registry part 5and6
DESCRIPTION
MRI Review by Amanda GolschTRANSCRIPT
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Physical/Logical Gradients and Spatial Localization
Part VAmanda Golsch BSc, RT(R)(MR)
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Physical Gradients• The physical coordinate system refers to the “physical body”.
• The gradient coil that varies the magnetic field from head to foot is the Z gradient.
• The gradient coil that varies the magnetic field from left to right is the X gradient.
• The gradient coil that varies the magnetic field from anterior to posterior is the Y gradient.
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Slice Selection and Physical Gradients
• If the Z gradient is utilized for slice selection, this results in an axial slice.
• Think of a loaf of bread. The blue arrow shows us the Z gradient coil. If we use the Z gradient coil for slice selection we get an axial slice.
Axial slice
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Physical Gradients and Slice Selection
• If the Y gradient coil is used for slice selection, the result will be a coronal slice.
• Back to the bread!
• The blue arrow shows us the anterior to posterior direction of the Y gradient. Therefore, when you cut the hoagie roll along the Y gradient the result is a coronal slice.
Note that the hoagie roll is sliced ina coronal orientation.
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Physical Gradients and Slice Selection
• If the X gradient coil is utilized in slice selection, the result will be a sagittal slice.
• Let’s look at a c-spine
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Logical Gradients• The logical notation is used when referring to the gradients by their function rather than their
direction/orientation.
• When speaking of logical gradients, the slice selection gradient is the Z gradient. This has This has nothing to do with physical direction or orientation!nothing to do with physical direction or orientation!
• The slice selection or Z gradient is the first gradient to be applied during the RF pulse. During a spin echo it is applied during the 90 and 180 pulses.
• The slice thickness is determined by the amplitude (slope) of the gradient and the transmit bandwidth of the RF pulse.
• Thinner slices = Higher amplitude and/or narrow bandwidth
• Thicker slices = Lower amplitude and/or wide bandwidth
• Slice location is determined by the transmit frequency of the RF pulse.
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Logical Gradients• Frequency encoding (X gradient) occurs during the sampling of the echo. The sampling of the
echo occurs during the readout at the TE time.
• The sampled data is then placed into K-space.
• Let us assume that we wish to obtain an image with a 512 X 512 spatial resolution:
– The system will sample the echo 512 times in the presence of the frequency encoding gradient. This will generate 512 frequency data points or columns to be plotted into K-space in the frequency (X) direction. This creates enough data to reconstruct an image with a spatial resolution of 512 X 1.
– In order to obtain 512 X 512, we need to collect 512 echos one for each line in k-space (phase direction). Each of these 512 echos must be sampled 512 times for each point along each line (frequency and direction).
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Logical Gradients• The phase encoding gradient is the Y gradient.
• The phase encoding gradient is applied during the FID and its purpose is to encode spatial information into the MRI signal. It provides the “line” in k-space on which the data will be plotted during the readout (frequency) period.
• It occurs after slice selection, but it is before the frequency encoding gradient.
• If you would like a phase resolution of 256, you would need to repeat the pulse sequence 256 times. This would occur at different amplitudes and polarities (128 positive and 128 negative steps).
• Spatial resolution is increased by acquiring more phase encoding steps. However, scan time is also increased.
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K-Space• Data is collected by sampling echos.
• The data that is collected is digitized and mapped in relation to the spatial encoding gradients which are the phase (Y) and frequency (X) gradients.
• The map is known as K-Space.
• In one direction of K-Space is frequency information, and the other direction holds phase information.
• Raw data is the signal information that is stored in K-Space.
• The letter K is used when referring to frequency.
• The number of data points in k-space is determined by the number phase and frequency encodings selected by the operator.
• The more data points selected, the better the detail. This results in longer scan times.
• Once all of the data is collected, the raw data is reconstructed by the Fourier Transform in the array processor.
• RAW DATA IS NOT THE MR IMAGE; IT IS DATA SIGNAL INFORMATION.
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Image Quality
Part VI
Amanda Golsch BSc, RT(R)(MR)
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Image Quality and Slice Thickness
• Slice Thickness is determined by the amplitude or slope of the z gradient.
• Higher amplitudes yield thinner slices.
• Lower amplitudes yield thicker slices.
