Module 1: Fundamentals of Patient Monitoring. Contents. Introduction. Why is monitoring in MR different? Exercise 1_1

Transcription

1 MRES MR Safety and Monitoring Module 1: Fundamentals of Patient Monitoring Contents Introduction Why is monitoring in MR different? Exercise 1_1 What parameters should we monitor? Selection of monitoring modality Communications I Communications II: Electrical noise reduction Communications III: Magnetic compatibility Video monitoring 1/10

2 Introduction The foremost concern in any medical imaging modality can be summarised by the adage: "primum non nocerum"(first, do no harm) Although MR permits unrivalled non-invasive imaging forethought and vigilance must be exercised to maintain patient safety. In all branches of clinical medicine observation and communication with the patient are the most basic procedures to be performed and an MR examination should provide no impediment to this rule. Observation of the patient outside of the magnet confines is often so trivial to be thought inconsequential but skin colour, pulse and respiration rate often indicate underlying disease or distress. Verbal communication is also taken for granted outside of the imaging environment and can obviously indicate to us the state of patient well-being. The next three modules will describe what types of clinical monitoring we should be familiar with and how these techniques are adapted for use in the MR environment. The following screens give an overview of these techniques and an outline of the types of technology we will examine to ensure hazards created by the MR imaging environment are minimised. Figure 1.1: Although MR imaging can tell us much about human structure and function, we have little knowledge of the well-being of our patient during the examination. 2/10

3 Why is monitoring in MR different? One could be excused for asking the question above. In most all other clinical spheres conventional monitoring equipment will at least operate without a creating a hazard for the patient. In an MR environment it is possible to cause serious burns and electrical shock if normally servicable electronic monitoring equipment is used. The presence of electrically conductive leads or wires exposed to field gradients and radio frequency pulses as we shall see is both the basis to the hazard and to the inability of the equipment to function correctly. Much state of the art electronic equipment relies on the presence or absence of static magnetic fields in its normal operation. The presence of magnetic fields greater than 1 Tesla cannot only cause faulty operation but can permanently damage equipment. Finally, electronic equipment has the potential to be a source of electrical noise due to alternating electrical currents present within leads or radiated E and B fields from encased circuitry. As the induced currents within the reception coil are minute, extraneous lectrical noise can seriously deteriorate image quality. Figure 1.2(a) demonstrate some common place items that cannot be taken into the magnetic enrvironment. Figure 1.2(b)... Figure 1.2(a): Seemingly innocuous electronic and mechanical components can be hazardous when combined with the MR environment Figure 1.2(b): Sometimes elaborate equipment is required to present a safe and calming environment or distraction to anxious or younger patients 3/10

4 Exercise 1.1 Q1. Which of the following items has the potential to cause deterioration fo an MR image if present in the magnet bore during scanning? (a) a non-magnetic 1.5 volt cell (b) a walkman style radio (c) an inserted intranenous catherter connected to a saline bag (d) a hearing aid false true false false This answer is incorrect. Consider if the device radiates a possible noise signal or contains ferromagnetic material. This is correct. A radio would cause deterioration of an MR image. This answer is incorrect. Consider if the device radiates a possible noise signal or contains ferromagnetic material. This answer is incorrect. Consider if the device radiates a possible noise signal or contains ferromagnetic material. Q.2 Which of the following items of medical equipment would be compatible with an MR examination due to the absence of electrical noise emissions? (a) an ECG machine false This answer is incorrect. Consider the possilbe radio frequency emissions due to internal circuitry. (b) a pulse oximeter false This answer is incorrect. Consider the possilbe radio frequency emissions due to internal circuitry. (c) a portable false This answer is incorrect. Consider the possilbe radio frequency emissions syringe pump due to internal circuitry. (d) a sphygmomanometer true This answer is correct. An ECG machine, a pulse oximeter and a portable syringe pump would all be incompatible with an MR examination due to emissions of radio frequency noise from integral electronic circuitry. 4/10

5 What parameters should we monitor? Recall the fundamental observations we make of our patients: their colour, pulse rate and, on occasion, breathing or respiratory rate. We will of course make more elaborate measurements of physiological function as the situation demands but these three parameters can be effectively monitored in order that we are always aware of the patient's well-being. The types of equipment used for these observations are : (1) Pulse oximeter measures blood oxygen concentration and thereby the patient's colour. It can also give an indication of heart rate. Figure 1.3 demonstrates a typical pulse oximeter attachment. (2) Electrocardiograph or ECG measure the electrical activity of the heart muscle on the skin surface and can measure heart rate and rhythm. (3) Respiratory bellows or pneumatic capsules measure the change in chest or abdomen size which occurs during the normal respiratory cycle. Such devices tell us both the amplitude and rate of breathing. The technology behind these monitoring techniques will be explored in further modules. Figure 1.3: As we shall see, the pulse oximeter is an example of a non-invasive monitoring technique easily adapted to the MR environment 5/10

