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MAGNETIC RESONANCE IMAGING (MRI) PHYSICS AND INSTRUMENTATION |
Title: MRI Image Quality
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ANIS YUSLIYANA BT YUSOF |
TABLE OF CONTENTS
2.0 SIGNAL TO NOISE RATIO (SNR)
3.0 CONTRAST TO NOISE RATIO (CNR)
5.0 THE RELATIONSHIP BETWEEN MRI PARAMETERS AND IMAGE QUALITY
Table 1: MRI parameters trade-offs (Proprofs.com, 2015)
1.0 INTRODUCTION
Image quality is the most important element in imaging radiography. According to Courses Washington Education, (2015) image quality must be assessed on the basis of average performance of some task of interest by some observer or decision maker. It was stated that image quality must be defined in terms of a task on what information to be retrieved from an image; and the observer on how the information will be extracted from the image.
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Since MRI image is a digital image, it is greatly depends on image contrast and its spatial characteristics. Nevertheless, one of the disadvantages of this flexibility is a greater difficulty in terms of the choice of scanning parameters. In general scan times are not negligible and there is a certain tendency towards artefact. However, the most fundamental limitation in MRI is the signal-to noise ratio (SNR) which is dependent upon the hardware, mostly the main field strength and radiofrequency (RF) coils, upon the relaxation properties of tissue and upon the choice of sequence parameters. Good image quality depends upon making good scanning parameter choices (McRobbie, 2007).
2.0 SIGNAL TO NOISE RATIO (SNR)
The signal to noise ratio can be defined as the ratio of the amplitude of the signal received to the average amplitude of the noise whereas the signal is the voltage induced in the receiver coil by the precession of the net magnetic vector in the transverse plane (Westbrook et al. 2011, pp.104).
Noise can be defined as an undesirable backgroundinterferenceor disturbance that affects image quality (Mr-tip.com, 2015). According to Weber (2015), noise is caused by two factors which are the electromagnetic noise in the body due to movement of charged particles; and small anomalies in the measurement electronics, which depends on the size of the RF coil and the bandwidth of the pulse sequence.
In addition, noise occurs at all frequencies and is also random in time and space. On the other hand, the signal is cumulative, and occurs at time TE, depends on many factors and it can be altered. The signal is therefore can be changed in relation to the noise. Increasing the signal increases the SNR, while decreasing the signal decreases the SNR. Therefore, any factor that affects the signal amplitude in gives effect to the SNR. The factors that affect the SNR include magnetic field strength, the proton density, voxel volume, repetition time (TR), echo time (TE), flip angle, number of excitations (NEX), receive bandwidth and coil type (Westbrook et al. 2011, pp.104).
According to McRobbie in his book, MRI from Picture to Proton (2007), images with a poor SNR will appear fuzzy. An important aspect of image optimization is to certify that there is a high enough SNR for the images to be diagnostically valuable yet low SNR may result in losing small details or the obscuring of subtle contrast changes. Therefore, contrast to noise ratio (CNR) is always taken into consideration in the aspect of image quality.
Figure 1: Increasing the basic resolution will increase the image quality. However, increasing the resolution more than the acceptable range will produce grains in the image due to low SNR and reducing it will produce a blurry image due to high SNR. Increasing basic resolution will result in prolonged time. (Image adapted from Mrimaster.com, 2015)
3.0 CONTRAST TO NOISE RATIO (CNR)
CNR can be defined as a measure to assess the ability of an imaging procedure to generate clinically useful image contrast. However, the image contrast itself is not precise enough to qualify an image, because in a noisy image it is uncertain where the contrast originates. It depends on two factors either due true tissue contrast, or it may be due to noise fluctuations. The human ability to distinguish between objects is proportional to contrast, and it decreases linearly with noise (KTH, 2015).
By improving CNR the perception of the distinct differences between two clinical areas of interest will be increased. In a simple word, acontrasttonoiseratio is a summary ofSNRandcontrast. It is the difference inSNRbetween two relevant tissue types (Mr-tip.com, 2015).