• As slices get thinner, SNR is reduced but spatial resolution improves.
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Image Quality and Spatial Resolution
• Spatial resolution allows you to distinguish one structure as separate from another.
• The only way for spatial resolution to be increased is to reduce voxel volume or pixel size.
• A voxel is a 3-dimensional object. Its size is determined by FOV, the acquisition matrix, and slice thickness.
• The pixel size is synonymous with the term in-plane resolution. The term pixel is often applied to the face of the voxel and is determined by the acquisition plane and FOV. Note that slice thickness does not play a role in determining the size of the pixel.
• Magnification can make a picture look blurry because it magnifies the pixel.
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SNR• Signal to noise is a ratio of MR signal to noise.
• Noise can occur from multiple sources. The patient, system electronics, and the environment are all sources of noise.
• Signal comes directly from the tissues within the voxel.
• In order to increase SNR, you need to reduce the noise and/or increase the signal.
• Operator acquisition parameters greatly effect the SNR.
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Contrast• TR,TE,TI, and flip angle are all parameters that can be used to manipulate MR contrast.
• Increasing the TR will increase the amount of longitudinal magnetization allowed to recover between excitation periods. This results in an increase in the SNR.
• Therefore, decreasing the TR will decrease the SNR.
• Increasing the TE increases the amount of transverse magnetization decay between the excitation pulse and the sampling of the echo and, therefore, will result in a reduction in the SNR.
• If you decide to null the signal from fat by adjusting the TI, and the area being imaged is primarily composed of fat, the image would have a low SNR because the tissue that is providing the signal has been nulled.
• Adjusting the flip angle can increase or decrease SNR. With a low flip angle, the amount of transverse magnetization is small, so the SNR will to low.
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Field Homogeneity and Shimming• The magnetic field must be as homogenious as possible. This is especially true at isocenter.
Homogeneity is maximized by shimming.
• Shimming can be accomplished actively or passively.
• Active shimming implies the use of addition coils within the magnet. Current applied within the shim coils either adds or subtracts from the static magnetic field to produce as homogeneous a field as possible.
• Passive shimming implies the use of small bits of ferrous material, known as shim plates. These plates are placed around the bore.
• Homogeneity is expressed in ppm.
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RF Shielding• RF from outside sources can interfere with MR studies.
• Outside RF sources can degrade image quality, so MR scan rooms are RF shielded.
• MR rooms use a lining of copper in the walls and around the doors to shield from outside RF sources.
• RF shielding is known as a “Faraday Cage”
• Holes or tears in the shielding can result in extraneous signals entering the scan room and can appear as zipper artifacts on images. This is known as a leak in the RF shielding and can occur when scanning with the room door open or not tightly closed.
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Question 1• The physical gradient coil that varies the magnetic field from left to right is the
– A. Y gradient
– B. Z gradient
– C. X gradient
– D. W gradient
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Question 2• To obtain a coronal slice, which physical gradient coil should be applied
– A. Y gradient
– B. F gradient
– C. D gradient
– D. X gradient
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Question 3• Which logical gradient is applied during the RF pulse?
– A. Y gradient
– B. X gradient
– C. Z gradient
– D. X and Y gradient
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Question 4• Which logical gradient is the frequency encoding gradient?
– A. Z gradient
– B. X gradient
– C. Y gradient
– D. A gradient
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Question 5• How many echos need to be sampled to obtain a spatial resolution of 256 X 1?
– A. 128
– B. 256
– C. 512
– D. 600
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Question 6• The gradient amplitude must be for a 3mm slice than a 5mm slice.
– A. Higher
– B. Lower
– C. Equal
– D. Parallel
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Question 7• K-space stores?
– A. The MR image
– B. The FID
– C. SNR
– D. Raw data
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Question 8• What mathematical process converts raw data into a reconstructed image?
– A. K-Space
– B. Faraday’s Law
– C. Fourier transform
– D. The array processor
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Question 9• If you increase your spatial resolution from 256 to 512 what happens to the SNR?
– A. It stays the same
– B. It decreases
– C. It increases
– D. It is the average of 256 and 512
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Question 10• Decreasing the TE does what to the SNR?
– A. Increases
– B. Decreases
– C. It doesn’t effect SNR
– D. Averages