Magnetic Resonance Imaging (MRI) Lecture 1 PDF

Magnetic Resonance Imaging (MRI) Dr. Mohammad Abu Mhareb  Introduction Magnetic Resonance Imaging (MRI) is a non- invasive imaging technique that provides excellent soft tissue contrast with high resolution and no ionizing radiation. MRI has revolutionized medic...

Magnetic Resonance Imaging (MRI) Dr. Mohammad Abu Mhareb  Introduction Magnetic Resonance Imaging (MRI) is a non- invasive imaging technique that provides excellent soft tissue contrast with high resolution and no ionizing radiation. MRI has revolutionized medicine. Directly visualizes soft tissues in 3D. Wide range of contrast mechanisms. It depends on the Nuclear Magnetic Resonance (NMR) phenomenon.  Introduction Con…. MRI provides a spatial map of the hydrogen nuclei (water and lipid) in different tissues. The image intensity depends upon: 1. The number of protons in any spatial location, as well as 2. Physical properties of the tissue such as viscosity, stiffness and protein content.  Introduction Con…. In comparison to other imaging modalities, the main advantages of MRI are: 1. No ionizing radiation is required, 2. The images can be acquired in any two- or three-dimensional plane, 3. There is excellent soft-tissue contrast, 4. A spatial resolution of the order of 1 mm or less can be readily achieved, and 5. Images are produced with negligible penetration effects. Pathologies in all parts of the body can be diagnosed, with neurological, cardiological, hepatic, nephrological and musculoskeletal applications all being widely used in the clinic. In addition to anatomical information, MR images can be made sensitive to blood flow (angiography) and blood perfusion, water diffusion, and localized functional brain activation.  The main disadvantages of MRI are: 1. MR image acquisition is much slower than CT and ultrasound, and is comparable to Positron Emission Tomography (PET) technique: a typical clinical protocol might last 30–40 minutes with several different types of scan being run, 2. A significant percentage of patients are precluded from MRI scans due to metallic implants from previous surgeries, and 3. Systems are much more expensive than CT or ultrasound units.  The MRI system comprises three major hardware components: 1. A superconducting magnet, a set of three magnetic field gradient coils, and 2. A radiofrequency transmitter and 3. A receiver. The superconducting magnet typically has a strength of 3 Tesla, approximately 60 000 times greater than the earth’s magnetic field.  This magnetic field causes the protons to precess at a frequency proportional to the strength of the magnetic field, i.e. there is a ‘resonance’ frequency. The magnetic field gradients make this resonance frequency dependent upon the spatial location of each proton in the body, thus enabling an image to be formed.  A tuned radiofrequency (RF) coil transmits energy into the body at ~128 MHz for a 3 Tesla magnet, and the MRI signal is induced in the same or other RF coils which are placed close to the body. There is excellent contrast between the grey and white matter of the brain, with the protons in lipid seen as a bright signal outside the skull.  Protons in very rigid structures such as bone are normally not visible using MRI: this is evident by the thin dark line between the lipid layer and brain surface 1. Effects of a strong magnetic field on protons in the body In an MRI scanner the patient lies on a patient bed which slides into a very strong magnet. A typical value of the magnetic field, B0, is 3 Tesla (30 000 Gauss), roughly 60 000 times greater than the earth’s magnetic field of ~50 μT (0.5 Gauss). Patients must undergo a thorough check to ensure that they have no magnetic metal implants or surgical clips.  1.1 Proton energy levels In clinical MRI the image is formed by the signals from protons (hydrogen nuclei) in water and lipid. At the atomic level, since the proton is a charged particle which spins around an internal axis of rotation with a given value of angular momentum (P), it also has a magnetic moment (μ), and therefore can be thought of as a very small bar magnet with a north and south pole. The internal rotation of a proton creates a magnetic moment, and so the proton acts as a magnet with north and south pole In the absence of a strong magnetic field, the orientations of the magnetic moments are completely random. When there is a strong magnetic field present the magnetic moments must align at an angle θ = ±54.7° with respect to the direction of B0.  The magnitude of the angular momentum of the proton is quantized and has a single, fixed value. The magnitude of the proton’s magnetic moment is proportional to the magnitude of the angular momentum: where γ is a constant called the gyromagnetic ratio, and has a value of 267.54MHz/Tesla for protons.  1.2 Classical precession Having determined that the proton magnetic moments are all aligned at an angle of 54.7° with respect to the direction of B0, the motion of these magnetic moments can most easily be described using classical mechanics. The B0 field attempts to align the proton magnetic moment with itself, and this action creates a torque, C, given by the cross product of the two magnetic fields: where iN is a unit vector normal to both μ and B0. The direction of the torque, shown in Figure 1, is tangential to the direction of μ and so causes the proton to ‘precess’ around the axis of the magnetic field, while keeping a constant angle of 54.7° between μ and B0.  The effect of placing a proton in a magnetic field, therefore, is to cause it to precess around B0 at a frequency directly proportional to the strength of the magnetic field. This frequency, termed ω, is termed the Larmor frequency after the renowned Irish physicist Joseph Larmor. Example: What are the frequencies, expressed in MHz, for B0 fields of 1.5 Tesla, 3 Tesla and 7 Tesla? 2. Effects of a radiofrequency pulse on magnetization As with all such multi-level systems, to obtain an MR signal, energy must be supplied with a specific value ∆E to stimulate transitions between the energy levels. (left) In the absence of a strong magnetic field, the energies of all the random orientations of the magnetic moments are the same. (right) When a strong magnetic field is applied, the single energy level splits into two levels, one corresponding to the magnetic moments being in the parallel state, and the other the anti-parallel state.