Magnetic resonance imaging is a medical imaging process that uses a magnetic field and radio frequency signals to produce images of various organs of the human body. MRI produces images that are distinctly different from the images produced by other imaging modalities such as X-Rays and CT scans. The image and resolution produced by MRI is quite detailed and can detect any minute changes of structures within the body . The contrast agents such as gadolinium are used in some procedures to increase the accuracy of the images. The cutaway view of an MRI machine is shown in Figure 4.1.
Figure 4.1Cutaway view of an MRI machine
MRI process can selectively image several different tissue characteristics which makes it different from other modalities. A potential advantage of this is that if a pathologic process does not alter one tissue characteristic and produce contrast, it might be visible in an image because of its effect on other characteristics. This causes the MRI process to be more complex than most other imaging methods. For optimizing an MRI procedure for a particular clinical examination, the user must have a good knowledge of the characteristics of the magnetic resonance (MR) image and how these characteristics can be controlled. In this chapter, we present a basic knowledge and overview of the MR image, how the image relates to specific tissue characteristics, and how image quality characteristics can be controlled.
The MR Image
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MRI is based on the principle of nuclear magnetic resonance (NMR). The principle of NMR for medical imaging was first demonstrated by Raymond Damadian in 1971 and Paul Lauterbur in 1973 . The MR image displays certain physical characteristics of human tissue. The MR image is the display of radio frequency (RF) signals that are emitted by the tissue during the image acquisition process. The source of the signals is a condition of magnetization that is produced in the tissue when the patient is placed in the strong magnetic field as shown in Figure 4.2. The tissue magnetization depends on the presence of magnetic nuclei in it. The specific physical characteristics of a tissue or fluid that is visible in the image depends on how the magnetic field is being changed during the acquisition process.
An image acquisition consists of an acquisition cycle, such as a heartbeat, which is repeated number of times. For every cycle, the tissue magnetization is forced with a series of changes. Different tissues and fluids progress through these changes at different rates. It is the level of magnetization that is present at a special "picture snapping time" at the end of each cycle which determines the intensity of the RF signal produced and the resulting tissue brightness in the image. MR images are generally identified with specific tissue characteristics or blood conditions that are the predominant source of contrast. These characteristics determine the level of tissue magnetization and contrast present at the time the "picture is snapped." The equipment operator, who sets the imaging protocol, determines the type of image that is to be produced by adjusting various imaging factors.
Figure 4.2 Gradient Magnetic Parts of the MRI
The characteristics which are commonly used as a source of image contrast fall into three different categories. The first, and most widely used, category is the magnetic characteristics of tissues. Characteristics of fluid (usually blood) movement fall in the second category. The spectroscopic effects related to molecular structure form the third category.
Principles of MRI
NMR is a phenomenon of magnetic systems that possesses both magnetic moment and an angular moment. All materials are made of atoms. Atoms consist of nuclei which contains protons and neutrons. Nuclei with odd number of these components possess a nuclear spin and a magnetic moment such as 1H, 2H, 13C, etc. Protons and neutrons combine with opposite spins to form nucleus. So when there is an even number of protons and neutrons, then there is no net spin where as odd occurrences possess a net spin. The Hydrogen nucleus which has single proton possesses a net spin. The fat and water present in the human body contain many hydrogen atoms. In MR images, the NMR signal from the hydrogen nuclei in the body tissues are captured.
