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How radiographers can improve MR image - optimizingfactors affecting signal-to-noise ratio(SNR)

Poster No.: C-1658

Congress: ECR 2013

Type: Educational Exhibit

Authors: M. minov1, M. Popovska2, A. Doreski3, G. Markoski3, S.

Jovanoska4; 1Stip/MK, 2Kocani/MK, 3Skopje/MK, 4Bitola/MK

Keywords: Artifacts, Technical aspects, Physics, Education, MR, MR physics

DOI: 10.1594/ecr2013/C-1658

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Learning objectives

Managing important parameters that affect signal-to-noise (SNR). A high signal isdesirable in MRI, but here are number of factors which produce noise and degrade theMR signal.

Background

The relationship between the MR signal and the amount of image noise present isexpressed as the signal-to-noise ratio (SNR).

SNR is the quotient of the signal intensity measured in a region of interest(ROI)and thestandard deviation of the signal intensity in a region outside the anatomy or object beingimaged.

Imaging findings OR Procedure details

The signal to noise ratio depends both on some factors that are beyond the operator'scontrol and on factors that the user can change.

The SNR is dependent on the following parameters:

- Slice thickness and receiver bandwidth

- Number of acquisitions

- Magnetic field strength

- Field of view and image matrix

- Selection of the transmit and receive coil

- Scan parameters (TR, TE, flip angle)

To achieve optimal image resolution, thin slices with a high SNR are desirable, but thinnerslices produce more noise and decreasing SNR.

This loss of signal can be compensated by increasing the number of excitations(averages) or by a longer TR but this changes will extend image acquisition time.

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The receiver bandwidth is the range of frequencies collected by an MR system duringfrequency encoding.

A higher bandwidth is used for the reduction of chemical shift artifacts (lower bandwidth -more chemical shift - longer dwell time but better SNR bandwidths accentuate this waterfat shift by assigning a smaller number of frequencies across the MRI image. This effectis much more significant on higher field strengths.

Number of Excitations

The number of excitations (NEX) or number of signal averages (NSA) denotes how manytimes a signal from a given slice is measured or each line of k-space data is acquiredduring the scan.

Doubling the number of ac quisitions will increase the SNR. The approximate amount ofimprovement in SNR is calculated as the square root of the number of excitations.By using multiple averages, respiratory motion can be reduced in the same way thatmultiple averages increase the signal to noise ratio. NEX will increase SNR but will notaffect contrast unless the tissues are being lost in noise (low CNR). Scan time scalesdirectly with NEX and SNR as the square root of NEX.

Magnetic Field Strength

Applying a higher magnetic field strength increases longitudinalmagnetizationbecausemore protons align along the main axis of the magnetic field,resulting in an increasein SNR. MR systems with higher magnetic field give as more space for combinationsand changes in imaging parameters how we can obtain acceptable SNR for relativelyreasonable scan time. With this kind of systems we can generate fast imaging pulsesequence with improved spatial resolution and SNR.

Field of View and image matrix

There is a close relationship between field of view (FOV) and SNR. Smaller FOV dropdown the SNR but examination time stay unchanged.

A smaller FOV results in a smaller pixel size as long as the matrix is unchanged. Pixelsize is very important for the spatial resolution of the image so when we use matrix withmore pixels-the spatial resolution are improved, SNR drop down, and examination timeincreases. Conversely with the same FOV and matrix with fewer pixels result in poorerspatial resolution but SNR go up and examination time decreases.

The trick is to achieve high spatial resolution in reasonable time. This can be done byreducing of the "rectangular FOV" (phase encoding direction), because spatial resolution

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is determined by the frequency encoding direction, so changes in rectangular FOV orphase encoding just determine scan time and slight drop down SNR. Commonly this"trick" is used in imaging of the spine, extremities or MR angiography. Radiographersmust be very carful when make changes in FoV because this can be associated withwraparound (aliasing) artifacts from part of the body outside of FoV in phase encodingdirection.

Radiofrequency coil (RF coil)

Radiofrequency coil is very important in the MR image quality and choice of adequatecoil for different organs are preferable. I will just briefly describe below.

RF coil should be as close as possible to the anatomy being imaged and surround thetarget organ.

Volume coils completely surround the target organ and allows homogeneous signal onobject under examination. Head and extremity coils are examples of volume coils.

Surface coils are used for spinal MRI and imaging of small anatomic structures.

Intracavity coils are small coils inserted into body cavities and improves SNR imagequality of the target organ. Endorectal coil is example of intracavity coil and is use forexamination of the prostate and rectum.

Phased-array coils yield images with a high spatial resolution and allow imaging with alarger field of view as they improve both SNR and signal homogeneity.

Imaging Parameters

Other parameters affecting the SNR are the sequence used, echo time (TE), repetitiontime (TR), and the flip angle. The SNR increases with the TR but the T1 effect is alsolost at longer TRs. Conversely, the SNR decreases as the TE increases. With a shortTE, the T2 contrast is lost. For this reason, the option of shortening TE to improve SNRis available only for T1-weighted sequences

Images for this section:

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Fig. 1: Picture one show how SNR reacting when we change slice thickness from 5mm(A) to 3mm (B)

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Fig. 2: When we increase NEX/NSA from 1(A) to 3(B), signal-to-noise ratio increases butwith same changes increases and scan time.

Fig. 3: Image "B" show better SNR than image "A" because of higher magnetic field.

Fig. 4: Image "A" show how Coarser matrix allows better spatial resolution but lower SNRthan in case when we use fine matrix(image B).

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Fig. 6: Phase encoding direction and decreases of rectangular FoV allows better imagewithout artifacts and slightly decreases in SNR (image B)

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Fig. 5: Contrast agent allows extreme high SNR in T1 pulse sequence than nativ series.

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Conclusion

With the proper knowledge of the physics of the factors affecting SNR and operating skillsof the radiographers, they can significantly improve the quality of the examination andtherefore have an active role in good patient diagnostics.

References

1. Elster AD, Burdette JH (2001) Questions and answers in magneticresonance imaging 2nd ed. Mosby, St. Louis

2. Mitchell DG, Cohen MS (2004) MRI principles, 2nd ed. Saunders,Philadelphia Hendrick RE (1999) Image

3. Contrast and noise. In: Stark DD, Bradley WG Jr (eds) Magnetic Resonance

imaging, 3rd ed. Mosby-Year Book no 43. Mosby, St. Louis4. How does MRI Work 2nd edition Dominik Wieshaupt, Viktor D. Kocli Borut

Mrincek5. MR-Technology Information Portal6. MRI in practice Catherine Westbrook, Carolyn Kayt

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