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CONTENTS
Preface (Help) About the author
- Introduction NMRI or MRI? Opportunities in MRI Tomographic Imaging Microscopic property responsible for MRI
- The Mathematics of NMR Exponential Functions Trigonometric Functions Differentials and Integrals Vectors Matrices Coordinate Transformations Convolutions Imaginary Numbers The Fourier Transform
- Spin Physics Spin Properties of Spin Nuclei with Spin Energy Levels NMR Transitions Energy Level Diagrams Continuous Wave NMR Experiment Boltzmann Statistics Spin Packets T 1 Processes Precession T 2 Processes Rotating Frame of Reference Pulsed Magnetic Fields Spin Relaxation Bloch Equations
- NMR Spectroscopy Time Domain NMR Signal +/- Frequency Convention 90- FID Spin-Echo Inversion Recovery Chemical Shift
- Fourier Transforms
Introduction The + and - Frequency Problem The Fourier Transform Phase Correction Fourier Pairs The Convolution Theorem The Digital FT Sampling Error The Two-Dimensional FT
- Imaging Principles Introduction Magnetic Field Gradient Frequency Encoding Back Projection Imaging Slice Selection
- Fourier Transform Imaging Principles Introduction Phase Encoding Gradient FT Tomographic Imaging Signal Processing Image Resolution
- Basic Imaging Techniques Introduction Multislice imaging Oblique Imaging Spin-Echo Imaging Inversion Recovery Imaging Gradient Recalled Echo Imaging Image Contrast Signal Averaging
- Imaging Hardware Hardware Overview Magnet Gradient Coils RF Coils Qadrature Detector Safety Phantoms
- Image Presentation Image Histogram Image Processing Imaging Coordinates Imaging Planes
- Image Artifacts Introduction RF Quadrature Bo Inhomogeneity Gradient
[ Contents | Help | Glossary | Symbols ]
The Basics of MRI
Chapter 8
BASIC IMAGING TECHNIQUES
Introduction Multislice imaging Oblique Imaging Spin-Echo Imaging Inversion Recovery Imaging Gradient Recalled Echo Imaging Image Contrast Signal Averaging Problems
Introduction
In the previous chapter you learned the principles of Fourier transform magnetic resonance imaging. The examples presented were for a simplified 90-FID imaging sequence. Although the principles were correct, some aspects were simplified to make the presentation easier to understand. Some of these principles will be presented in a little more depth in this section. The 90-FID imaging sequence will be presented as a gradient recalled echo sequence in this section. The principles of multislice imaging and oblique imaging will be introduced. Two new imaging sequences called the spin-echo sequence and inversion recovery sequence will be introduced.
Multislice Imaging
An imaging sequence based on a 90-FID was introduced in Chapter 7. Based on this presentation, the time to acquire an image is equal to the product of the TR value and the number of phase encoding steps. If TR were one second and there were 256 phase encoding gradient steps the total imaging time required to produce the image would be 4 minutes and 16 seconds. If we wanted to take 20 images across a region of interest the imaging time would be approximately 1.5 hours. This will obviously not do if we are searching for pathology. Looking at the timing diagram for the imaging sequence with a
one second TR it is clear that most of the sequence time is unused. This unused time could be made use of by exciting other slices in the object. The only restriction is that the excitation used for one slice must not affect those from another slice. This can be accomplished by applying one magnitude slice selection gradient and changing the RF
frequency of the 90o^ pulses. Note that the three frequency bands from the pulses do not overlap. In this animation there are three RF pulses applied in the TR period. Each has a different center frequency 1 , 2 , and 3. As a consequence the pulses affect different
and 180o^ RF pulses.
The frequency encoding gradient is applied after the 180o^ pulse during the time that echo
is collected. The recorded signal is the echo. The FID, which is found after every 90o
pulse, is not used. One additional gradient is applied between the 90o^ and 180o^ pulses. This gradient is along the same direction as the frequency encoding gradient. It dephases the spins so that they will rephase by the center of the echo. This gradient in effect prepares the signal to be at the edge of k-space by the start of the acquisition of the echo.
The entire sequence is repeated every TR seconds until all the phase encoding steps have been recorded.
Inversion Recovery Imaging
In Chapter 4 we saw that a magnetic resonance signal could be produced by an inversion recovery sequence. An advantage of using an inversion recovery sequence is that it allows nulling of the signal from one component due to its T 1. Recall from Chapter 4 that the
signal intensity is zero when TI = T 1 ln 2. Once again, this sequence will be presented in
the form of a timing diagram only, since the evolution of the magnetization vectors from the application of slice selection, phase encoding, and frequency encoding gradients are similar to that presented in Chapter 7.
An inversion recovery sequence which uses a spin-echo sequence to detect the magnetization will be presented. The RF pulses are 180-90-180. An inversion recovery sequence which uses a 90-FID signal detection is similar, with the exception that a 90-FID is substituted for the spin-echo part of the sequence.
The timing diagram for an inversion recovery imaging sequence has entries for the RF
pulses, the gradients in the magnetic field, and the signal. A slice selective 180o^ RF
pulse is applied in conjunction with a slice selection gradient. A period of time equal to
TI elapses and a spin-echo sequence is applied.
The remainder of the sequence is equivalent to a spin-echo sequence. This spin-echo part recorded the magnetization present at a time TI after the first 180o^ pulse. (A 90-FID sequence could be used instead of the spin-echo.) All the RF pulses in the spin-echo sequence are slice selective. The RF pulses are applied in conjunction with the slice
selection gradients. Between the 90o^ and 180o^ pulses a phase encoding gradient is applied. The phase encoding gradient is varied in 128 or 256 steps between G (^) m and -G (^) m.
