Question

Describe how the magnetic fields in z, x, and y; and the pulses and other timing are used to localize and influence the nuclear spins in a small vowel of tissue with an entire body placed in a MRI machine. Diagram the fields and timings and explain why. Explain the physics (Bloch Equations, etc.) of how an MRI image is made. Explain T1 and T2 weighting and what they measure best.

Answer 1

Answer: MRI is its excellent tissue-differentiating capabilities, made possible because the biochemical characteristics of the nuclei within their microscopic environment alter the information (called signals) received during an MRI acquisition. MRI acquisitions may further alter and differ contrast. These signals are not influenced by the amount of bladder filling, the size of a patient, or the amount of gas in the surrounding bowel, but these factors have an important role in the quality of an ultrasonographic image. 8o net magnec moner vector Do Fig. Diagram showing individual nuclei aligning with the main magnetic field Bo Certain atomic nuclei in molecules possess nuclear angular momentum, or spin. Such nuclei may be considered as spinning charged particles that create a magnetic field or moment (u) 'H,31p, 1'C, N, and "F are nuclei with spins. 'H nucleus, the proton, is currently the only nucleus extensively used for MRI because of its relative abundance in the human body. In the absence of an external magnetic field, individual hydrogen nuclei are randomly oriented within a tissue. When a uniform external magnetic field (Bo) is applied to these nuclei, the nuclei tend to align in the direction of the applied field because this is the lowest energy state. The aligned nuclei will spin, or precess, about E at a frequency that is determined by the strength of the magnetic field being sensed by the nuclei--that is, the larger the external magnetic field, the higher the frequency of precession. The magnetic moments of multiple similarly aligned nuclei produce a net magnetic moment that is used to produce the MRI signal (Fig. A) Alignment in a direction opposite to the external magnetic field requires nuclei to be in a higher energy state. The quantity of energy required to transform the proton to its higher energy state is determined by quantum mechanical principles and is directly related to the precession frequency of the proton. Such transfer of energy may be achieved by applying an electromagnetic field (B,) (which for imaging is in the range of RF wavelengths) orthogonal to the main magnetic field (Bo). The net magnetic moment vector will rotate about the original magnetic field, the degree of rotation being determined by the strength and duration of the applied electromagnetic field. Assuming Eo to be oriented in the Z direction, then the net magnetic moment is usually rotated 90°, thus placing the net magnetic moment vector into the X-Y plane perpendicular to the direction of Eo (Fig B). When the electromagnetic field is discontinued, the net magnetic moment vector precesses in the X-Y plane This precession of the net magnetic moment vector is also known as transverse magnetization

Bloch began with the assumption that the millions of individual nuclei in a sample could be represented by a single vector M. (Bloch called M the "polarization" but today it is more frequently known as the net magnetization.) Just as individual nuclei possess spin angular momentum, so does the vector M. As Joseph Larmor showed a half century before, an external field (B) produces a torque or "twisting force" on M resulting in its precession at angular frequency ω Since M is a vector rotating in space it can be resolved into have three components-Mx(), My(V), and Mz(t)- each a function of time. Mx() and My() are the transverse components, Mz() is the longitudinal component For the case of simple precession of M in a cone around B, the longitudinal component isa constant, Mz()-C. The transverse components rotate sinusoidally around the z-direction at angular frequency w. For simplicity we can adjust their phases so Mx() - at 0, allowing us to write Mx(t) "A sin ωt and My() A cos cot, where A is a scaling factor. ngutar Fig. A torque or twisting force (r) results from the interaction of net magnetic angular momentum (M) and the magnetic field (M), defined as the vector cross product-M x B. The direction of τ is perpendicular to both Mand B according to the right-hand rule. This torque causes M to precess around B This precession in the transverse plane, the X-Y plane, will induce a voltage within coils or loops of wire that are aligned with the transverse plane. This induced voltage is then translated into a signal that results in a magnetic resonance image. It is the ability of MRI to assess variations in the net magnetization of the nuclei within different tissues that allows for superb tissue contrast, a unique feature. These variations in net magnetization decrease exponentially over time and are characterized by Tl and T2 relaxation times that are unique to the specific tissue The longitudinal relaxation time, TI, therefore represents the time required for Mz to grow from 0 to (1-le), or about 63% of its final value. Similarly, the iransverse relaxation time, 12, represents the time required for Mr or My to decay down to 1/e (about 37%) of their initial maximum values. 63% M TL TIMB T2

Tl relaxation: After electromagnetic excitation, nuclei absorb energy and a higher percentage of the nuclear population exists at a higher energy state. By transferring this energy from the excited nuclei to surrounding molecules, commonly referred to as the lattice, the nuclei return to a lower energy state, realigning with the main magnetic field (Bo) This return to the low energy state is called Tl relaxation or spin lattice relaxation. The efficiency with which energy is transferred depends on the molecular structure of the surrounding tissues (i.e., relaxation of protons in fat would differ from relaxation of protons in muscle). The transfer of energy is described by an increasing exponential function in which the time constant (i.e., the time at which 63.7% of the previously displ realign with the main magnetic field) is the TI relaxation time. Shorter T1 relaxation times imply faster realignment with the main magnetic field after excitation. aced spins T2 relaxation: After excitation, the nuclei initially precess coherently within the X-Y plane However, as a result of local fluctuations in the magnetic field caused by nearby nuclei, energy is transferred from one nucleus to another. This transfer of energy results in loss of phase coherence. Macroscopically, the net magnetic moment vector decreases, resulting in a smaller signal. T2 relaxation describes this loss of phase coherence or transverse magnetization. It is also known as spin- spin relaxation, because energy is transferred from one excited spin (nucleus) to another and not to the lattice as in Tl relaxation. This transfer of energy is characterized by an exponential decay function in which T2 relaxation is the time necessary to lose 63.7% of the signal. T2 relaxation typically occurs more rapidly than Tl relaxation and is the primary cause of the exponential decay in transverse magnetızation. Tissue contrast: By adjusting the TR and TE intervals, it is possible to make either Tl or T contrast more prominent within an image. Short TR(repetition time) and TE (echo time) combinations (commonly written TRTE-e g., 500/20 msec) accentuate Tl contrast, whereas long TR and TE combinations (e g., 2500/80 msec) preferentially select for contrast based on differences in T2 relaxation. Accordingly, short TR/TE images are considered Tl weighted and long TR/TE images are considered more T2 weighted The appearance of an image changes markedly depending on the emphasis of Tl or T2 relaxation times. For example, on a Tl-weighted image, fat appears bright (white), whereas fluid appears dark. On a more T2-weighted image, fat appears gray, whereas fluid appears bright (white). Most but not all pathology, because of its increased water content, mimics fluid-that is, it is dark on Tl-weighted images and bright on T2-weighted images in the following table Tissue Ti-Weighted Images T2-Weighted Images Fat Increased Slightly inereased Blood Variably incrasedVariably increased or decreased or decreased Pus Stightly decreased Increased Inflammation Decreased Increased Edema Decreased Serous fluid Decreased Increased