Patient monitoring during magnetic resonance imaging

Practice advisory on anesthetic care for magnetic resonance imaging: an updated report by the american society of anesthesiologists task force on anesthetic care for magnetic resonance imaging

The American Society of Anesthesiologists Committee on Standards and Practice Parameters and the Task Force on Anesthetic Care for Magnetic Resonance Imaging presents an updated report of the Practice Advisory on Anesthetic Care for Magnetic Resonance Imaging.

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Anesthesia for magnetic resonance imaging

PURPOSE OF REVIEW

This review focuses on the technological principles, safety considerations, monitors and equipment, patient issues, and a general overview of the anesthetic management of both conventional and intraoperative magnetic resonance imaging based on the most recent literature.

RECENT FINDINGS

As a diagnostic imaging modality, magnetic resonance imaging remains unparalleled in its diagnostic and clinical value. The clinical applications for magnetic resonance imaging continue to evolve, and include its latest use in minimally invasive procedures as well as in the operating room. Intraoperative magnetic resonance imaging is steadily gaining acceptance for neurosurgical procedures. The safety considerations, monitor and equipment issues for intraoperative magnetic resonance imaging are similar to the conventional setting. However, they differ in their focus on anesthesia management. Most monitoring compatible with magnetic resonance imaging has been available for many years. In the USA, the newest available monitoring option during magnetic resonance imaging is for temperature. This option has been available in other countries for a number of years. A fiberoptic surface sensor provides a safe and accurate monitor of adult, pediatric, and neonatal body temperature.

SUMMARY

The magnetic resonance imaging suite is a challenging environment for the anesthesiologist, and carries inherent risks. Several factors account for this, including the remote location, the unique features of the magnetic resonance imaging scanner, and patient-related factors. Understanding the implications of the magnetic resonance imaging environment will facilitate ensuring the safety of the patient and personnel.

MR imaging in cervical spine trauma

Continual improvements in MR imaging, technology and MR imaging-compatible monitoring and fixation devices have allowed the incorporation of this relatively new imaging modality into standard algorithms for cervical spine trauma assessment. The ability of MR imaging to define the type of spinal cord injury, the cause and severity of spinal cord compression, and the stability of the spinal column is unmatched. The heavy reliance of the spinal surgeon on MR imaging for decisions regarding the type of therapy, the timing, the approach of surgical intervention, and for predicting patient outcome attests to the usefulness of this modality.

Recording of EEG during fMRI experiments: patient safety

The acquisition of electroencephalograms (EEG) during functional magnetic resonance imaging (fMRI) experiments raises important practical issues of patient safety. The presence of electrical wires connected to the patient in rapidly changing magnetic fields results in currents flowing through the patient due to induced electromotive forces (EMF), by three possible mechanisms: fixed loop in rapidly changing gradient fields; fixed loop in a RF electromagnetic field; moving loop in the static magnetic field. RF-induced EMFs were identified as the most important potential hazard. We calculated the minimum value of current-limiting resistance to be fitted in each EEG electrode lead for a representative worst case loop, and measured RF magnetic field intensity and heating in a specific type of current-limiting resistors. The results show that electrode resistance should be > or = 13 k(omega) for our setup. The methodology presented is general and can be useful for other centers.

Assessment of cerebral fat embolism by magnetic resonance imaging in the acute stage

The authors assessed the characteristic appearance of magnetic resonance imaging (MRI) of cerebral fat embolism in three patients. The MRI features in the acute stage were characterized by widespread, spotty lesions in the white matter, which appeared hyperintense on T2-weighted images and iso- or hypointense on T1-weighted images. The relation between clinical features and MRI findings are discussed.

Sedation, anesthesia, and physiologic monitoring during MR imaging: evaluation of procedures and equipment

The authors developed safe standard sedation and general anesthesia procedures for adults and children, including adequate physiologic monitoring, during magnetic resonance (MR) imaging. Six-year results are reported from one institution; 75% of the 600 patients per year who require sedation or anesthesia are children who require sedation only. Testing was done to determine MR compatibility of various types of equipment essential for monitoring and supporting sedated or anesthetized patients in 1.0- and 1.5-T MR imagers. Use of sedation procedures that include oral chloral hydrate after sleep deprivation resulted in a failure rate of 3.8% in sedating outpatient children. Every physiologic parameter that can be monitored under normal circumstances in the critical care unit or operating room can be monitored during MR imaging. Our experience indicates that with careful consideration of the unique MR environment and with rigorous testing of monitoring equipment, MR imaging can be performed safely in sedated or anesthetized patients.

