MRI Registy Review 6 Motion Artifact Control Techniques
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how do motion artifacts look on an image?
ghosting or smearing of the signal along the phase-encoding direction of the image regardless of the direction of the motion
why does motion artifact occur in the phase encoding direction?
read page 5-7
what is gradient moment nulling?
- flow compensation
- partially compensates or minimize artifacts arising from motion in the images
- also known as gradient motion rephasing or flow compensation
how does gradient moment nulling work?
- by adding extra gradient pulses to rephase the signal in moving protons
- it uses extra gradient pulses in the slice select direction and the frequency direction
when is flow comp frequently chosen?
for brain and spine studies in which there is significant blood flow and CSF pulsation. Also for knees
what are saturation pulses?
- addition of RF pulses to a pulse sequence
- its to eliminate the signal from specific regions of anatomy that we prefer not to have displayed in the images
- first is a 90 degree pluse with a gradient pulse then a spoiler is applied to dephase the magnetization just placed into the transverse plane
whenever saturation pulses are used to reduce motion artifacts, what is important?
to completely cover the moving structures with the saturation region
what are the two drawbacks to using sat pulses?
- increased power to the patient, which increases the pt's SAR (specific absorption rate)
- a reduction in the maximum number of slices within a given TR period because additional time is required to apply each saturation pulse and spoiler, and that time restricts the number of slices that can be acquired within the TR period
what is gating?
echoes are only collected during certain selected periods of the physiological cycle
what is triggering?
a particular event during the physiological cycle commences the collection of echoes which then continues through the entire TR
what is ecg synchronization?
- used to acquire images for cardiac or coronary artery studies to eliminate motion artifacts
- the idea is to achieve a good ECG waveform with a tall, narrow R-wave. The R-wave is the event that is used for synchronization
what are the 3 types of cardiac synchronization that are routinely applied?
- ECG triggering
- ECG gating
- retrospective cardiac synchronization
how does ECG triggering work?
- since ECG triggering is a triggering technique, rather than a gating technique, the sequence waits to detect an R-wave before it begins. It then continues to collect echoes until it completes the TR (even if another R-wave occurs)
- The TR and thus, the contrast of the images, depends partly upon the pt's R-R interval (or some multiple of the R-R interval), which becomes the effective TR
how does ECG gating work?
ECG gating also uses the R-wave as a synchronization starting point, but specific time period during echo collection can be selected. For example, echoes can be collected during only systole or only diastole. This is advantageous for some MR angiography studies and some imaging of the heart during specific cardiac phases. Being a gating technique, ECG gating always increases the scan time compared to a scan with no synchronization
how does retrospective cardiac synchronization?
it uses the ECG electrodes to encode the phase of the cardiac cycle in each of the echoes. During the scan, there is no stopping and starting of echo collection. The echoes are collected as normal. The synchronization occurs after the scan, retrospectively. All of the echoes from each heart phase are sorted out and averaged together. In this way, motion artifacts are removed from the images. RCS is often used for cine studies of the heart. A cine study is one in which a single slice is depicted but you can view it through each of the phase of the cardiac cycle, like a beating heart on a movie
what is respiratory synchronization?
- is used to minimize artifacts from the pts breathing
- usually for abdomen, thorax and lumbar spine scans
- pts respiration rate is measured with a transducer that expands and contracts as the patient breathes
- like other synchronization techniques, the success depends on a reasonably consistent physiological cycle. If the pt has very irregular breathing, respiratory compensation may fail to eliminate motion artifacts
what are the 3 types of respiratory synchronization techniques?
- respiratory triggering
- respiratory gating
- alternative respiratory compensation
how does respiratory triggering work?
similar to ECG triggering, respiratory triggering waits for an event to begin echo collection and then continues to collect echoes until the TR period is complete. The even that begins echo collection is usually the end of expiration. The TR is usually assigned to be the time between expiration and the next inspiration. As the TR is adjusted for the patient's rate of respiration, the contrast of the images is also affected
how does respiratory gating work?
the TR can be set to any value, and echoes will only be collected during the selected period, usually the time between expiration and the next inspiration. Since echoes are only collected during a fraction of the respiratory cycle, with respiratory gating, the scan time is increased.
how does alternative respiratory compensation work?
- it is not a gating or triggering technique
- it can use a reordered rawdata imaging scheme
- it places the echoes collected during the most severe motion in the outer lines of the rawdata since the central lines of rawdata are more sensitive to motion. The central lines of rawdata are more sensitive to motion. The central lines of rawdata are filled in during a more stationary portion of the respiratory cycle. Respiratory compensation is an effective method for reducing artifacts due to patient breathing and it has virtually no effect on the scan time
what are the 3 techniques available that result in fat suppression?
- fat suppressed inversion recovery
- spectrally selective fat suppressed inversion recovery
- spectral fat saturation
what is fat suppressed inversion recovery?
- begins with an inverting 180 RF pulse
- one draw back is its ineffectiveness when used with MR paramagnetic contrast agents since contrast shorten the T1 relaxation time of any perfused tissues, and fat suppressed inversion recovery sequences reduce the signal of short T1 tissues, the pulse sequence will act to suppress the pathology that the contrast agent is working to enhance
what is spectrally selective fat suppressed Inversion recovery?
is like fat suppresed inversion recovery, except that it applies the 180 inverting pulse only to the protons of fat molecules. It can differentiate between the protons in fatty tissues and the protons in watery tissues by their chemical shift properties, and thus their different resonant frequencies. Since only the magnetization of fat protons is deflected by the frequency selective 180 RF pulse, it is easier to suppress only the fat signal with this technique, compared to the fat suppressed inversion recovery method in which all tissues with a similar T1 relaxation time to fat are suppressed. Plus this is effective when giving contrast
what is spectral fat saturation?
- does not employ an inversion recovery technique, but does rely upon the chemical shift properties of fat and water. A 90 RF pulse is transmitted at the precise precessional frequency of the magnetization of hydrogen protons located in fat molecules. Because the transmission frequency is matched to that generated by the fat protons, the magnetization specifically of fat protons is the only magnetization disturbed and moved into the transverse plane. Any type of pulse sequence may then be run. Similar to the way in which the saturating 90 degree pulse reduces the tissue's signal when selecting spatial saturation, the 90 degree pulse applied just to the magnetization of fat protons causes a reduction of signal from only the fatty tissues in the images
- may be used to collect T1 and T2 images, also effective when giving contrast
- Shimming has to be done to have a homogenous signal. The extra added RF pulse increases the SAR of pts
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