FACTORS AFFECTING THE SIGNAL TO NOISE RATIO ARE DESCRIBED IN DETAILIN THIS PPT. CONTENT TAKEN FROM MUTIPLE BOOKS AND GENERALS.

1. FACTORS AFFECTING
THE SIGNAL-TO-NOISE
RATIO
MR. ROHIT BANSAL
ASSISTANT PROFESOR (RADIO-PHYSICS)
MAMC,AGROHA

2. Signal-to-noise ratio (SNR) is a standard used to describe the
performance of an MRI system. An MRI image is not created by
pure MRI signals but from a combination of MRI signals and
unavoidable background noise.
MRI image = signal + noise
Noise in an MRI image does not contribute useful
information toward image formation and is produced by the
static fluctuation of signal intensity, usually appearing as grains
or irregular patterns. Noise in MRI is from two main sources:
1. Molecular movement - charged particles in the human body
create electromagnetic noise.
2. Electrical resistance from the receiver coils, data cables and
the electronic components of the measurement system.

3. Noise produced in the MRI image depends on:
1. The coil - the number of elements, type and size and of the
coil e.g., 8-channel body coil, 4channel flex coil.
2. Bandwidth differs in each pulse sequence.
In MRI the SNR is mainly used for image evaluation and
measurement of contrast enhancement. SNR is also used for
quality assurance, pulse sequence comparison and
radiofrequency (RF) coil comparison.

4. A high SNR is desirable in MRI.The SNR is dependent on the
following parameters:
– Slice thickness and receiver bandwidth
– Field of view
– Size of the (image) matrix
– Number of acquisitions
– Scan parameters (TR,TE, flip angle)
– Magnetic field strength
– Selection of the transmit and receive coil (RF coil)

5. SliceThickness and Receiver Bandwidth
To achieve optimal image resolution, very thin slices with a high
SNR are desirable.
However, thinner slices are associated with more noise, and so the
SNR decreases with the slice thickness.
Conversely, thicker slices are associated with other problems such
as an increase in partial volume effects.
The poorer SNR of thin slices can be compensated for to some
extent by increasing the number of acquisitions or by a longerTR.
Yet this is accomplished only at the expense of the overall image
acquisition time and reduces the cost efficiency of the MR imaging
system. The receiver bandwidth is the range of frequencies collected
by an MR system during frequency encoding.
The bandwidth is either set automatically or can be changed by the
operator.

6. A wide receiver bandwidth enables faster data acquisition and minimizes
chemical shift artifacts but also reduces SNR as more noise is included.
Halving the bandwidth improves SNR by about 30%.
With a narrow bandwidth, on the other hand, there will be more chemical
shift and motion artifacts and the number of slices that can be acquired for a
givenTR is limited.

8. Field ofView (FOV)
There is a close relationship between field of view (FOV) and SNR. When
matrix size is held constant, the FOV determines the size of the pixels.
Pixel size changes with the FOV. A smaller FOV results in a smaller pixel
size as long as the matrix is unchanged.
Pixel size is crucial for the spatial resolution of the MR image. With the
same FOV, a finer matrix (i.e., a matrix consisting of more pixels) results in an
improved spatial resolution.
Conversely, a coarser matrix (i.e., one with fewer pixels) results in a poorer
spatial resolution when the FOV is held constant.

10. Number of excitations (NEX) or number of signal averages/acquisitions
(NSA) is a measurement parameter. It is used to represent the number of
times each line of k-space data is acquired and is primarily used to improve
signal-to-noise (SNR) ratio. Doubling the NEX only improves the SNR by the
square root of two (√2) because random noise is also sampled. The noise in
the MRI image is from electromagnetic noise in the body resulting from
molecular movement and electrical resistance of the receiver coils.
Therefore, as the number of NEX increases, noise begins to be cancelled
out. Repeated sampling and the accumulation of signal results in a high
signal image.
Doubling the NEX will increase the SNR by 140% and double the scan
time. For example, if you take a sequence with a scan time of 2 minutes,
NEX 1 and SNR of 1 (100%) and increase the NEX from 1 to 2, this will
increase the scan time to 4 minutes and SNR by 140% or 1.4.
Number of Excitations

12. Other parameters affecting the SNR are the sequence used, echo time
(TE), repetition time (TR), and the flip angle.
The SNR increases with theTR but theT1 effect is also lost at longer TRs.
Conversely, the SNR decreases as theTE increases.
With a shortTE, theT2 contrast is lost.
For this reason, the option of shortening TE to improve SNR is available
only forT1-weighted sequences.
Imaging Parameters

14. Applying a higher magnetic field strength increases
longitudinal magnetization because more protons align along
the main axis of the magnetic field, resulting in an increase in
SNR.
The improved SNR achieved with high-field systems can be
utilized to generate images with an improved spatial resolution
or to perform fast imaging.
Magnetic Field Strength

15.  An effective means to improve SNR, without increasing voxel size or
lengthening scan time, is selecting an appropriate radiofrequency (RF) coil.
In general, an RF coil should be as close as possible to the anatomy being
imaged and surround the target organ.
The nearer the coil can be placed to the organ under examination, the
better the resulting signal.
RF coils can be used either to transmit RF and receive the MR signal or to
act as receiver coils only.
In the latter case, excitation pulses are delivered by the body coil.
VOLUMECOIL
SURFACE COIL
INTRACAVITY COILS
PHASED-ARREY COIL
Coils