6 Selection of monitoring modality. Given the ease of application of some monitoring equipment it may be standard practice in some centres to routine apply a device such as the pulse oximeter to all patients. Make an informed choice as to what you would consider prudent in the following clinical scenarios: (1) An anxious patient with a history of episodic asthma (a respiratory condition). (2) A patient with a history of unstable angina. (3) An obese patient with a history of snoring who requires a prolonged MR examination. (You may need to investigate the problems suffered by subjects with sleep apnoea.) Solution Pre-existing anxiety can be heightened by exposure to the MR environment. One of the most common manifestations of anxiety is an elevated respiratory rate or hyperventilation. Many asthmatic patients have a psychological dimension to their illness and may have an episode precipitated by an anxiety provoking experience such as an MR examination. In such patients it is worthwhile to monitor the subject's respiration to both assist in image quality and offer some reassurance to the subject during the procedure. Angina or chest pain due to inadequate blood flow to the heart can also be precipitated by an anxiety producing experience. The physical environment of the magnet and subsequent procedures can provoke episodes of angina. The presence of ECG changes and observation of heart rate and rhythm are of value in the detection and management of this condition. A single lead electrocardiogram (ECG) would be prudent in this instance. A prolonged supine examination on a comfortable bed can result in some patients falling asleep. Although this is often seen as a benefit to those performing the image, some snoring patients experience obstructive sleep apnoea or cessation of breathing due to a floppy nasopharynx when asleep. Blood oxygenation is compromised and ischaemic damage to brain and heart are thought to occur. It would be wise to monitor such patients with pulse oximetry with an alarm set at >90% saturation. Respiratory monitoring would confirm the cause of desaturation. 6/10

7 Communications I The close proximity of the patient to the technologist in many other imaging modalities makes verbal communication a trivial concern. The obvious remote position of the patient and the attendant noise from the pulsing field gradients makes communication extremely difficult without the use of an electronic intercom system. The associated electrical noise generated by an electronic intercom system is also likely to introduce noise into the image in an identical manner to which the monitoring equipment discussed earlier would behave. Manufacturers produce integrated intercom systems and third party vendors (Figure 1.4) supply intercom systems which are MR compatible and also permit playing of audio material from radio or compact disc. The essential features in the design of such apparatus are: (1) the marked reduction in electrical noise and (2) the absence of ferromagnetic materials. Figure 1.4: Third party vendors cater for communications and patient alarm systems with compatible equipment, which are electrically 'quiet' and lack ferromagnetic components 7/10

8 Communications II: Electrical noise reduction Speech and music consist of signals in the frequency range of 20 to Hz. It is possible that harmonics or multiples of audio frequencies may be present, particularly if the amplification system is not highly linear. These harmonics and the fundamental signal can masquerade as an induced MR signal given the sensitivity of the coil and receiver in the MR system. Such noise may present as banding or striping in the reconstructed image. In order that such noise be attenuated a device known as a lowpass filter is employed at the point of entry of the signal through the Faraday cage. The filter plate will contain a number of electrical filters which function to restrict the signals passing into the room to their fundamental frequencies and no more. This is often technically demanding in the case of high current, high voltage gradient cabling. With the help of a service engineer you should try to identify the lowpass filters on your MR system. 8/10

9 Communications III: Magnetic compatibility A conventional loudspeaker (Figure 1.5) operates on the basis of the passage of an alternating current through a coil placed in a static magnetic field. The coil which is physically connected to a rigid cone will move according to the force due to the alternating current flow. The presence of the permanent magnet precludes the use of a conventional loudspeaker in the vicinity of the superconducting magnet. Similarly a moving coil microphone in which a small voltage is induced in the coil when the cone moves with speech is also incompatible. An alternative transducer for these roles is found in piezo crystals. Crystals of barium titanate or lead zirconate exhibit mechanical deformation when an electrical field is applied across their structure (Figure 1.6). Similarly a minute piezo voltage is present when they are deformed by a signal which may be in the audio range. The piezo speaker and microphone can thereby form the basis of an MR compatible speakers and microphones. Figure 1.5: Conventional loudspeakers use an integral permanent magnet which is not compatible with use in an MR system. It is possible to use the stray field of the magnet to fulfil the same function Figure1.6: The piezo electric effect is ideally suited to use around the magnet for audio transducers 9/10

10 Video monitoring Where a direct line of vision cannot be maintained with the patient or an alternative view on the proceedings within the magnet room is desired a remote video camera and monitor is often installed. The concerns of electrical noise and magnetic compatibility still apply in this technology. The operation of a modern closed circuit television camera (CCTV) is based on so-called solid state electronics and does not possess the older electron beam type camera tube. Both complementary metal oxide silicon (CMOS) and charged coupled device (CDD) cameras are suitable for use in the vicinity of a magnet. 10/10