Feature | November 20, 2012 | Irving Weinberg, M.D.

Breaking the MRI Gradient Sound Barrier

Magnetic gradients are responsible for setting the spatial resolution in a magnetic resonance imaging (MRI) device. A recent clinical study shows that gradients could be increased a hundred-fold without causing peripheral nerve stimulation.

A typical magnetic resonance imaging (MRI) system includes a cryogenic magnet (that creates a static magnetic field), a radiofrequency (RF) coil and resistive electromagnetic gradient coils. The examination time and spatial resolution of MRI are determined by the gradient coils’ speed and strength. 

The ability to perform fast pulse sequences (like the echo-planar images used in functional neuroimaging) is determined by the gradient slew rate, which is the gradient strength divided by the rise-time to achieve that strength. In 1998, the U.S Food and Drug Administration (FDA) set limits on MRI slew rates because of reports of visual disturbances (phosphenes) and other unpleasant sensations caused by electrical nerve stimulation from the rapidly changing magnetic gradient fields. 

In the United States, the slew rate limits are set at 200 T/m/s. A National Institutes of Health (NIH)-funded study recently published in Medical Physics by lead author Stanley Fricke, M.D., of the Children’s National Medical Center in Washington, D.C., has shown that these unpleasant sensations go away when the gradients operate at really high speeds, potentially opening up new markets and applications for ultra-fast MRI systems.

The prospective, controlled study looked at a group of 26 adults whose hands were exposed to rapidly-changing magnetic fields. Prior work had predicted that at least half of all subjects would experience nerve stimulations at maximum magnetic field strengths of 400 milliTeslas, no matter how fast the fields were changing. The new study results showed that at frequencies above about 100 kHz (i.e., rise-times below 10 microseconds and corresponding to a slew rate of 160,000 T/m/s), no nerve stimulation occurred in any of the tested subjects at this field strength (see Figure 1). The results of the published study, along with prior studies conducted by Fricke, applied to diverse pulse shapes, including triangular and rectangular pulses.

Potential Uses for Fast MRI Systems

Many potential applications exist for MRI systems with stronger, faster gradients (see Table 1). A high spatial resolution prostate imaging system is being built as a potential competitor to prostate-specific antigen (PSA) screening, by the study’s co-authors at Weinberg Medical Physics LLC (a medical device design company in Bethesda, Md.). Functional brain imaging also uses fast sequences, and with high spatial resolution could potentially examine individual neurons. 

Coronary artery imaging has been a holy grail for body MRI developers and would be facilitated with high-resolution systems that could capture an image within a millisecond. Cortical bone has a very short natural magnetization decay time, and so it would be a natural fit for ultra-fast MRI. 

In keeping with Children’s National Medical Center’s commitment to reduce public radiation exposure, Fricke and his colleagues are building new nonionizing imaging systems for bone mineral density measurements and dental imaging. The system could potentially visualize short-lived free-radicals formed during radiation therapy, using electron spin resonance imaging.  Finally, magnetic nanoparticles require high slew rates in order to achieve high spatial resolution (for imaging) and high gradient field strengths (for therapy).  itn

Irving Weinberg, M.D., president of Weinberg Medical Physics, is a practicing radiologist who received a Ph.D. in experimental plasma physics and has cofounded several medical imaging companies (whose products have been used by more than 500,000 women for detecting and delineating breast cancers). Currently, he is working on applications for ultra-fast MRI for high-resolution dental and prostate care, and (with academic investigators) on image-guided therapy with magnetic nanoparticles.

For more information, contact Weinberg at [email protected]

Related Content

 Many patients with severe coronavirus disease 2019 (COVID-19) remain unresponsive after surviving critical illness. Investigators led by a team at Massachusetts General Hospital (MGH) now describe a patient with severe COVID-19 who, despite prolonged unresponsiveness and structural brain abnormalities, demonstrated functionally intact brain connections and weeks later he recovered the ability to follow commands

Getty Images

News | Coronavirus (COVID-19) | July 08, 2020
July 8, 2020 — Many patients with severe coronavirus disease 2019 (...
A patient implanted with the Axonics System can undergo MRI examinations safely with radio frequency (RF) Transmit Body or Head Coil under the conditions outlined in the Axonics MRI Conditional Guidelines.

A patient implanted with the Axonics System can undergo MRI examinations safely with radio frequency (RF) Transmit Body or Head Coil under the conditions outlined in the Axonics MRI Conditional Guidelines.