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The net spin of the nucleus gives an angular moment which results in a current loop. This current generates a magnetic field. The combined effect of the angular moment and the magnetic field provides a magnetic dipole moment to the proton. When a material with net magnetic moment is placed inside a magnetic field, a proton with its magnetic dipole moment precesses around the field axis as shown in Figure 4.3. The frequency of precession is called as Larmor frequency, and is proportional to the applied magnetic field strength and defined by
where http://www.cs.sfu.ca/%7Estella/papers/blairthesis/main/_4056_tex2html_wrap5256.gif is the gyromagnetic ratio and http://www.cs.sfu.ca/%7Estella/papers/blairthesis/main/_4056_tex2html_wrap5292.gif is the strength of the applied magnetic field. The gyromagnetic ratio is a nuclei specific constant. For hydrogen,http://www.cs.sfu.ca/%7Estella/papers/blairthesis/main/_4056_tex2html_wrap5260.gif. To obtain an MR image of an object, the object is placed in a uniform magnetic field , between 0.5 to 3.0 Tesla. As a result, the object's hydrogen nuclei align with the magnetic field. It creates a net magnetic moment http://www.cs.sfu.ca/%7Estella/papers/blairthesis/main/_4056_tex2html_wrap5180.gif, parallel to
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Figure 4.3. (a). Alignment of odd magnetic moment of hydrogen nuclei in the
absence of a strong magnetic field. (b) Alignment of the hydrogen
nuclei when the strong magnetic field, , is applied about
the direction of the field.
A radio-frequency (RF) pulse, , is applied perpendicular to . This pulse, causes http://www.cs.sfu.ca/%7Estella/papers/blairthesis/main/_4056_tex2html_wrap5180.gifto tilt away from . Some energy and a measurable RF signal is gained by the nuclei in the realignment. The nuclei realign when RF signal is removed, their net magnetic moment, http://www.cs.sfu.ca/%7Estella/papers/blairthesis/main/_4056_tex2html_wrap5180.gif, is again parallel with . This process of returning to equilibrium is called as relaxation. During relaxation, the nuclei lose energy by emitting their own RF signal as shown in Figure 4.4. This signal is referred to as the free-induction decay (FID) response signal. A conductive field coil placed around the imaging object measures the FID response signal. This measurement is processed or reconstructed to obtain 3D grey-scale MR images.
Figure 4.4. (a) The net magnetic moment of the nuclei, http://www.cs.sfu.ca/%7Estella/papers/blairthesis/main/_4056_tex2html_wrap5180.gif, tilted away from by
the RF pulse, ,(b) Returning of nuclei to equilibrium such that http://www.cs.sfu.ca/%7Estella/papers/blairthesis/main/_4056_tex2html_wrap5180.gif
is again parallel to .
4.4 Components of MRI system
Figure 4.5 shows a schematic representation of the mahor systems of a magnetic resonance imager. The magnet produces the field for the imaging procedure. Within the magnet are the gradient coils for producing a gradient in in the X, Y, and Z directions. Within the gradient coils is the RF coil. The RF coil produces the Brf magnetic field necessary to rotate the spins by 900, 1800, or any other value selected by the pulse sequence. The RF coil also detects the signal from the spins within the body. The patient is positioned within the magnet by a computer controlled patient table. The table has a positioning accuracy of 1mm. The scan room is surrounded by an RF shield. The shield prevents the high power RF pulses from radiating out through the hospital. It also prevents the various RF signals from television and radio stations from being detected by the imager. Some scan rooms are also surrounded by a magnetic shield which contains the magnetic shield from extending too far into the hospital. In newer magnets, the magnetic shield is an integral part of the magnet.
Figure 4.5 Schematic representation of MRI scanner
The heart of the imager is a computer. It controls all components on the imager. The RF components under control of the computer are the radio frequency source and pulse programmer. The source produces a sine wave of the desired frequency. The pulse programmer shapes the RF pulses into apodized sine pulses. The RF amplifier increases the pulse power from millli watts to kilo watts. The computer also controls the gradient pulse programmer which sets the shape and amplitude of each of the three gradient fields. The gradient amplifier increases the power of the gradient pulses to a level sufficient to drive the gradient coils. The image processor, located on some imagers, is a device which is capable of performing a two-dimensional Fourier transform in fractions of a second. The computer off loads the Fourier transform to this faster device. The operator of the imager gives input to the computer through a control console. An imaging sequence is selected and customized from the console. The operator can see the images on a video/image display or can make hard copies of the images on a film printer. A typical MRI suite with main components like MRI scanner, image acquisition system, and movable table is shown in Figure 4.6.