CNR is controlled by the same factors that affect SNR. However, it is considered as the most critical factor affecting image quality (scrsl.weebly.com, 2015).
4.0 RESOLUTION AND SCAN TIME
In MRI imaging, the scan time is advisable to be as short as possible. This is because the longer the patient lies on the table; the more likely it is that they will move. Moreover, if the patient has moved during the scan, the image produced will have a great SNR (Westbrook et al. 2011).
The minimum scan time in MRI imaging is affected by TR, matrix size and NEX while the spatial resolution is determined by matrix size, FOV and slice thickness. By increasing matrix size or decreasing FOV and slice thickness increases spatial resolution at the expense of either decreased signal-to-noise or increased scan time. In order to obtain images of high resolution with high signal-to-noise requires longer scan times. All of the scan parameters affect signal-to-noise ratio. However, the signal within an image can be enhanced either by increasing TR, FOV, slice thickness and NEX or by decreasing TE and matrix size. The most direct way to increase signal is by increasing NEX, but increasing NEX from two to four which doubles the scan time, increases the signal by only the square root of two. Lastly, TE does not affect scan time; however, it does determine the maximum number of slices in multi-slice mode. Increasing the TE or shortening TR decreases the number of slices that can be obtained with one pulse sequence (Spinwarp.ucsd.edu, 2015).
5.0 THE RELATIONSHIP BETWEEN MRI PARAMETERS AND IMAGE QUALITY
An image that is obtained in a short scan time, with a good spatial resolution and high SNR is preferable yet is hardly to achieve as increasing one factor certainly reduces one or both of the other two (Westbrook et. al, 2011).
Trade-offsexists when changing imaging parameters to obtain the best images possible. For instance, the SNR, resolution, and acquisition time, are all interconnected. Changing one will affect the others. It is important to decide what factors are more important for an examination of a particular body part, patient and suspected abnormality.
For example, when looking at the pituitary or cranial nerves, some SNR may need to be less considered or use longer acquisition time to improve the spatial resolution. However, in a claustrophobic of patient in pain who may be moving around, both resolution and SNR for the shortest possible examination time need to be considered to produce better image quality and preventing motion artefact (Ballinger, 2015). Thetable below summarizes the trade-offs in MRI between SNR, resolution, time, maximum number of slices and distance covered.
|
Parameters |
Effect on MRI Image |
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Increased TR |
Increased Longitudinal Relaxation Increased SNR Increase of maximum allowable slices Increased scan time Decrease T1 weighted contrast |
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Increased TE |
Increase transverse relaxation Increased T2 weighted contrast Decrease SNR Decrease number of allowable slices Increased gradient echo susceptibility |
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Increased Inversion Time |
Decreased T1 weighted contrast Very short: null fat (STIR) Long: null CSF (FLAIR) |
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Larger Flip Angle |
Better SNR |
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Increased Slice Thickness |
Increased SNR More anatomical coverage Increased partial volume artefact Decreased spatial resolution |
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Increasing Gap |
Decreased cross talk Increased anatomical coverage Increased risk of missing important anatomy or pathology |
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Increased Matrix: Phase Steps (Y) |
Decreased SNR Increased spatial resolution Increased scan time |
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Increased Matrix: Frequency steps |
Decreased SNR Increased spatial resolution No effect on scan time |
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Increased NEX |
Increased SNR Increased scan time Decrease motion artefacts |
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Increased Receiver Bandwidth |
Decreased SNR Decreased chemical shift artefact No effect on scan time |
Table 1: MRI parameters trade-offs (Proprofs.com, 2015)
6.0CONCLUSION
In conclusion, the quality of an MR image depends on several factors which include the spatial resolution and image contrast, SNR and CNR and also artefacts. An MR examination is cooperation between scan time and image quality and its sequence parameters will have to be optimized in function of the organs and pathology.
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Moreover, the signal intensities and contrast are determined by the timing parameters TR and TE and also the flip angle. Besides, to produce a good image in MRI the scan time should always be as short as possible to avoid patient movement by using the shortest TR possible, select the coarse matrix possible and reduce the NEX to a minimum.
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