MRI physics ch 4
Increasing the FOV size ____ spatial resolutin; however a small FOV results in ____ voxels and therefore _____SNR
decreases smaller reduced.
Any selection that increases the voxel _____ (decreases/increases) the SNR
increases
Reducing bandwitch _____ chemical shift artifact
increases
When magnetic field strength increases, SNR ______
increases
Increasing the signal _____ the SNR, while decreasing the signal _____ the SNR Therefore any factor that affects signal amplitude in turn affects _______
increases decreases SNR
How do phased array coils affect SNR?
incresase even more than quadrature because data from several coils are added together
A course matrix is....
one with allow number of frequency encodings and/or phase encodings and results in a low number of pixels in the FOV
If flip angle is reduced from 90 to 10, what will happen to SNR =?
significantly reduced.
The ______ aslo detemines the pixel dimensions.
size of the FOV
The pixel area is determined by the...... Therefore....
size of the For and the number of pixel in the matrix pixel area = FOV dimensions / matrix size
Volume imaging is advantages in that..... because....
very small lesions can be demonstrated because slice thickness can be drastically reduced compared with conventional imaging, and there is no slice gap.
The voxel represents a _____ within the patient t and is determined by the....
volume of tissue by the pixel area and the slice thickness
SNR is proportional to.....
voxel volume and any parameter that alters the size of th voxel changes the SNR
anisotropic
voxels that are not the same direction in all three planes
Isotropic
voxels that are the same in all three planes
Any selection that decreases the size of the voxel decreases the SNR and vice versa. This is achieved in what ways
1. Changing the slice thickness 2. Changing the image matrix 3. Changing the FOV
How does changing the image matrix increase or decreases the SNR?
1. Image matrix = number of pixels in image 2. Identified by two numbers: a. number of pixels in frequency direction . (usually short axis of image) b. number of pixels in the phase direction (usually the short axis of image) 3. If phase matrix is increased from 128 to 256, FOV will remain the same, but image will be better because there are smaller pixels and therefore voxels. See page 110.
Factors that affect SNR include....
1. Magnetic field strength of the system 2. Proton density of the area under examination 3. voxel volume 4. TR, TE, and flip angle 5. NEX 6. Receive bandwidth 7. Coil type
Describe how proton density affects SNR?
1. Number of protons in area under examination determines amplitude of signal received 2. Areas of low proton density (like lungs) have low signal ---> low SNR 3. High proton density (like pelvis) have high signal ---> high SNR
The four main considerations determining image quality are...
1. Signal to noise ratio (SNR) 2. contrast to noice ratio (CNR) 3. Spatial resolution 4. Scan time
Factors that affect scan time....
1. TR 2. Phase matrix 3. NEX
CNR is increased in what ways?
1. Using T2 weighted image 2. Using contrast agents 3. Using chemical pre-saturation technique 4. Using magnetization transfer contrast
Describe how TR, TE, and flip angle affect SNR
1. Usually parameters that affect image contrast, but influence SNR and overall image quality.
Voxel
1. building unit of a digital image is a pixel 2. brightness of the pixel represents the strength of the MRI signal generated by a voume of patient tissue. 3. Voxel = unit of volume in the patient tissue
Voxel size if affected by...
1. slice thickness 2. FOV 3. Number of pixels in the matrix
Typically, the operator controls the geometry of a voxel by selecting what?
1. the fov dimensions 2. the image matrix 3. the slice thickness
To reduce likelyhood of patient movement, the scan time shold always be as short as possible, to acheive the shortest scan time, what must happen?
1. use the shortest TR possible 2. select the coarsest matrix possible 3. reduce the NEX to a minimum
To double the SNR we need to increase the NEX and the scan time by a factor of.... To triple it requires ______ increase in NEX and scan time
4 ninefold
Halving the bandwidth increases SNRF by about ___%, but it ______ the sampling time or acquisition window. As a result, reducising bandwidth _____ minimum TE
40 increases increases
How does using a T2 weighed image increase CNR
Although a T2 weighed image often has a lower SNR than at T1 weighed image (due to longer TE), the ability to distinguish tumor from normal tissue is often much greater because of the high signal of tumor compared with the low signal surrounding the anatomy, i.e CNR is higher.