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Figure 4.6 MRI scanner with movable table
Imaging coordinates and planes
Clinical imagers do not use the XYZ magnetic resonance coordinate system for collection and presentation of images. Instead the anatomic coordinate system is used. In this system the axes are referenced to the body as shown in Figure 4.7. The three axes are left-right (L/R), anterior-posterior (A/P), and superior-inferior (S/I).
Similarly, on clinical imagers the terminology XY, YZ, and XZ are not used to indicate the imaged planes. Instead they are called as axial or transverse, sagittal and coronal respectively as shown in Figure. An imaged plane perpendicular to the long axis (Z-axis) of the body is called an axial plane. The sides of this plane are L/R and A/P. A plane bisecting the left and right sides of the body is called a sagittal plane. This plane is perpendicular to the X-axis and parallel to the field of gravity (g). The sides of this plane are S/I and A/P. A plane bisecting the front of the body from the back is referred to as a coronal plane. The sides of this plane are L/R and S/I. This plane is mutually perpendicular to both axial and sagittal plane.
Figure 4.7 Anatomical coordinates and planes
MRI signal intensity produced by a particular tissue depends on at least three intrinsic tissue parameters: the proton density, which determines , and the relaxation times T1 and T2. Hence, a number of MR imaging techniques("weightings") are available to choose from, which accentuate some properties and not others. They are PD-weighted or PD, T1-weighted or T1, and T2 weighted or T2 images.
Two more additional parameters that control the image contrast are repetition time (TR) and echo time (TE). When MR images are acquired, the RF pulse is repeated at a predetermined rate. The period of the RF pulse sequence is TR that is measured in milliseconds. For a given type of nucleus in a given environment, TR determines the amount of T1 relaxation. The longer the TR, the more the longitudinal magnetization is recovered. Tissues with short T1 have greater signal density than tissues with a longer T1 at a given TR. A long TR allows more magnetization to recover and thus reduces differences in the T1 contribution in the image contrast.
The FID response signals can be measured at various times within the TR interval. The time between which the RF pulse is applied and the response signal is measured is the TE. The TE is the time when the spin echo occurs due to the refocusing effects of the 180 degree refocusing pulse applied after a delay of TE/2 from the RF pulse. TE determines how much decay of the transverse magnetization is allowed to occur before the signal is read. It therefore controls the amount of T2 relaxation. The TR and TE control the local tissue relaxation times, T1 and T2, affect the signal. The application of RF pulses at different TRs and receiving of signals at different TEs produce variation in contrast in MR images. By adjusting TR and TE the acquired MR image can be made to contrast different tissue types. Table 4.1 shows the set of conditions necessary to produce the MR weighted images in terms of TR and TE values.
Table 4.1 : MRI imaging techniques in terms of TR and TE
Table 4.2: Image quality characterized by the imaging parameters
Increase Parameters below
Max. Number of slices
FOV (Field of view)
NEX (Number of excitation or acquisition)
Inter slice gap
In MRI, the quality of image is generally described in terms of signal to noise ratio (SNR), spatial resolution, contrast and acquisition time. They all are interrelated and the change in one affects the others. Hence, a radiologist has to decide what factors are more important for an examination. The trade off between these factors are given in Table 2. This table shows the effect on the top row of parameters, of increasing the variables in the column on the right. A plus (+) sign indicates an increase, a minus (-) sign indicates a decrease, and (nc) indicates no change. Many of these parameters are interdependent. so we are looking at the direct effects of changes of single variables.
Image Types and Tissue Characteristics
Proton Density (PD) Images
One of the tissue characteristic that can be imaged most directly is the concentration or density of protons (hydrogen). In a proton density image the tissue magnetization, RF signal intensity, and image brightness are determined by the proton (hydrogen) density of the tissue. Tissues that are rich in protons will produce strong signals and have a bright appearance compared to the tissues with less protons.
