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Rad Tech’s Guide to MRI: Basic Physics, Instrumentation, and Quality Control

Rad Tech’s Guide to Mammography: Physics, Instrumentation, and Quality Control

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Rad tech’s guide to MRI: imaging procedures, patient care, and safety / by Carolyn Kaut Roth.

1. Magnetic resonance imaging—Outlines, syllabi, etc. 2. Radiologic technologists—Outlines, syllabi, etc. I. Title: Guide to MRI. II. Title.

[DNLM: 1. Magnetic Resonance Imaging—methods—Outlines. 2. Technology, Radiologic—methods—Outlines. WN 18.2 K21ra2001]

Blackwell Science’s Rad Tech Series in radiologic technology is intended to provide a clear and comprehensive coverage of a wide range of topics and prepare students to write their entry-to-practice registration examination. Additionally, this series can be used by working technologists to review essential and practical concepts and principles and to use them as tools to enhance their daily skills during the examination of patients in the radiology department.

The Rad Tech Series features short books covering the fundamental core curriculum topics for radiologic technologists at both the diploma and the specialty levels, as well as act as knowledge sources for continuing education as defined by the American Registry for Radiologic Technologists (ARRT).

The entry-to-practice series includes books on radiologic physics, equipment operation, patient care, radiographic technique, radiologic procedures, radiation protection, image production and evaluation, and quality control. This specialty series features books on computed tomography physics and instrumentation, patient care and safety, and imaging procedures; mammography; and quality management in imaging sciences.

In Rad Tech’s Guide to MRI: Imaging Procedures, Patient Care, and Safety, Carolyn Kaut Roth, a renowned educator and director technologist of MR programs of the University of Pennsylvania Medical Center, presents clear and concise coverage of patient care and safety issues of magnetic resonance imaging (MRI), as well as MR imaging procedures. Topics include patient care and safety, imaging procedures that describe MRI of the head and neck, spine, chest, musculoskeletal system abdomen, pelvis, and vascular system.

Carolyn Kaut Roth has done an excellent job in explaining significant concepts that are mandatory for the successful performance of quality MRI in clinical practice. Students, technologists, and educators alike will find this book a worthwhile addition to their libraries.

Enjoy the pages that follow; remember, your patients will benefit from your wisdom.

The purpose of Rad Tech’s Guide to MRI: Imaging Procedures, Patient Care, and Safety is to provide an easy reference for the study of magnetic resonance imaging (MRI) for the technologist who is preparing for the advanced level examination in MRI. This guide can also be used as a quick overview of MRI for the practicing technologist and physician. The outline format provides easy reference for each section of the text. The subtopics and bulleted text facilitate quick reference without “over reading” the material.

MRI safety and imaging procedures with anatomy have been discussed in this guide, and the basic principles and image contrast to pulse sequences and k-space are discussed in a partner guide in the Rad Tech series. The more complicated topics have hopefully been expressed in an understandable format that will encourage the reader to explore these topics, rather than run in the opposite direction. Purists may perceive our attempt at creating a “user-friendly” text as an oversimplification. However, we believe it important to disseminate difficult information to a variety of educational levels.

Notice: The indications and dosages of all drugs in this book have been recommended in the medical literature and conform to the practices of the general community. The medications described and treatment prescriptions suggested do not necessarily have specific approval by the Food and Drug Administration for use in the diseases and dosages for which they are recommended. The package insert for each drug should be consulted for use and dosage as approved by the FDA. Because standards for usage change, it is advisable to keep abreast of revised recommendations, particularly those concerning new drugs.

First, I would like to thank God for the opportunity to be involved in this project, the wisdom to undertake it, and the determenation to see it through.

Next, I greatfully acknowledge the encourgament of those indivudals who have given me the support and patience to complete this guide. These inculding my loving husband, scott, and the rest of my family—my mom, dad, brothers, in-laws, nieces, and nephews. I love u all.

My thanks, however, cannot end with my family. My extended “HUP” family was also instrumental in providing information and images for the text. In particular, I would like to thank Lisa Desiderio, Paula Malagoli, Tony Festa, Dave Flint, Jorge Forero, Camille gallen, Chirsty Lennen, Joe Shea, Lena Inerso, Doree Schrann, Russell Boucher, Lee Cohen, Doris Caine-Edwards, Beverly Farrar, Nacy Fedullo, Jim Garrisson, Christine Harris, Dave Yost, Mike Irvin, Ralph Magee, Ray Chemiewlewslki, Ted Czwoski, and Ann Kopp, my office mate. Without your support, this project would have been virtually impossible.

