How much does patient motion affect your equation?
Many people have trouble sitting still, whether its in school, at work, or during a medical examination. So, it is not a big surprise that patient motion has a huge influence on medical imaging scans like a magnetic resonance scan. Magnetic resonance imaging (MRI) provides detailed images of and information about the different tissue structures and organs in the body. Because of the method used to create the images through an MRI scan, patient movement can cause artifacts and sometimes leads to longer or repeat scans. As a result, there are a few methods that can be used to reduce patient motion.
How much can motion artifacts affect one facility?
Magnetic resonance images are created using data collected throughout the scan. However, there is a large amount of data that must be collected to create detailed images, meaning that scans often take longer to complete than is desired. When a patient moves, it can cause distortion on the image, which is referred to as a motion artifact. Motion artifacts may appear as a blurring of contrast or edges, replication of part or all of a structure, signal loss or undesired strong signals.
In one randomly selected calendar week in 2014 that was retrospectively reviewed, 192 completed scans occurred using MRI.1 Of those scans, 7.5% of the outpatient and 29.4% of the inpatient or emergency patient exams showed significant motion artifacts. 19.8% of all MR exams had to be repeated. For that hospital, the extra cost caused by these motion artifacts was about $592 an hour.1
Although many people may prefer to eliminate motion altogether during an MR examination, it is not always possible. There is not only unintentional motion but also biological processes that involve motion such as movement in the lungs and abdomen for breathing and heart-beating. Some techniques allow for the technologist to accelerate the scan, which can reduce the amount of time required for the scan.2 Because the data is collected over less time, there may be less motion that occurs throughout because patients may find it easier to lie still for this shorter length of time, which can reduce the effects of the artifacts.
Parallel imaging (PI) can be used as an acceleration technique and may shorten acquisition times by a factor of two or three. PI acquires less k-space data, which is called under-sampling, and can lead to faster image acquisition.3 Specialized algorithms are then used to reconstruct the image where the data was under-sampled. In many cases, adequate images are produced.
Compressed sensing (CS) provides an acceleration technique that may be used to compensate for motion, sparking the introduction of a few motion-compensation applications.1 CS can be done by under-sampling data for the image during acquisition or by retrospectively under-sampling data. CS may also reduce artifacts that both parallel imaging and traditional imaging may be susceptible to.1
Triggering and gating
Motion from the heartbeat and breathing can cause artifacts, if it is not taken into account.2 Because of this, some scans may utilize breath-hold techniques. These techniques attempt to minimize the motion during the scan by acquiring the data during the breath-holds of a patient. Unfortunately, all patients are not able to hold their breath for as long as the scan requires.
Triggering is marked by data collection at the same point in the cycle over multiple cycles.2 Triggering is a form of prospective gating and often requires additional signal information. Because of this, patients whose scans are done using triggering can include the use of ECG (electrocardiogram) leads. This allows the scanner to track the cardiac or respiratory motion and cycle and acquire data during a specific phase.
Retrospective gating involves a continuous collection of data that is then reordered to provide information on one cycle.2,5 This method also may involve ECG leads or may involve information input that corresponds to pulse or respiration. It is typically used for cine-cardiac motion studies. The image is produced by aligning the phases of multiple cycles.
Motion correction techniques
Radiology departments have also begun to use motion correction techniques that are used either prospectively or retrospectively.2 These techniques are often referred to as MR navigator methods and can be used for 1D, 2D and 3D images. When navigator methods are used retrospectively, they can help to correct the acquired image data to reduce motion artifacts.
One self-navigated imaging method can help to reduce motion sensitivity due to over-sampling, which can increase the scan time.1 This technique can be combined with other techniques, including under-sampling, acceleration techniques. After the scan, the images are created relying more on the data obtained without motion, which can reduce the artifacts created.
All three types of techniques, acceleration, gating and motion-corrected, can lead to a reduced impact of artifacts on the magnetic resonance scan. The acceleration techniques may reduce the scan time, allowing the facility to possibly fit in more scans or providing lee-way for repeat scans. Gating techniques may be used to mitigate respiratory or cardiac motion by targeting certain phases of the patient's cycle. Motion-correction techniques and software can be particularly useful for scans that have already occurred or those that involve patients unable to hold still for long periods of time. With these techniques, facilities may find it easier to avoid inconclusive or repeat scans.
- Toward Quantifyiing the Prevalence, Severity, and Cost Associated With Patient Motion During Clinical MR Examinations. JACR. doi: 10.1016/j.jacr.2015.03.007. Last accessed June 28, 2019.
- Motion Artefacts in MRI: a Complex Problem with Many Partial Solutions. J Magn Reson Imaging. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4517972/ Last accessed July 1, 2019.
- Parallel MR imaging. J Magn Reson Imaging. https://www.ncbi.nlm.nih.gov/pubmed/22696125. July 1, 2019.
- Compressed sensing MRI: a review of the clinical literature. Br J Radiol. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4984938/. July 1, 2019.
- Intro to Gating/Triggering: How are cardiac and respiratory gating performed? MRIQuestions.com. http://mriquestions.com/gating-methods.html. July 1, 2019.