News | Magnetic Resonance Imaging (MRI) | July 02, 2020
July 2, 2020 — Axonics Modulation Technologies, Inc., a medical technology company that has developed and is commerci
This data represents wave 2 of a QuickPoLL survey conducted in partnership with an imagePRO panel created by The MarkeTech Group (TMTG), regarding the effects of COVID-19 on their business

Getty Images

Feature | Coronavirus (COVID-19) | July 01, 2020 | By Melinda Taschetta-Millane
Researchers reviewed results of prostate biopsies on over 3,400 men who had targets identified on prostate MRI and found that the positive predictive value of the test for prostate cancer was highly variable at different sites
News | Prostate Cancer | July 01, 2020
July 1, 2020 — Prostate MRI is an emerging technology used to identify and guide treatment for...
R2* maps of healthy control participants and participants with Alzheimer disease. R2* maps are windowed between 10 and 50 sec21. Differences in iron concentration in basal ganglia are too small to allow visual separation between patients with Alzheimer disease and control participants, and iron levels strongly depend on anatomic structure and subject age. Image courtesy of Radiological Society of North America

R2* maps of healthy control participants and participants with Alzheimer disease. R2* maps are windowed between 10 and 50 sec21. Differences in iron concentration in basal ganglia are too small to allow visual separation between patients with Alzheimer disease and control participants, and iron levels strongly depend on anatomic structure and subject age. Image courtesy of Radiological Society of North America

News | Magnetic Resonance Imaging (MRI) | July 01, 2020
July 1, 2020 — Researchers using magnetic...
In new QuickPoLL survey on imaging during the pandemic, responses were tallied from around 170 radiology administrators and business managers, who are part of an imagePRO panel created by The MarkeTech Group (TMTG), regarding the effects of COVID-19 on their business. TMTG is a research firm specializing in the medical device, healthcare and pharmaceutical industries.
Feature | Coronavirus (COVID-19) | June 30, 2020 | By Melinda Taschetta-Millane
Cardiac MR can offer data above and beyond anatomical imaging, which is the main reason why this system was installed at Baylor Scott White Heart Hospital in Dallas. The system is a dedicated heart MRI scanner.

Cardiac MR can offer data above and beyond anatomical imaging, which is the main reason why this system was installed at Baylor Scott White Heart Hospital in Dallas. The system is a dedicated heart MRI scanner.

News | Pediatric Imaging | June 29, 2020
June 29, 2020 — A type of smart magnetic r...
This image of DCE-MRI reveals persistent blood brain barrier disorder in American football players. Using brain imaging techniques and analytical methods, researchers can determine whether football players have CTE by measuring leakage of the blood-brain barrier. Image courtesy of Ben-Gurion University

This image of DCE-MRI reveals persistent blood brain barrier disorder in American football players. Using brain imaging techniques and analytical methods, researchers can determine whether football players have CTE by measuring leakage of the blood-brain barrier. Image courtesy of Ben-Gurion University

News | Magnetic Resonance Imaging (MRI) | June 22, 2020
June 22, 2020 — Chronic traumatic encephalopathy (CTE), a neurodegenerative disease caused by repeated...
Axial FLAIR in four different COVID-19 patients. A) 58-year old man with impaired consciousness: FLAIR hyperintensities located in the left medial temporal lobe. B) 66-year old man with impaired consciousness: FLAIR ovoid hyperintense lesion located in the central part of the splenium of the corpus callosum. C) 71-year old woman with pathological wakefulness after sedation: extensive and confluent supratentorial white matter FLAIR hyperintensities (arrows). Association with leptomeningeal enhancement (stars

Axial FLAIR in four different COVID-19 patients. A) 58-year old man with impaired consciousness: FLAIR hyperintensities located in the left medial temporal lobe. B) 66-year old man with impaired consciousness: FLAIR ovoid hyperintense lesion located in the central part of the splenium of the corpus callosum. C) 71-year old woman with pathological wakefulness after sedation: extensive and confluent supratentorial white matter FLAIR hyperintensities (arrows). Association with leptomeningeal enhancement (stars) D) 61-year old man with confusion: hyperintense lesions involving both middle cerebellar peduncles. Image courtesy of the journal Radiology

News | Coronavirus (COVID-19) | June 16, 2020
June 16, 2020 — Current data on central nervous system (CNS) involvement in...