Optical fibre-based goniometer for sensing patient position and movement within a magnetic resonance scanner using chromatic modulation

An optical fibre-based goniometer is described. This instrument was designed to measure the angular position of patients' limbs within the core of a magnetic resonance body scanner, via a 40 m remote fibre-optic link. The sensor exploits the advantages of optical fibre-based sensing, which include immunity to electromagnetic interference, intrinsic safety and chemical immunity. The detection electronics and signal processing are based on the principles of chromatic modulation, an inexpensive, intensity-independent technique in which a change in the spectral power distribution is measured over a broad bandwidth, by photodetectors with differing spectral responses. The optical fibre goniometer has an angular range of 90 degrees, with an average resolution of 2'. The long-term accuracy is within +/- 1 degree, the specified accuracy for the physiological application.

Magnetic resonance for the anaesthetist. Part II: Anaesthesia and monitoring in MR units

Anaesthetists are increasingly involved in patient care during magnetic resonance imaging and spectroscopy. This paper describes a system which has been developed for the management of critically ill patients and the conduct of anaesthesia in a magnetic resonance unit with a 1.6 tesla whole body magnet. Difficulties which arise from working in a confined space in a high magnetic field are highlighted. Different approaches to anaesthesia, sedation and the modification of equipment for use in this environment are reviewed. The problems associated with patient monitoring within a magnetic field are discussed and some solutions are suggested. A transport system for critically ill patients is described and a protocol for management is outlined.

Policies, guidelines, and recommendations for MR imaging safety and patient management. SMRI Safety Committee

The following are policies, guidelines, and recommendations from the Safety Committee of the Society for Magnetic Resonance Imaging (SMRI) concerning various issues related to magnetic resonance (MR) imaging safety and patient management. These policies, guidelines, and recommendations were developed to provide standardized and consistent information for use by health practitioners involved in clinical MR imaging.

Remote auscultatory patient monitoring during magnetic resonance imaging

A system for patient monitoring during magnetic resonance imaging (MRI) is described. The system is based on remote auscultation of heart sounds and respiratory sounds using specially developed pickup heads that are positioned on the precordium or at the nostrils and connected to microphones via polymer tubing. The microphones operate in a differential mode outside the strong magnetic field to reduce various sources of interference from the MRI equipment. After amplification, the signal is transmitted as infrared light to a small, battery-operated receiver and a headphone set. Thus, the patient can be simultaneously auscultated both inside and outside the shielded MRI room by infrared transmission through a metal mesh window. Bench tests of the system show that common mode acoustic noise is suppressed by approximately 30 dB in the frequency region of interest (100-1,000 Hz), and that polymer tubing having a diameter of approximately 2 mm can be used for efficient sound transmission. Recordings in situ show satisfactory detection of both heart sounds and respiratory sounds, although the signal is somewhat masked by noise during imaging. A clinical test incorporating 17 sedated or anesthetized patients was also performed. In all but four cases, the quality of the breath and heart sounds was regarded as acceptable or better.

Successful use of a Fortec II vaporizer in the MRI suite: a case report with observations regarding magnetic field-induced vaporizer aberrancy

Conducting a general anaesthetic within a magnetic resonance imaging (MRI) suite poses many problems for the anaesthetist. Ferromagnetic substances are contained in most anaesthetic and monitoring equipment. Their presence within the magnetic field may cause hazards and artifacts during imaging. This report describes the testing and use of a free-standing Fortec II vaporizer within an MRI suite. The Fortec II vaporizer's function was altered depending upon its distance from and orientation to the magnetic field. The MRI images were not affected by this vaporizer's presence within the MRI suite. We conclude that an inhalational anaesthetic can be administered, using a pretested free-standing anaesthetic vaporizer and a Bain circuit, within the magnetic field of a magnetic resonance imager.