Why does minimum TE increase when gradient slopes and rise times increase?
Because the system cannot collect the signal until all the gradient functions have been completed.
When scanning anatomy t hat has a smaller dimension in the phase axis than frequency, why would you desire a rectangular FOV instead of a square FOV?
Because to acquire a square FOV, high resolution image is costly in time, and rectangular FOV maintains spatial resolution but reduces the scan time as only a portion of the total number of phase encoding that are normally required are performed
How does using chmical pre-saturation technique affect CNR?
By saturating out normal anatomy, pathology is often seen more clearly
What is the most critical factor affecting image quality? Why?
CNR because it directly determines the eyes ability to distinguish areas of high signal from areas of low signal
CNR
Contrast to Noise Ratio difference in the SNR between two adjacent areas and it is controlled by the same factors that control SNR
Refresher: what is NEX
Controls amount of data stored in K space Chest of drawers analogy: number of times each drawer is filled with date
How does changing the FOV increase or decrease the SNR?
Example, if you half the FOV, you half the pixel dimension along both axes. Therefore, the voxel volume and SNR are reduced to one quarter of their original value (1000mm to 250mm). SNR will be reduced, but resolution will be increased. See page 112-113 figures 4.10 and 4.11
How does changing the thickness increase or decreases the SNR
For example, if voxel size is halved fro by having slice thickness of 10mm to 5mm, doing this halves the voxel volume from 1000mm^3 to 500mm^3, and hence the SNR the 10mm slice thickness (the thicker) will have better SNR than the 5mm slice thickness (the thinner). See page 108 for example
How do quadrature coils affect SNR?
Increase is because two coils are sued to receive signal
Reducing the slice thickness does what to spatial resolution? Increasing the slice thickness does what to spatial resolution? And does what to partial volume
Increases reduces; and increases
_____ the matrix increases the spatial resolution ____ the matrix increases spatial resolution However, fine matrices result in smaller voxels, and threfore _____ SNR
Increases Increasing Reduced
Magnetic field strength cannot be altered, when imaging low field systems, what can happen to SNR and what can you do about it?
It can be compromised and steps may have to be taken to boost the SNR that are not necessary in high field systems. This usually manifests in longer scan times.
Increasing TR does what to scan time? It also increases the chances what?
It increases scan time and increases chance of patient movement
What will happen if you angle the coil (this sometimes happens when using surface coils)?
It results in a reduction of SNR
_____ voxels result in low spatial resolution, because...
Large small structures are not resolved so well.
Why are scan times important in maintaining image quality?
Long scans give the patient more chance to move during the acquisition
Data contains both....
signal and noise
Describe MTC
Magnetization transfer contrast 1. Only protons that have sufficiently long T2 times can be imaged in MR 2. Other protons whose transverse components decay before the signal can be collected cannot be visualized adequately. 3. These protons, mainly bound to large proteins, membranes, and other macromolecules are called bound protons. 3. The protons that have longer T2 times can be visualized are are termed free protons. 4. There is always a transfer of mag between the free protons and the bound which causes a change in T1 values of the free protons. 5. This can be exploited by selectively saturating bound protons, which reduces the intensity of the signal from the free protons due to MTC 6. The MTC saturation band is applied before the excitation pulse at a bandwidth that selectively destroys the transverse components of magnetization of the bound protons. 6. The use of MTC increases CNR between pathological and normal tissues and is useful in many areas, including angiography and joint imaging.
Uses of volume imaging
Many potential applications Widely use fro imaging joints, esp. knees were anatomy is confusing and not strictly in plane. Very useful fro flooring ligaments or other structures that cross over the imaging plane. Lesions in the temporal lobes or posterior fossa
SNR
Ratio of the amplitude of the signal received to the average amplitude of the noise
Protocols in MRI
Set of rules In MRI these rules are a variety of parameters that are selected by the operator Include extrinsic contrast parameters, geometry parameters and a variety of imaging options.
TR determines the amount of....
T1 and proton density
Spatial resolution
The ability to distingues between two points as seperate and distinct, and is controlled by voxel size.