CHAPTER 1

Patient Care and Safety for Magnetic Resonance Imaging

Chapter at a glance

Introduction to Patient Care and Safety for MRI

Screening Patients and Personnel

Pre-MRI Screening Form or Questionnaire

Pre-MRI Screening Interview

Prescreening for Metallic Implants

Pregnant Patients

Pregnant Employees

Ancillary Equipment and Implants

Implants and Prostheses

Torque and Heating

Artifacts from Metallic Implants

Hemostatic Vascular Clips

Intravascular Coils, Filters, and Stents

Carotid Artery Vascular Clips

Vascular Access Ports

Artifacts from Implanted Vascular Access Ports

Heart Valves

Dental Devices and Materials

Penile Implants

Otologic Implants

Ocular Implants

Intraocular Ferrous Foreign Bodies

Metallic Foreign Objects

Bullets, Pellets, and Shrapnel

Orthopedic Implants, Materials, and Devices

Surgical Clips and Pins

Halo Vests and Other Similar Externally Applied Devices

Electrically, Magnetically, or Mechanically Activated or Electrically Conductive Implanted Devices

Pacemakers

Assessing and Monitoring

Sedated Patients in MRI

Claustrophobia

Contrast Agents for MRI

Gadolinium Side Effects and Reactions

Dose for Gadolinium

Precautions for Gadolinium

Iron Oxide Contrast Agents

Life-Threatening Situations

Safety Precautions for Placement of Electrical Conductors

Environmental Considerations: Temperature and Humidity

Gauss Line and Magnetic Field Strength

Magnetic Field Shielding

RF Shielding

Emergency Procedures

Quench

Evacuation

Biologic Considerations

Radio Frequency Fields

Specific Absorption Rate

FDA Guidelines for RF Exposure

Potential Bioeffects to RF Irradiation

Static Field Strength

Projectiles

Prescreening for Projectiles

Tesla

Static Fields Below 2 T

Static Fields Above 2 T

Biologic Effects

FDA Guidelines for Static Magnetic Fields

Gradient Magnetic Fields (Time-Varying Magnetic Fields)

Biologic Effects of TVMF

Acoustic Noise

FDA Recommendations for TVMF

Future Safety Considerations

INTRODUCTION TO PATIENT CARE AND SAFETY FOR MRI

To date, there have been virtually no long-term adverse biologic effects of extended exposure to magnetic resonance imaging (MRI) in general. However, when separate components of the MRI process are examined, several inconsequential and reversible effects of magnetic, gradient, and radio frequency (RF) fields can be observed. When MRI systems began to be used in the United States, the Food and Drug Administration (FDA) issued guidelines to hospital’s Investigational Review Boards (IRBs) in “Guidelines for Evaluating Electromagnetic Exposure Risks for Trials of Clinical NMR Systems,” on February 25, 1982. Follow-up was presented in December of that same year, not intending to provide limitations, but rather to evaluate the need for a risk assessment. Therefore the need to evaluate MRI for potential risks and hazards is clear and, to validly discuss long-term biologic effects of MRI, all of the components of the imaging process should be considered. These elements include not only the main magnetic field known as the static magnetic field (B₀), but also time-varying magnetic fields caused by magnetic field gradients and RF fields (B₁) created by RF transmitters and receiver coils.

SCREENING PAFIENTS AND PERSONNEL

Conducting a careful screening procedure is crucial to ensure the safety of anyone who enters the area of the magnetic resonance (MR) system. Careful questioning and education of patients and personnel help to maintain this controlled environment. Patient and personnel screening is, to date, the most effective way to avoid potential health hazards to patients involved in MRI. Patients and MR personnel with questionable ferromagnetic foreign objects either in or on their bodies should be rigorously examined so as to avoid any serious health risks or accidents.

All individuals, including patients, volunteer subjects, visitors, MR health care providers, and custodial workers, must be thoroughly screened by qualified personnel before being exposed to the MRI environment. In addition, routine preventative maintenance checks by the service engineer, as well as continuing education is also important. Therefore careful planning and diligent upkeep of the MR facility can provide a safe environment for patients, visitors, and employees.

Most MR-related injuries have been a direct result of deficiencies in screening methods. Unfortunately, not all MR users perform a rigorous screening procedure and there is a lack of agreement on what constitutes an appropriate or necessary protocol that will ensure the safety of individuals and patients in the MR setting. One other note is warranted: If a patient has previously had an MR examination, this is not an indication that they are safe to undergo another.

In 1994 the safety committee of the International Society for Magnetic Resonance in Medicine (previously designated as the Society for Magnetic Resonance Imaging) published screening recommendations and a questionnaire that encompassed all of the important aforementioned issues. These recommendations were developed from a consensus from an international panel of MR experts and were intended for use as a standard of care at all MR centers. Elster and others (1994) also published a screening recommendation. This information was somewhat similar to the content of the recommendations provided by the safety committee, which is not surprising since many of the same MR clinicians and scientists were involved in the development of both documents. A comprehensive pre-MRI screening form may be downloaded from the Internet (Mrsafety.com) and used at MRI facilities. This form was recently developed in collaboration with Frank Shellock and Anne Sawyer-Glover (1999).

Pre-MRI Screening Form or Questionnaire

The initial screening process should involve completion of a questionnaire that is specifically designed to determine whether there is any reason that the individual would have an adverse reaction to the MRI environment.