Design and evaluation of a pneumatic pulse monitor for use during magnetic resonance imaging

We describe a portable, battery-operated instrument designed to monitor pulsations in the finger or toe. The monitor is based on pneumatic principles and is suitable for use in the electromagnetically harsh environment of magnetic resonance imaging. A compliant Silastic chamber is deformed with each cardiac pulsation; the resulting pressure wave is then transmitted along a 25-ft (7.5-m) microbore tube to a pressure sensor in a remote electronic monitor.

Circulatory monitoring of children during anaesthesia in low-field magnetic resonance imaging

Anaesthesia for patients in a Magnetic Resonance Imaging (MRI) scanner provides some problems for the design of both the anaesthetic and the monitoring equipment. This report presents a technique for continuously displaying the heart rate during anaesthesia for children in an MRI scanner. The monitoring system used light to detect differences in skin capillary circulation, and the light was transferred to and from the patient via fiberoptic cables. After amplification, the signal was displayed "on line" on a cardioscope, thus continuously presenting heart rate and, in part, a qualitative view of the skin vascular resistance.

Patient anesthesia and monitoring at a 1.5-T MRI installation

This paper describes the facilities successfully used to provide patient monitoring and anesthesia support in a 1.5-T imaging installation. The requirements for the MRI site for anesthetic gases, shielded power, and radiofrequency ports are outlined. Specific modifications in anesthesia machines, anesthesia cart, laryngoscope, mercury sphygmomanometer, oximeter, and remote blood pressure devices are described. Additional aspects of patient monitoring and support, e.g., electrocardiogram and pressure infusion, are also discussed.

Magnetic resonance imaging in acute head injury

Using cardiorespiratory monitoring and support equipment compatible with a low field (0.15 T) system, magnetic resonance imaging (MRI) of patients suffering acute head injuries proved to be both feasible and safe. An abnormality was demonstrated by magnetic resonance imaging in 46 of 50 patients examined within 7 days of head injury using T2 weighted (SE2200/80) and T1 weighted (IR2000/600/40) multislice sequences. IN contrast, computed tomography (CT) demonstrated abnormalities in only 31 of the 50 patients. Intracranial extracerebral space-occupying collections of blood were well shown by magnetic resonance imaging which provided especially clear definition in the posterior fossa, subtemporal and subfrontal regions. Magnetic resonance imaging was more sensitive to cerebral abnormalities associated with traumatic unconsciousness and detected parenchymal lesions both in patients in coma and in those who had lost consciousness for only a few minutes. Lesions seen with MRI but not with CT included non-haemorrhagic contusions and abnormalities thought to reflect shearing injuries of white matter and intracerebral vessels. Magnetic resonance imaging is an effective alternative to CT; the additional information it can provide should be valuable in increasing the understanding of the early effects and late consequences of a head injury.

Electrocardiographic gating and monitoring in NMR imaging

ECG gating and monitoring during NMR imaging may be achieved reliably by applying the principles in this tutorial. In order to use the ECG signal both for triggering and for patient monitoring it must have a prominent R-wave, while at the same time must have little artifact from gradient switches or the Lorentz voltage across the aorta, and not be significantly distorted by gradient switching artifacts. The twin goals of no image artifacts and minimal ECG artifacts may be achieved by the following means: (1) using ECG electrodes with minimal metal, (2) selecting electrodes and cables with no ferrous metals, (3) placing the limb electrodes close together, (4) placing the line between the limb electrodes and the leg electrode parallel to the magnetic flux lines and, if possible, parallel to the transverse component of the gradient flux lines, (5) keeping the area between the limb electrodes and the leg electrode small, (6) placing that area in the center of the imager and (7) twisting or braiding the cables. Following these principles allows artifact-free images and reliable ECG monitoring during ECG-gated NMR imaging examinations.

