brain during certain activities.
Disadvantages
• More expensive and less accessible than CT
• Longer scan times (patient dyscomfort is sometimes an issue).
• Length of the exam and decreased visualization of the patient throughout the exam can put certain patients at risk for clini- cal decompensation.
• Claustrophobia and patient size may preclude the ability to obtain images.
• Ability to lay still may be challenging for some neurological patients; intubation and/or conscious sedation is possible, but risks and benefits should be weighed carefully.
• MRI scanning is contraindicated for patients with some metal implants, cochlear implants, spine stimulators, cardiac pacemakers (relative contraindication) and metallic foreign bodies close to neurovascular structures. Careful attention to safety measures is necessary to avoid serious injury to patients and staff. This requires special MRI compatible
equipment and stringent adherence to safety protocols. A comprehensive list of MR safe equipment is available at- www.mrisafety.com
• Consideration needs to be given for how continuous infusions and ventilator support will be managed as not all equipment is MRI compatible.