Magnetic Resonance (Medical Imaging) - The Essentials: Part1

The lecture focuses on the principles of MR signal generation. Discussed topics:

• Sample in magnetic field

• Bloch equation and free precession

• RF field and excitation

• Brief overview of signal detection

MRI is the most versatile medical imaging tool in terms of achievable contrast types and tissue information. This lecture discusses some of the most important ones of these including the following topics:

• Closer look at the RF excitation, resonance condition&rotating frame and the B1-field

• Bloch equations in the rotating frame - free precession

• Bloch equations in the rotating frame - RF excitation

• Signal decay and recovery - introducing the relaxation times

Having set the scene in the previous lecture here we discuss relaxation in details including how to measure the relaxation time constants.

• Bloch equations with relaxation

• T1, T2, T2* relaxation

• IR and CPMG pulse sequences

• Properties of T1 and T2

• Dependence of T1 and T2 on the tissue type

• Pulse-acquire sequence

• Spin echo

• Signal and contrast types of spin echo: PD-weighted, T1-weighted, T2-weighted images

The lecture describes the principles of:

• Precessing net magnetization, flux and induction

• Phase sensitive detection and demodulation

• Mathematical form of the acquired signal

• Coil sensitivity, chemical shift, B0-field inhomogeneity

• Shimming and the Fourier relationship between the detected signal and the spectrum

Since the Fourier-transform is the most important mathematical tool in magnetic resonance this lecture give an introduction and a brief overview about its essential properties:

• Signal vs frequency components relationship

• Fourier- and inverse Fourier-transform

• Properties of the Fourier-transform

• Consequences of discretisation and finite sampling

• Nyquist sampling criterion explained

• Sampling, bandwidths and isochromats

Introduction of:

• Complex spectrum - phasing, absorption, dispersion and absolute value modes

• Signal - dependence on physical parameters

• Noise -  dependence on physical parameters

• Noise statistics - white noise, Gaussian, Rayleigh and Rician distributions

• SNR

The lecture introduces the concept of steady-state and its properties. We discuss:

• What is steady-state, pros and cons

• The two types of steady-state: incoherent and coherent

• Concept of spoiling

• Signal equation for incoherent steady-state

• Ernst-angle, maximum signal, maximum contrast, contrast types

  
  

Continuing the previous lecture, here we discuss:

• What coherent steady-state is

• Spin echos, stimulated echos

• Illustrative description of bSSFP

• Signal equation and contrast type

• Frequency profile and the banding artefact

  
  

The lecture gives a broad overview of the RF pulses and their properties and discusses the most often used pulse shapes within the Fourier-framework:

• RF parameters: shape, pulse length, peak amplitude, phase, duty cycle, SAR

• Frequently used shapes and their frequency profile: hard, sinc, gauss, hermitte, fermi

• Limitation of the Fourier-design

Since the Fourier-design is limited to low flip-angles the SLR technique is an important tool for designing RF pulses with larger flip-angle. The lecture introduces the concept of the SLR design:

• Extra steps relative to the Fourier-design

• Beta-polynomial design

• Finding matching alpha-polynomial

• SLR and iSLR transforms, hard pulse approximation

The SLR algorithm discussed in the previous lecture is a versatile technique thanks to the wide range of possible beta-polynomials aka. SLR filters in the design procedure. Here we discuss:

• Linear phase

• Minimum phase

• Maximum phase

• Quadratic phase SLR filters

In this lecture the problem of imperfect B1-field is introduced in conjunction with a potential remedy in the form of adiabatic pulses through discussing:

• Transmit gain calibration and the problem with miscalibration and/or B1-field inhomogeneity

• Concept of B1-insensitive rotations

• Frequency sweep and the z-component of the B1-field

• Effective B1-field and its adiabatic change

• Adiabatic inversion example

Here we extend the concept of adiabatic inversion to refocusing and excitation:

• Problem with naive adiabatic refocusing - odd echo effect

• Phase compensation for adiabatic refocusing

• Adiabatic excitation - BIR4 pulses

• Mathematical form of HS, HSn and BIR4 pulses and their simulated performance

So far all the RF pulses were frequency selective. In this lecture we learn how to convert this frequency selectivity to spatial selectivity for spatially localised acquisition. The foci of this lecture are:

• The need for spatial localisation

• How gradient field converts frequency selectivity to spatial selectivity

• Slice thickness considerations

• Isodelay

• Slice cross talk and potential remedies

In the previous lecture we have learnt how to trade off frequency selectivity to spatial selectivity. However, sometimes we want to do both. For this, the lecture introduces the concept of SpSp pulse discussing the following topics:

• The need for SpSp pulses - the chemical shift artefact

• Fat-water problem in in vivo imaging and the slice position ambiguity

• SpSp pulses - selective excitation both in the spatial and frequency domain

• Link with the hard pulse approximation employed in the SLR framework

• Excitation profile and the problem of odd lobes

• Flyback design to eliminate odd lobes and the parameter relation of SpSp pulses

The final lecture discusses the hardware components of MR scanners that has anything to do with RF waves. This includes a closer look at the:

• Components of the Tx chain - how the RF pulses are generated

• Components of the Rx chain - the way of the signal from the RF coil to processed information

• Principles of RF coils via the simple but useful surface coil example

• Simple and practical tips on surface coil building as well as how to used RF coils in general