[Anesthesia and cardiac pacing]

Nowadays, anaesthetists often have to deal with pacemaker patients. All the problems encountered in the anaesthetic management of such patients are discussed in this paper: the pacemaker, specific risks linked to the pacemaker, monitoring of such patients, and temporary pacing. The preoperative assessment of pacemaker function is an absolute necessity. The technical characteristics of the pacemaker can be found in the patient's booklet. The clinical history should reveal a possible malfunction (syncopes). The underlying cardiac disease should be known, as it will have repercussions on the anaesthetic and surgical risks. An electrocardiogram and measurement of blood electrolytes must be carried out. There are three major risks linked to the pacemaker during surgery: 1) the loss of pacing by threshold (drugs, dyskaliemia); threshold (drugs, dyskalemia); 2) ventricular fibrillation (the intracardiac electrode conducting the electrocautery currents); 3) reprogramming or damaging of the pacemaker by electrocautery, cardioversion or nuclear magnetic resonance. The only mandatory monitoring of these patients is the electrocardioscope. Other monitoring techniques will be dictated by the underlying cardiac disease or the surgery planned. Temporary pacing is indicated in the same conditions as permanent pacing. However the intracardiac electrode can be displaced by moving the patient; the efficacy of pacing must therefore be continuously checked. During cardiac surgery, with cardiopulmonary bypass, conduction disturbances can occur. Temporary pacing electrodes should therefore be sewn onto the ventricular epicardium for the duration of the surgery; atrial electrodes should be added if sinus troubles can be expected. Oesophageal pacing is possible in the operating theatre because it is easily and rapidly set up: a bipolar oesophageal electrode linked to an external pacer can speed up the heart (atrial dysfunction) or slow down a tachycardia. An oesophageal electrocardiogram can also be carried out with this electrode. Swan-Ganz catheters can be also used for temporary pacing: either with two pairs of electrodes, atrial and ventricular respectively--this system being useful in a patient who does not move--or with a newer system where a single small electrode is introduced into the right ventricle by a special lumen in the Swan-Ganz catheter. Although external pacing was historically the first technique to be developed, it was abandoned because of the muscle pains it gave. Recently, a new technique of external pacing, with large electrodes and longer stimuli, has been developed for use in emergency situations.(ABSTRACT TRUNCATED AT 400 WORDS)

[Reprogramming of a pacemaker induced by electrocautery]

A case is reported of reprogramming of a ventricular unipolar permanent pacemaker induced by electrocautery during biliary surgery. After skin incision and use of the unipolar electrosurgery unit, the CPI model 505 multiprogrammable pulse generator previously set at 70 b X min-1 abruptly fired at 120 b X min-1. Application of a magnet over the pacemaker reduced the heart rate to 100 b X min-1. After surgery, the pulse generator was successfully reprogrammed to a rate of 65 b X min-1. Based on the analysis of this case and of previous reports, it is suggested, so as to avoid such complications, that the unipolar electrocautery be avoided when the surgical field is near the pulse generator or lead: that the bipolar electrocautery be preferred; that a magnet and non-invasive programmer be available during and after surgery; and that a postoperative assessment of the pulse generator be carried out.

A simple fiber optic monitor of cardiac and respiratory activity for biomedical magnetic resonance applications

A fiber optic reflectometer, capable of monitoring physiological parameters in a high magnetic field without significant disturbance of either rf or static fields, is described. Both cardiac and respiration rates were monitored with this optical device while 31P NMR spectra of the liver of an anesthetized rat were acquired.

Nuclear magnetic resonance. Its implications for the anaesthetist

The theory and practice of clinical nuclear magnetic resonance (NMR) imaging is reviewed. Problems which the anaesthetist will encounter are considered, and recommendations are proposed. Possible uses of NMR in anaesthesia are discussed.

Imaging of articular pathology: MRI, CT, arthrography

The purpose of this review article is to present the advantages and limitations of magnetic resonance imaging (MRI), computed tomography (CT) and arthrography for evaluating articular pathology. Techniques, patient selection, indications and contraindications for each modality are reviewed. MRI provides superior soft tissue contrast and image plane selection, making it the ideal technique for most articular abnormalities. However, obese patients and patients with certain electrical or metallic implants cannot be examined with MRI. CT provides superior detail for fine cortical bone and subtle calcifications, but lacks the soft tissue contrast provided by MRI. Arthrography is an invasive technique with minimal risk of allergy to contrast material or infection. This technique permits accurate measurement of capsular volume, allows for fluid aspiration for laboratory studies and permits injection of anesthetic and/or steroid compounds for purposes of treatment or confirming the site of the patient's pain. Magnetic resonance imaging, computed tomography and arthrography are useful techniques for evaluating articular pathology. Careful review of the clinical symptoms and patient's condition is necessary to select the most appropriate technique.