(DWI) gives MRI the ability to detect acute stroke and chronic infarct. Functional MRI (fMRI) is a popular tool in neuroscience research which utilizes the Blood Oxygenation Level Dependent (BOLD) effect to map neural activation. White matter connectivity can be investigated using diffusion tensor imaging (DTI) or high angular diffusion imaging (HARDI) and tractography. In musculoskeletal MRI soft tissue components, muscle, bone marrow, fat, and cartilage are all visible. Whilst tendon, ligament, and cortical bone are inferred by their absence of signal, edema resulting from injury is highly conspicuous.
MRI has a major role in oncology through tumor imaging, often using DWI for diagnosis, staging, and treatment assessment. Applications include breast, bowel, prostate, liver, pancreas, female pelvis, in addition to brain and spine. MR spectroscopy (MRS) provides in‐vivo bio‐chemical information on tumor and tissue metabolism. Through sensitizing the MR signal to flow or by the injection of a gadolinium‐based contrast agent (GBCA) angiographic images can be obtained to investigate vascular disease. The development of rapid GRE sequences has facilitated cardiac MR for heart morphology and function studies. GRE using the in‐phase–out‐of‐phase technique is applied in liver and abdominal imaging of adenoma and cirrhosis, whilst TSE/FSE can indicate cysts, hepatocellular carcinoma and metastases. Single‐shot TSE/FSE is used for MR cholangio–pancreatography (MRCP) in the biliary system. Some of these are illustrated in Figures 1.8 and 1.10.
MRI HARDWARE
The heart of the MRI system is the magnet, creating the static B0 field that produces the tissue magnetization. The B1 pulse that manipulates the magnetization is generated by a RF transmit coil (Tx) or coils fed by high‐powered broadband RF amplifiers. The pulse shapes are generated digitally and converted to analog waveforms prior to amplification. The gradient coils provide the pulses used for spatial encoding of the signal. Finally, the MR signal is detected by receiver coils (Rx). These are usually positioned close to the anatomy of interest to maximize the signal received. Most receive coils are array or matrix coils formed from numerous minimally interacting smaller elements. The advantages of array coil technology include improved signal‐to‐noise ratio (SNR) and the ability to utilize parallel imaging techniques. Once detected, the MR signal is demodulated, i.e. the RF “carrier” component (at f0) is removed, as the spatial information is stored in the VLF signal region. With “direct digital” systems, demodulation and digitization are applied directly to the amplified RF signal, close to or at the coil. The ensuing signals are transmitted digitally or optically and stored for reconstruction. Figure 1.12 shows a schematic of a typical MR system. The gradient and RF amplifier systems, control and signal processing systems, and cooling equipment are situated in the equipment or technical room, external to the MR examination or magnet room. The console and host computer are located in the control room (Figure 1.13).
Figure 1.12 Schematic of principal MRI system components: DAC is digital‐to‐analog convertor; ADC is analog‐to‐digital convertor; Tx transmit; Rx receive; black lines denote digital signal; colored lines denote analogue signal. The magnet power supply is only required for the initial ramp‐up.
Figure 1.13 MRI operator’s console in the control room with observation window into the magnet room. Source: James Steidl/iStockphoto.
Magnet system
The magnet is the largest single component of the system. Most systems use superconducting magnets. Superconductivity is a quantum mechanical property whereby, below a critical temperature Tc, an electrical conductor loses its electrical resistance, enabling large electrical currents to be sustained in perpetuity without a driving voltage from a power supply. As long as the windings are kept sufficiently cold, the current and hence the magnet’s field persists indefinitely. Liquid helium with a boiling point of −269 °C (4.3 K or kelvin) is used for cooling. Safety consequences of this are considered in Chapter 12 .
Superconductivity
Superconductivity was discovered by Dutch scientist Heike Kamerlingh Onnes in 1911, but it took until 1957 for Bardeen, Cooper, and Schrieffer to formulate a quantum mechanical theory (BCS theory) to account for the phenomenon. The electrical resistance of a non‐superconducting metal, such as copper, depends upon its temperature, decreasing with lower temperatures, but possessing a finite resistance even at absolute zero (−273.15 °C). Below Tc the electrons in a superconductor pair up into “Cooper pairs” acting as a superfluid resulting in zero resistance (FIGURE 1.14a).
Figure 1.14 Superconductivity: (a) resistance v temperature for a superconductor and a non‐superconductor; (b) Superconducting phase diagram: each of temperature, current density and magnetic field must be below a critical value Tc, Jc, Bc to maintain the superconductive state.
Niobium titanium (Nb‐Ti) alloy used in MR magnets is a type 2 superconductor.1 It has a second, higher critical temperature at which some magnetic flux may exist within the material. Superconductivity behaves as a thermodynamic phase with a relationship between temperature, field, and current density (Figure 1.14b). There is a critical field Bc and current density Jc above which the superconductive state cannot exist. This puts an ultimate limit on the field strength that can be achieved. Nb‐Ti has a Tc of 9.5 K and Bc of 15 T. Niobium‐tin (Nb‐Sn) alloy can sustain higher fields.
High temperature superconductors can have Tc above 90K and can be cooled using liquid nitrogen (N) with a boiling point of 77 K (−196 °C) or with cooled helium gas. These have been used to produce 0.5 T MRI magnets, but not operating in a persistent current mode. Research is ongoing with the prospect of simplifying the cooling system and reduced dependence upon helium. Helium is a by‐product of natural gas extraction, a limited resource. Nitrogen can be produced from the atmosphere.
Superconducting MR magnets
Superconducting magnets are capable of generating magnetic flux densities, colloquially referred to as “magnetic field strength”, up to 7 T (tesla) in currently available systems. 1.5 and 3 T systems are most common. B0 is orientated horizontally
