Magnetic resonance imaging in emergency medicine

Introduction

Magnetic resonance imaging (MRI), like each of the other imaging modalities, has its unique strengths and weaknesses. Emergency diagnostic imaging algorithms are complex and vary according to the clinical condition, the question being asked, patient factors, local availability and expertise. In Australasia, there are rapidly increasing numbers of MRI machines, together with appropriately trained staff. Many diagnostic algorithms are being revised to include MRI as a realistic imaging alternative. While the indications for urgent MRI are increasing, the major indication for emergency MRI remains suspected acute spinal cord pathology.

The main strength of MRI lies in its ability to image soft tissues at extremely high resolution, both spatially and with unparalleled levels of soft tissue contrast. It also has the ability to create two-dimensional slices in any plane; these multiplanar capabilities allow comparison of adjacent tissues from any angle.

Different MRI techniques allow different anatomical and pathological features to be demonstrated. MRI is particularly good at imaging the structure and pathology of brain, spinal cord and nerves, muscle, tendons and ligaments, cartilage, bone marrow and solid abdominal organs. The advent of magnetic resonance angiography (MRA) makes it an alternative to computed tomography angiography (CTA), particularly in those with contraindications to CT contrast media or those more vulnerable to ionizing radiation. Finally, the ability to perform ECG-gated imaging has enabled unsurpassed dynamic noninvasive cardiac imaging.

MRI has imaging limitations in cortical bone and air-filled spaces (particularly lung) and thus tends not to be the imaging choice for assessing these tissues.

The lack of ionizing radiation makes MRI an attractive alternative to CT, particularly in younger patients, especially children and women of childbearing age. MRI’s apparent safety in pregnancy is another advantage.

Currently, the main limitations of MRI lie in its lack of availability, expertise and associated high costs. From the perspective of emergency medicine, even when availability is not an issue, a major disadvantage is the time it takes to complete a scan. The patient is in an inaccessible, confined space, usually with limited monitoring for the duration of the 30- to 60-minute scan. MRI is therefore unsuitable for the unstable patient. If a patient is stable and intubated, specialized anaesthetic and monitoring equipment and expertise in using it is required to ensure patient safety.

Patients with metallic foreign bodies or electronic implants are often unable to have MRI, which can also limit its use.

Technical issues

Put very simply, the steps of an MRI involve putting the patient into a powerful magnet, sending in a radio wave and turning the radio wave off; the patient then emits a signal which is received and used for reconstruction of the image.

Components

The traditional MRI suite is centred around the MRI scanner, with its mobile patient table that moves the patient in and out of the MRI tunnel ( Fig. 23.3.1 ). Current machines have a bore (internal diameter of the tunnel) of up to 70 cm and utilize short bore architecture that allows the tunnel to be approximately half the length of that required in the previous generation of MRI scanners. The machine houses a superconductor magnet, the strength of which is measured in Tesla. Most current machines operate at 1.5 or 3 Tesla. A 3-Tesla magnet creates a magnetic field around the patient 60,000 times the strength of the earth’s magnetic field. In addition to the magnet, there are radiofrequency transmitter and receiver coils that send and receive radiofrequency pulses. These briefly disturb the magnetic field and ultimately create the MRI image. Another three sets of gradient coils provide additional linear electromagnetic fields important for spatial information—determining the origin of the signal in the three-dimensional space. It is these coils banging against their anchoring devices that cause the loud noises associated with MRI.

The high magnetic field generated by MRI means metallic objects within range can become projectile missiles, and great care has to be taken to ensure metal objects are well secured or do not enter the room. The magnet can also interfere with electronic equipment, such as computers, monitors and medical equipment such as pacemakers, and these must be kept away. Finally, the receiver coils are highly sensitive and are designed to detect very minor fluctuations in returning radio waves (which are a form of electromagnetic radiation). External radio waves can interfere with the waves received by the coils, and this noise will create artefacts interfering with image production. To minimize this, the entire MRI room is secured inside a Faraday cage, whose external conducting surface blocks or markedly attenuates any outside potentially interfering radio waves. The MRI control room and computer terminals with operating console are located immediately adjacent to, but outside the MRI room, in a similar fashion to the CT control room.

Creating an image

MRI depends on the alignment of hydrogen nuclei or positively charged protons within organic compounds in the body. Hydrogen nuclei act like tiny bar magnets. Under the influence of the external MRI magnet, mobile hydrogen ions align and spin in the orientation of the MRI magnet’s field, creating a magnet of the patient’s body. In addition to aligning and spinning on their own axis, protons also rotate or ‘precess’, as would a spinning top with a slight wobble, around a central axis.

Pulses of electromagnetic energy, called radiofrequency or RF pulses, are then sent into the area being imaged. This briefly disturbs the orientation and precession of the aligned protons. A transient reduction in the longitudinal magnetic field results, and a new magnetic vector in the transverse direction, called transversal magnetization, is created. Once the RF pulse is stopped, the protons relax back to their initial aligned state and the longitudinal and transverse magnetic vectors return to their original state. The realignment rate depends on tissue characteristics and water content. As the magnetic vectors realign, electric currents are induced and the MRI signal and signal intensity created. The receiver coils receive these minute pulses of newly created electromagnetic radiation, and these are interpreted to create the ultimate image.

Different magnetic resonance imaging techniques

Numerous different MRI imaging sequences and techniques have been developed to create the optimal images for varying body tissues and pathology. The following is not an exhaustive list.

T1 and T2 imaging

These are the most common MRI images with which we are familiar.

T1-weighted images (anatomical) create high-definition anatomical images with optimal tissue contrast resolution. In these images, fat is white and water is black. The resultant image gives detailed representation of the internal structure of soft tissue organs. T1 is a time constant that refers to the time it takes for the changes in longitudinal magnetization induced by the RF pulse, to return toward the original state. Measuring this tends to define structural tissue proteins and fats optimally.

T2-weighted images (pathological) highlight pathological processes where there is increased water content within tissues ( Fig. 23.3.2 ). Most pathological processes involve an element of tissue oedema, and, whether it be trauma, infection, infarction or neoplasia, these images highlight water. Water is seen as white in these images. T2 is a time constant that refers to the time it takes for the changes in transversal magnetization induced by the RF pulse, to return toward their initial state. Measuring this tends to highlight water optimally.

Other MRI techniques each aimed at highlighting other anatomical or pathological features are shown in Fig. 23.3.3 . The left image (A) is a FLAIR (fluid-attenuated inversion recovery) sequence that nulls fluid and can highlight periventricular demyelination; the central image (B) is a T2 gradient image which detects haemoglobin and its breakdown products; the right -hand image (C) is a diffusion ADC image detecting cell injury in early stroke.

Angiography and gadolinium

MRA can be done with or without contrast media ( Fig. 23.3.4 shows non-contrast MRI). Flow itself alters the MR signal simply by moving the protons that have been exposed to the RF pulse. This can leave what is called a flow void phenomenon, and, using this, the machine can create an angiographic image.

Where a contrast agent is used, the paramagnetic rare earth gadolinium (Gd) is the agent of choice. Its use creates excellent angiographic images. In addition, Gd does not cross the normal blood–brain barrier. However, if this is disrupted, as can occur in many pathological processes, Gd improves lesion detection and diagnostic accuracy. Gd is not an iodinated contrast medium and is generally very well tolerated.

The sagittal images of the lumbar spine shown in Fig. 23.3.5 demonstrate a tumour involving the L1, causing some cord compression. The left image is T1, the centre T2 and the right a T1 fat-saturated post-Gd image where the tumour with its abnormal vasculature is most obvious.