Archive for the ‘Allg. techn. Grundlagen’ Category

MRT aktive Implantate

Samstag, 29. Juni 2013

Cave:   Niederfeld-Kernspintomographen 

Funktionsverlust eines Titanmagnetic-Insert auf Zahnimplantat bei/nach MRT

Gestern konsultierte mich eine Patientin, der ich vor 28 Jahren meine ersten 3 Implantate von Straumann einsetzte. Sie (die 80-jährige Patientin) und die Impl….

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also kurzum: Patienten mit mechanischem Herzklappenersatz können zum MRT bis 3T überwiesen werden! Herzliche Grüße 

 

(Zitat)

Die Schlussfolgerung, dass derzeit unfallfreie Untersuchungen „purer Zufall“ seien, ist angesichts unserer Ergebnisse nicht richtig. In einem Zentrum mit eingespieltem Team mit entsprechender Erfahrung (erfahrene Radiologen, Schrittmacherexperte, Herzchirurg und enger Kommunikation mit den Aggregat-Herstellern) sind diese Untersuchungen sicher durchführbar. Für Schrittmacherpatienten haben wir folgendes Prozedere eingeführt:

  • Aufklärung, kardiologische Anamnese
  • Basisabfrage, Erhebung und Dokumentation aller Parameter inklusive Impedanzen
  • Umprogrammierung des Schrittmachers auf asynchronen Modus oder Aus.
  • MRT-Untersuchung unter EKG-Kontrolle und Pulsoxymetrie
  • Post-MRT, Erhebung und Dokumentation aller Parameter noch im umprogrammierten Modus
  • Umprogrammierung des Schrittmachers auf ursprüngliche Parameter, Endkontrolle, nochmalige Erhebung und Dokumentation aller Parameter.
  • ImplantateMRT
  • ImplantateMRT1
  • MRT_Schrittmacher!!!
  • Medtronic_HandbuchMRTfähig
  • Mrt_Schrittmacher_Entscheidungsparameter

MRI Magnetic Field Stimulates Rotational Sensors of the Brain

Montag, 24. Oktober 2011

October 12, 2011 — The puzzle of what causes vertigo in patients undergoing magnetic resonance imaging (MRI), which has baffled researchers for years, may now be solved, a new study suggests.

Johns Hopkins University investigators believe that the key to the dizziness that almost everyone undergoing MRI experiences lies in a phenomenon known as the Lorentz force. This force results from the interaction between a magnetic field and naturally occurring ionic currents that flow into the hair cells in the labyrinthine endolymph fluid.

„You have this fluid in your inner ear that helps you sense your balance, and there are tiny electric currents flowing through this fluid all the time,“ explained study author Dale C. Roberts, MS, senior research systems engineer in the laboratory of David Zee, MD, a neurologist in the Department of Neurology at the Johns Hopkins University School of Medicine, Baltimore, Maryland. „If you put in an electric current in the magnetic field, you will feel a sideways force, which is the Lorentz force.“

The force occurs in the fluid, which then pushes against the rotational sensor in the inner ear and creates the feeling of spinning, he added.

These findings may have important implications for functional MRI (fMRI) studies, especially those investigating movement, the investigators point out.

„When the brain’s balance system is stimulated, the stimulation goes into the brain and activates many areas associated with the eye movements and balance and motion, muscle control, and limb movement,“ said Roberts. „If you’re doing an fMRI that’s looking at these sensations, the machine itself could be affecting that a little bit, so people have to take this into account.“

The study appears in the October 11 issue of Current Biology.

In the Dark

To measure vestibular stimulation, the researchers used nystagmus, or slow eye movements in 1 direction followed by quick movements in the other direction, which reflects the brain’s detection of motion. Ten healthy volunteers demonstrated robust nystagmus while in an MRI machine, but 2 volunteers who had lost vestibular function did not. „When they go in the magnet, they have no sensation of spinning, and they show no signs in their eye movements,“ said Roberts.

