1 Sign up to receive ATOTW weekly - email ! Understanding MAGNETIC RESONANCE. IMAGING. ANAESTHESIA TUTORIAL OF THE WEEK 177. 3RD MAY 2010. ! Dr Grant Stuart, Specialist Registrar, Chelsea and Westminster Hospital, London Correspondence to ! ! ! QUESTIONS. 1. The following is true about magnetic fields: a) MRI scanners generate magnetic fields between and Tesla b) 1 Tesla is the equivalent of 10 000 Gauss c) The Earth's magnetic field is approximately 1 Gauss d) MRI superconductor coils are immersed in liquid oxygen in order to keep them below Kelvin e) The strength of the magnetic field remains the same as you move away from an MRI scanner 2. With regards to magnetic resonance imaging: a) The T-weighting of an image refers to the exact dosage of contrast agent given to the patient.
2 B) In T1 weighted images fluid structures will appear darker c) In T2 weighted images fluid structures will appear white d) The negative charge of the hydrogen atom in water is most commonly used to generate MR images e) The contrast agent Gd-DTPA can commonly cause anaphylaxis in patients undergoing MRI scans 3. The following are significant safety concerns in the MRI. examination room: a) Patient injury from metal projectiles b) Hypothermia due to the cold environment c) Burn wounds related to equipment cables d) Anaphylaxis from the use of intravenous contrast agent e) Hearing damage due to high noise levels 4. With regards to the management of patients for MR. imaging: a) In the event of a cardiac arrest, the magnet should be switched off in order for safe resuscitation to take place b) Patients with neurovascular aneurysm clips should never have MRI scans c) Cardiac pacemakers are at significant risk of failure within the 5 Gauss line d) Patients with angina should keep their GTN patches on during anaesthesia for MRI scans e) Intensive care patients should not have MRI if their tracheal tube contains a metal spring in the pilot balloon !
3 ATOTW 177 Understanding Magnetic Resonance Imaging, 03/05/2010 Page 1 of 12. Sign up to receive ATOTW weekly - email ! ! INTRODUCTION. ! Magnetic resonance imaging (MRI) produces high quality images of the body in cross section and in three-dimension. It detects the effects of induced changes in the nuclei of specific elements within the body and is particularly useful for the imaging of soft tissues, providing greater contrast between different types of soft tissue than computerised tomography (CT). It is the technique of choice for many neurological, cardiovascular, oncological and musculoskeletal conditions. ! ! ESSENTIAL KNOWLEDGE FOR MAGNETIC RESONANCE IMAGING. ! The physics of magnetic resonance imaging !
4 MRI relies on the fact that some atoms within the human body possess an odd unpaired proton. The proton nucleus of the hydrogen atom is one of the most abundant examples, being a major constituent of water. It responds particularly well to the application of an external magnetic field and is therefore one of the simplest atom to use for MRI. Another example is phosphorus, which as a component of adenosine triphosphate, allows for many metabolic processes to be studied. These nuclei possess a spin that results in a local magnetic field because of their charge, allowing them to act like small magnets. The alignment of these nuclei is usually random (Figure A), however when a strong electromagnetic field is applied to the body they align themselves with that field.
5 (Figure B). ! These nuclei can be turned out of alignment with the magnetic field by applying brief bursts of radio- frequency energy, creating an electromagnetic field perpendicular to the first magnetic field. When the electromagnetic field is removed, the radio-frequency energy taken up by the nuclei is released slowly as they relax back into alignment. The rate at which realignment takes place depends on the type of nucleus, or element being measured, and thus the emitted signal depends on the molecular properties of the This low radiofrequency radiation that is emitted induces an electrical signal within a set of three orthogonal gradient coils in the MRI machine. They are positioned in the transverse (X and Y) and longitudinal (Z) planes allowing for encoding of spatial information.
6 The detected signals are therefore able to form a three-dimensional image of the body. It is these gradient coils that are rapidly turned on and off during an MRI study that is responsible for the loud banging ATOTW 177 Understanding Magnetic Resonance Imaging, 03/05/2010 Page 2 of 12. Sign up to receive ATOTW weekly - email ! ! ! Different tissues within the body have different relaxation rates. T' refers to the relaxation time constant, and images may be T1 weighted (generated a few milliseconds after the electromagnetic field is removed) or T2 weighted (generated later than T1), depending on the characteristics of the tissue you wish to look at. Nuclei in hydrogen take a long time to decay to their original position, so fluid will appear dark (minimal signal) in a T1 weighted (early) image (Figure 1), but white in the later T2 image as the signal (Figure 2).
7 ! ! Because the signal that makes up the final MR image is very weak, any external radiofrequency sources can greatly interfere with its detection by the gradient coils. To prevent this the MRI machine is contained within a radiofrequency shield called a Faraday cage. This is built into the fabric of the MR room. To allow infusion lines or monitoring cables to enter the MR room, a hollow brass tube or waveguide' is built into the Faraday cage passing through into the control ATOTW 177 Understanding Magnetic Resonance Imaging, 03/05/2010 Page 3 of 12. Sign up to receive ATOTW weekly - email ! The Magnetic field MRI requires strong magnetic fields between and Tesla that are generated by superconductors.
8 To minimise the electrical resistance of the superconducting coils, they are immersed in liquid helium and cooled to below 1 Tesla = 10 000 Gauss (Earth's magnetic field = Gauss). = 1 weber/m2. The magnetic field strength falls away exponentially from the magnet. A safety line is usually demarcated at the level of (5 Gauss) within which pacemakers will malfunction, and therefore unscreened personnel should not enter (see hazards section below).2. A second line is demarcated at 50 Gauss within which a significant attractive force will be encountered on all ferromagnetic objects, which risk becoming dangerous projectiles. Such items include gas cylinders, needles, watches, floor cleaners and patient trolleys.
9 Within this line anaesthetic infusion pumps (or any electronic or mechanical equipment) may fail due to the effects of the magnetic While these lines of demarcation are often referred to theoretically, in practice many MRI units are simply divided into a safe zone' outside the scanner, and the controlled hazardous zone' within the MR examination room. INDICATIONS FOR THE USE OF MAGNETIC RESONANCE IMAGING. MRI is usually the preferred imaging technique in the following cases: Posterior fossa and infratentorial pathology Sinus and orbit pathology, sensorineural hearing loss and cranial nerve pathology Cerebral inflammatory disease including encephalitis, myelitis and meningitis Brain abscess Acute ischaemic strokes Spinal cord soft tissue pathology including congenital, traumatic, neoplastic and vascular abnormalities and disc pathology Demyelinisation and the myelopathies Airway malformations Vascular malformations Liver vascular pathology Joint soft tissue pathology CT scanning remains more useful for bony pathology, chest examinations.
10 Intracranial haemorrhage and abdominal and pelvic applications HAZARDS AND SAFETY CONSIDERATIONS FOR PATIENTS AND STAFF. IN THE MRI UNIT. 1. The presence of a strong magnetic field The strong magnetic field poses by far the most important hazard related to anaesthesia and care of patients requiring MRI. These powerful magnetic fields are able to exert large forces on any ferromagnetic materials in close proximity. They may also induce currents in metallic objects causing local heating and may interfere with monitoring equipment. Conversely, ferromagnetic objects and electrical fields in the vicinity of the magnet will degrade the quality of the MR images produced. The safety aspects related to ferromagnetic objects as projectiles, implants, foreign bodies and as equipment will be discussed in further detail below.