The phase encoding gradient could not be applied after the first 180o^ pulse because there is no transverse magnetization to phase encode at this point. The frequency encoding
gradient is applied after the second 180o^ pulse during the time that echo is collected.
The recorded signal is the echo. The FID after the 90o^ pulse is not used. The dephasing gradient between the 90o^ and 180o^ pulses to position the start of the signal acquisition at the edge of k-space, as was described in the section on spin-echo imaging. The entire sequence is repeated every TR seconds.
Gradient Recalled Echo Imaging
The imaging sequences mentioned thus far have one major disadvantage. For maximum signal, they all require the transverse magnetization to recover to its equilibrium position along the Z axis before the sequence is repeated. When the T 1 is long, this can
significantly lengthen the imaging sequence. If the magnetization does not fully recover to
equilibrium the signal is less than if full recovery occurs. If the magnetization is rotated
by an angle less than 90o^ its Mz component will recover to equilibrium much more
rapidly, but there will be less signal since the signal will be proportional to the Sin. So
we trade off signal for imaging time. In some instances, several images can be collected and averaged together and make up for the lost signal.
The gradient recalled echo imaging sequence is the application of these principles. Here is
its timing diagram. In the gradient recalled echo imaging sequence a slice selective RF
pulse is applied to the imaged object. This RF pulse typically produces a rotation angle
of between 10o^ and 90o. A slice selection gradient is applied with the RF pulse.
A phase encoding gradient is applied next. The phase encoding gradient is varied between G (^) m and -G (^) m in 128 or 256 equal steps as was done in all the other sequences.
A dephasing frequency encoding gradient is applied at the same time as the phase encoding gradient so as to cause the spins to be in phase at the center of the acquisition
period. This gradient is negative in sign from that of the frequency encoding gradient turned on during the acquisition of the signal. An echo is produced when the frequency encoding gradient is turned on because this gradient refocuses the dephasing which
occurred from the dephasing gradient.
A period called the echo time (TE) is defined as the time between the start of the RF pulse
and the maximum in the signal. The sequence is repeated every TR seconds. The TR period could be as short as tens of milliseconds.
Image Contrast
In order for pathology or any tissue for that matter to be visible in a magnetic resonance image there must be contrast or a difference in signal intensity between it and the adjacent tissue. The signal intensity, S, is determined by the signal equation for the specific pulse sequence used. Some of the intrinsic variables are the:
Spin-Lattice Relaxation Time, T 1
Spin-Spin Relaxation Time, T 2
Spin Density,
T 2 *
The spin density is the concentration of signal bearing spins. The instrumental variables are the:
SA and SB are determined by the signal equations given above. For any two tissues there
will be a set of instrumental paramenters which yield a maximum contrast. For example in a spin-echo sequence the contrast between two tissues as a function of TR is graphically
presented in the accompanying curve.
A contrast curve for tissues A and B as a function of TE is presented in the accompanying
curve.
To assure that signals from all phase encoding steps possess the same signal properties a few equilibrating cycles through the sequence are added to the beginning of every image acquisition. The necessity of this can be seen by examining the MZ and MXY components
as a function of time in a 90-FID type sequence. Note that the amount of transverse magnetization from a 90o^ pulse reaches an equilibrium value after a few TR cycles. This practice lengthens the imaging time by a few TR periods.
The magnetic resonance community has adopted nomenclature to signify the predominant contrast mechanism in an image. Images whose contrast is predominantly caused by differences in T 1 of the tissues is called a T 1 -weighted image. Similarly for T 2 and , the
images are called T 2 -weighted and spin density weighted images. The following table
contains the set of conditions necessary to produce weighted images.
Weighting TR Value TE Value T 1 < = T 1 < < T 2
T 2 > > T 1 > = T 2
T 1 < < T 2
It is impressive to see how the choice of the instrumental parameters TR, TE, TI, and
affect the contrast between the various tissues of the brain. In the accompanying set of graphics you can select an imaging sequence and the imaging parameters, the resultant image will be displayed in the graphics window. The spin-echo images are actual magnetic resonance images of the human brain. The remaining images are calculated images based on the signal equations above and a set of measured overall T 1 , T 2 , and
images of the human brain. The two bright circles to the bottom right and left sides of each calculated image are spin density standards, or phantoms, placed next to the head.
Spin-Echo Images
TE (ms) TR (ms) 20 40 60 80 250 500 750 1000 2000
Inversion Recovery Images (180-90)
TR (ms) TI (ms) 1000 2000 50 100 250 500 750
Gradient Recalled Echo Images ( TE=5 ms )
TR (ms) ( o^ ) 25 50 100 200
Signal Averaging
The signal-to-noise ratio (SNR) of a tissue in an image is the ratio of the average signal for
the tissue to the standard deviation of the noise in the background of the image. The signal-to-noise ratio may be improved by performing signal averaging. Signal averaging is the collection and averaging together of several images. The signals are present in each of the averaged images so their contribution to the resultant image add. Noise is random so it does not add, but begins to cancel as the number of spectra averaged increases. The signal-to-noise improvement from signal averaging is proportional to the square root of the number of images averaged (Nex). Nex is more commonly referred to as the number of excitations.
SNR Nex1/
Compare the results of averaging together the following number of images of a bottle of water.
Nex Nex1/2^ Image 1 1. 2 1. 4 2. 16 4.