The experiments were conducted in the dark with an infrared camera because the brain can suppress eye movement if there is light, although stimulation still occurs. „If you’re in the light, this nystagmus can be suppressed, so the key to measuring these eye movement is to occlude vision; what we did is turn off the lights in the room and put a piece of black felt over the subjects‘ heads so they couldn’t see anything,“ said Roberts.

To test the effect of motion or direction of the magnetic field on the volunteers‘ balance centers, the researchers used MRI machines with magnetic field strengths of 3 and 7 T. They varied the speed and direction of the patients‘ movements into the bore, their duration of time inside the tunnel, and the static field strength.

The degree of vertigo experienced by the patients depended on the strength of the magnetic force of the MRI machine. Higher magnetic field strengths caused significantly faster nystagmus, which persisted throughout the time participants spent in the machine.

Using specific complex calculations, the researchers demonstrated that the vestibular stimulation is static and requires neither head movement nor dynamic change in magnetic field strength, and that it is sensitive to magnetic field polarity and the position of the head.

It is almost impossible to avoid vertigo when in an MRI machine unless the patient does not have labyrinthine function, although there may be a position that minimizes the feeling, said Roberts. „We found that the amount of horizontal eye movement varies with your head pitch, so it might be that people could pitch their head, as they would do when nodding their head up and down in agreement, and it may be possible to find what’s close to a null point.“

However, the spinning sensation would not disappear altogether, as the eye can move vertically as well as horizontally, „so there could be stimulation in another direction,“ he added.

The study results suggest that the Lorentz force, resulting from the interaction between the magnetic field and the ionic currents in the inner ear fluid, pushes on the semicircular canal cupula, leading to nystagmus.

Research Implications?

Roberts said he does not want to raise an alarm about the accuracy of fMRI studies, but said that the effects uncovered by the study could „introduce a systemic bias.“

During the course of their experiments, the researchers noticed that not all MRI magnets go in the same direction, which means they cause different eye movements and activate different parts of the brain. „This could really confound functional imaging, especially when you’re interested in motion perception,“ said Dr. Zee.

Asked to comment on these findings, Steven Laureys, MD, PhD, from the Coma Science Group, Cyclotron Research Center, University of Liege, Belgium, and clinical professor, Neurology, Liege University Hospital, said that the article explained MRI-related dizziness in „a wonderful and controlled way,“ and that „it’s good to finally study how this is happening.“

He wryly added, however, that the results will make his life more difficult because he will have to take into account these possible biases in his extensive work using fMRI.

„It’s something that so far we didn’t need to deal with, and now we’ll need to deal with it, but that’s of course a very good thing because understanding it will help all of us to use MRI in a more controlled way, and I think that’s only good news, especially as researchers use stronger magnetic fields,“ said Dr. Laureys.

Still, he pointed out, „many things that we’ve done with this tool remain valid.“

Another expert, Ravi S. Menon, PhD, Canada research chair in functional and molecular imaging and professor, medical biophysics, medical imaging, and psychiatry, Schulich School of Medicine and Dentistry, and professor, physics, at the University of Western Ontario, London, Canada, noted that the effect detailed in the article has never been observed for those doing careful eye tracking in fMRI systems during active tasks.

„As the authors point out, it’s likely to only manifest in situations where the subject is doing nothing in a dark tube, such as resting-state fMRI,“ said Dr. Menon in an email. „Their implication is that in such cases, eye movement network activations are presumed to be suspect, except that these same networks have been observed in fMRI on nonhuman primates where the eyes have been immobilized.“

Although interesting, the article’s observations need substantial replication, said Dr. Menon.

Joseph S. Gillen was supported in part by National Institutes of Health/National Center for Research Resources grant. Charles C. Della Santina is a founder of Labyrinth Devices, LLC, and a member of its scientific advisory board.

Current Biology. 2011;21:1635-1640. Abstract

Technik Grundlagen MRT

Sonntag, 09. September 2007

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