Magnetic Relaxation Times
During an MRI procedure, the tissue magnetization is cycled by flipping it into an unstable condition and then allowing it to recover. This recovery process is known as relaxation. The time required for the magnetization to relax varies depending on the tissue type. This relaxation time is commonly used to differentiate (i.e., produce contrast) normal and pathologic tissues. Every tissue is characterized by two relaxation times: T1 and T2. Images can be created in which one among these two characteristics is the predominant source of contrast. It is impossible to create images in which one of the tissue characteristics (e.g., PD, T1, or T2) is the only pure source of contrast. Usually, there is a mixing or blending of the characteristics. But, an image will be more heavily weighted by one of them. When an image is described as a T1-weighted image, it means that T1 is the predominant source of contrast. There is also some possible contamination from the PD and T2 characteristics and similarly for PD as well as T2. Figure 4.5 shows 2d slices from the weighted MRI volumes.
Figure 4.3. (a) A proton density (PD) weighted MR image slice. (b) The same T2-weighted slice.
When the imaging protocol is set to produce a T1-weighted image, it is the tissues with the short T1 values that produce the highest magnetization and are the brightness in the image.
When the imaging protocol is set to produce a T2-weighted image, it is the tissues with the long T2 values that are the brightest. This is because they have a higher level of magnetization at the picture snapping time.
A typical examination will consist of at least one set of contiguous slices. In most cases the entire set of slices is acquired simultaneously. However, the number of slices in a set can be limited by certain imaging factors and the amount of time allocated to the acquisition process. The slices can be oriented in virtually any plane through the patient's body. The major restriction is that images in the different planes cannot generally be acquired simultaneously. For example, if both axial and sagittal images are required, the acquisition process must be repeated.
Visual noise is a major issue in MRI. The presence of noise in an image reduces its quality, especially by limiting the visibility of low contrast objects and differences among tissues. Most of the noise in MR images is the result of a form of random, unwanted RF energy picked up from the patient's body. The amount of noise can generally be controlled through a combination of factors. However, many of these factors involve compromises with other characteristics.
Artifacts are undesirable objects, such as streaks and spots, that appear in images which do not directly represent an anatomical structure. They are usually produced by certain interactions of the patient's body or body functions (such as motion) with the imaging process. There are numerous kinds of artifacts that can occur in MRI. Some of them affect the quality of the MRI but some do not affect the diagnostic quality. Artifacts are categorized based on the source of cause presented in the Table 4.3.
Table 4.3 : MRI artifacts
Failure of the RF detection circuitry
Metal object distorting B0 field
Failure in a magnetic field gradient
Failure of RF coil
Movement of imaged object during the sequence
Movement of body fluids during the sequence
Large B0 and chemical shift difference between tissues
Large voxel size that is averaged by many tissues
Improper chosen FOV
Lack of sampling data
There is a selection of techniques that can be used to reduce the presence of artifacts.
The general spatial characteristics of the MR image are described previously. However, when setting up an imaging protocol the spatial characteristics must be considered in the general context of image quality.
Image Acquisition Time
When considering and adjusting MR image quality, attention must also be given to the time required for the acquisition process. In general, several aspects of image quality, such as detail and noise, can be improved by using longer acquisition times.
An optimum imaging protocol is one in which, there is a proper balance among the image quality characteristics described above and also a balance between overall image quality and acquisition time. The imaging protocol that is used for a specific clinical examination has a major impact on the quality of the image and the visibility of anatomical structures and pathologic conditions. Therefore, the users of MRI must have a good knowledge of the imaging process and the protocol factors and know how to set them to optimize the image characteristics. The five major image quality characteristics such as contrast sensitivity, detail, noise, artifacts, and spatial can be controlled to a great extent by the settings of the various protocol factors. MRI is a powerful diagnostic tool because the process can be optimized to display a wide range of clinical conditions. However, maximum benefit requires a staff with the knowledge to control the process and interpret the variety of images.
Advantages of MRI
MRI is preferred over other modalities especially for children and patients since it is not using ionizing radiation
MRI can produce the image with greater detail thereby depicting the abnormalities with more sensitive
MRI is so flexible that it can image in any plane without altering the position of the patient
MRI agents cause less harm compared with others
MRI can overcome the artifacts by bones in CT images
So, it is concluded that MRI is the best source for any of the clinical studies and researches related with medical science