Flip angle controls
The amount of T1 and proton density weighing
The flip angle controls The maximum signal amplitude is created with flip angles of....
The amount of transverse mag that is created which induces a signal in the coil 90 degrees
Noise is constant in every patient. It depends on what?
The build of the patient the area under examination the in hereunto noise of the system
How do you induce maximum signal for SNR with coil
The coil must be positioned in the transverse plane perpendicular to B0.
In conventional imaging the slice thickness affects the SNR. IN volume imaging..... The other main advantage of volumes is...
The entire volume of tissue is excited and the volume contains no gap, the SNR is superior and so fewer NEX can be used. Data are collected from a slab, the slab can be manipulated to look at the anatomy within the volume in any plane and at any angle of obliquity
How does phase matrix affect scan time?
The number of phase encodings determines the number of lines in k space or number of drawers that are filled with data to complete the scan. If number phase phase encoding doubles, scan time doubles
How does NEX affect scan time?
The number of times data are collected with the same slope of phase encoding gradient or number of times each drawer is filled wit data. Dobling the NEX doubles the scan time
256 x 128 matrix means what?
The pixels are rectangular and it results in decreased spatial resolution
How does using contrast agents increased CNR?
The purpose of administering contrast agents is to increased the CNR between pathology (which enhances) and normal anatomy (which does not)
What if the voxels are not isotropic in volume imaging?
The volume has poorer resolution in the planes other than the one in which it was acquired. For example, if FOV of 240mm and matrix of 256 x 25, each pixel has a dimension of 0.9mm (FOV/matrix), if slice thickness selected n 3mm, resolution is worse when the voxel is viewed from the side. This means voxel is anisotropic
A long TE allows for...
a considerable amount of decay of transverse mag to occur before echo is collected, while a short TE does not
Large coils increase the likelyhood of...
aliasing
Noise occurs at ________ and is also ______ in time and space The signal however is _______, occurs at time of ______, and depends on ______ and can be ______
all frequencies randomly cumulative TE many factors altered
Signal is not random, it's....
always occurs at the same place when it is collected.
The type of coil used affects _______ and therefore the _____
amount of signal received SNR
In large voxels, individual signal intensities are...
averaged together and not represented as distinct within the voxel
Any selection that decreases the size of the voxel _____ (decreases/increases) the SNR
decreases
As TE increases, SNR_____. This is because... This is why...
decreases there is less transverse mag available to be rephrased in the echo. T2 weighted images have lower SNR than T1 or PD weighed sequences that use a short TE
Doubling the NEX does what? Halving the K space does what?
doubles the amount of data stored in each line halves the amount of data
the size of the FOV is determined by the _____ gradient. To achieve small FOV the _______ gradient slope must be _____
frequency encoding gradient frequency encoding gradient steep
Spin echo pulse sequences generally Gradient echo pulse sequences... In addition...
have more signal than gradient echo sequences as the longitudinal mag is converted into strangers mag by 90 degree flip angle only convert a proportion of longitudinal mag into transverse mag as they use flip angle other than 90 The 180 degree rephrasing pulse is more efficient at rephrasing than the rephrasing gradient of gradient echo sequences so the resultant eho has greater signal amplitude
Large voxels have _____ (higher/lower) than small voxels
higher
Steep gradients result in ______ rise times for the gradient than shallow gradient slopes Steep gradient slopes therefore......
higher stress the gradient coils more than shallow gradient slopes
Usually the frequency matrix is the _____ number and the phase matrix is........
highest altered to change scan time and the resolution
The dimension of the FOV in the phase direction is reduced compared to that in the frequency direction and so should be used when...
imaging anatomy that fits into a rectangle, for example, an L spine
What are the disadvantages of volume imaging?
in general - long scan time - so used in conjunction with after pulse sequences
A small FOV, thin, slicces, and fine matrices ______ the minimum TE and may result in.... If the TE increases, the selection and encoding of each slice is ______ and therefore
increase fewer slices being available longer fewer slices can be excited in a given TR.
How do surface coils placed close to the area under examination affect SNR
increase it
If signal bandwidth decreases (narrow bandwidth) what happens to SNR
it increases as less noise is sampled as a proportion of signal
If TR is increased from 140ms to 700 ms, what happens to SNR? Why?
it will improve as the TR increases. This is because as TR increases more longitudinal mag is available to create transverse mag after excitation.
For a given matrix, a large FOV results in ______ pixels, while a small FOV results in ____ pixels
large small
Increasing FOV results in ______ voxels and therefore ______ SNR
large reduced
For a given FOV a course matrix results in....
large pixels and voxels
Reducing receive bandwidth results in....
less noise being sampled relative to signal
In volume imaging, the greater the slices the perspired, the ______ the scan time. However, this is offset by.....
longer the fact that the greater the slice number the greater the SNR and so the NEX can be reduced.
TR controls the amount of A long TR allows A short TR...
longitudinal mag can occur before next excitation pulse is applied full recovery of longitudinal mag so that more is variable to be flipped in the next rep does not allow full recovery of longitundal mag so less is available to be flipped.
Large voxels contain ____(more/less) than small voxels, and therefore have ____ (more/less) nuclei within them to contribute toward signal
more more
By applying a filter over the frequency encoding gradient...
noise frequencies are much higher and lower than signal frequencies filtered out
A fine matrix is...
one with a high number of frequency encodings and/or phase encodings and results in a large number of pixels in the FOV
The matrix size in the phase direction is determined by the _____ performed. To achieve a fine matrix a high proportion of the ______ gradient slopes must be ______.
phase encodings phase encodings steep
Noise is ____, as it is in a different position each time data are stored
random
Receive bandwidth
range of frequencies that are sampled application of readout gradient
When scanning anatomy that has a smaller dimension in the phase axis than frequency, a _______ FOV may be desired
rectangular
Noise What is noise in MR context
represents frequencies that exist randomly in space and time Noise is generated by the presence of the patient in the magnet and the background electrical noise of the system
In volume imaging, slices are sectioned out by a technique known as..... Therefore.... Formula
slice encoding - another series of phase encoding steps along the slice axis. just as the number of phase encoding steps increases the scan time in conventional spin echo, number of slices also affects scan time in volume imaging scan time = TR X NEX X number of phase encodings X number of slice encodings
____ voxels result in good spatial resolution, because....
small small structures can easily be differentiated.
For a given FOV a fine matrix results in.....
small pixels and voxels
Pressence of random noise means that doubling the NEX only increases the SNR by... Therefore...
square root of 2 (=1.4) Increasing NEX is not the best way to increase SNR
Do square or rectagular pixels provide better spatial resolution? Why?
square, because the image is equally resolved along both the frequency and the phase axis
Slice thickness is determined by the slope of the slice select gradient; therefore, the to achieve thin slices the slice select gradient is _______
steep
In order to obtain equal resolution in every plane and at every angle of obliquity, each voxel should be ___________ (_______)
symmetrical; isotropic
The proton density of a tissue is inherent to that tissue and cannot be change, but as the SNR is likely to be compromised when imaging areas of low proton density, what you do?
take steps to boost SRN that may not be necessary when scanning areas with high proton density
The thinner the slice, the greater.....
the ability to resovle small structures in the slice select plane.
T2 controls
the amount of T2 weighting
TE controls
the amount of decay that is allowed to take place before an echo is completed.
If the FOV is rectangular, the pixels will be square if...
the matrix selected produces a pixel with the same dimensions along phase as well as frequency
The matrix determines what/
the number of pixels in the FOV
rise time
the rise time of a gradient is the time required for it to achieve the correct slope
How does TR affect scan time?
the time of each repetition or MR experiment or times between filling consecutive drawers. Double TR doubles scan time and vice versa
Signal
the voltage induced in the receiver coil by the precession of the NMV in the transverse plane
In general the size of the receiver coil should be chosen such that.....
the volume of tissue imaged optimally fills the sensitive volume of the coil
The choice of pulse sequence determines....
the weighting and quality of the image.
Timing of the parameters determines
the weighting of the images.
Usually if the FOV is square and the phase matrix is less than the frequency matrix, this does what to the pixels? What does this do to spatial resolution?
they are longer in the phase direction than the frequency direction. The spatial resolution is therefore reduced along the phase axis