The questionnaire must include important questions concerning previous surgery, prior injury from a metallic foreign body, and whether the individual is pregnant.

In addition, the questionnaire should contain a means of determining whether the individual has any of the various implants, materials, devices, or objects that are considered to be a contraindication or problematic in the MR environment, including any device that is electrically, magnetically, or mechanically activated.

A diagram of the human body should be provided on the questionnaire for the individual to indicate the position of any object that would be potentially hazardous or would interfere with the interpretation of the MR procedure as a result of causing what is known as an artifact.

The pre-MRI screening questionnaire may also be used to obtain additional pertinent information related to the safe performance of the MR procedure. For example, questions may be asked concerning previous adverse reactions to contrast media that should alert the health care provider to potential problems.

Finally, pertinent questions should include information related to the phase of the menstrual cycle, as well as the use of contrast media and hormone treatment that are relevant to patients undergoing MRI examinations for breast abnormalities.

Pre-MRI Screening Interview

With the use of any form of written questionnaire, limitations related to incomplete or incorrect answers provided by the patient, guardian, or other individual preparing to enter the MRI environment are bound to exist. For example, there may be difficulties associated with individuals who are impaired with respect to their vision, fluency, or level of literacy. Therefore it may be necessary to have a version of the screening questionnaire in the individual’s native language or to have a direct verbal interaction with individuals who may routinely have problems with written questionnaires.

It is also recommended that the MR technologist or other trained staff member conduct an oral interview to further ensure the safety of the individual entering the MRI environment or undergoing an MR procedure. This allows a mechanism for clarification or confirmation of the answers to the questions posed to the individual so that there is no miscommunication.

The “oral phase” of pre-MRI screening is believed to be especially vital for establishing the reliability of the individual’s answer.

Prescreening for Metallic Implants

Every MRI facility must establish a standardized policy for pre-MRI screening of patients and individuals who are suspected of having metallic foreign objects. The policy should include guidelines concerning which individuals or patients require “work-up” by radiographic procedures and the specific procedure to be performed (e.g., number and type of views, position of the anatomy). Each case must be considered on an individual basis to assess the relative risk with regard to the metal object and the MRI environment. These basic precautions should be taken with respect to any type of MR system regardless of the field strength, magnet type, and the presence or absence of magnetic shielding.

Pregnant Patients

A patient who is pregnant or suspects that she is pregnant must be identified before exposure to the MRI environment to address the risks versus the benefits of the examination for the individual. To date, there are no known biologic effects of MRI on fetuses. However, a number of mechanisms exist whereby there could be a potential for adverse effects of the interaction of electromagnetic fields with developing fetuses. Cells undergoing division, which occurs during the first trimester of pregnancy, are more susceptible to a variety of effects. For this reason, many facilities choose to delay MR imaging until after the first trimester.

The FDA guidelines indicate that the safety of MRI when used to image the fetus has not been established or proved. Therefore patients should be provided this information and should also be informed that there are presently no known deleterious effects related to the use of MR procedures during pregnancy. However, according to the recommendations provided by the safety committee of the Society for Magnetic Resonance Imaging, in “MR procedures may be used for pregnant patients when other nonionizing forms of diagnostic imaging are inadequate or when the examination provides important information that would otherwise require exposure to a diagnostic procedure that requires ionizing radiation (e.g., computerized tomography, fluoroscopy).” For this reason, the American College of Gynecology and Obstetrics recommends that potential MR patients who are pregnant should be reviewed on a case-by-case basis. This policy has been adopted by the American College of Radiology and is considered to be the “standard of care” with respect to the use of MR procedures for pregnant patients.

Pregnant Employees

A recent survey revealed no increased incidence of spontaneous abortions among MR technologists and health care practitioners. (It should be noted that the incidence of spontaneous abortions makes up approximately 30% of all pregnancies.) After this survey, the following determinations were made:

The facility from which the data was observed changed their in-house policy from one which restricts pregnant technologists from being near the magnetic field to a policy which allows pregnant technologists to be in the room to set up the patient but not to remain in the room during image acquisition.

It has been suggested that informed workers make their own decision. For this reason, MRI facilities have established individual guidelines for pregnant employees in the MR environment. The majority of facilities have determined that pregnant employees can safely enter the scan room of superconductors or permanent magnets on which the magnetic field is contained, but must stay out while the scanner is running when the RF and gradient fields are employed.

A policy is recommended that permits pregnant technologists and health care workers to perform MR procedures, as well as to enter the MR system room and to attend to the patient during pregnancy, regardless of the trimester.

Importantly, the technologists or health care worker should not remain in the MR system room or magnet bore during the actual operation of the device. This recommendation is especially important for MR users involved in interventional MR-guided examinations and procedures to which to adhere since it may be necessary for them to be directly exposed to the MR system’s electromagnetic fields at levels similar to the fields used for patients. Notably, these recommendations are not based on indications of adverse effects, but rather from a conservative point of view and the notion that there are insufficient data pertaining to the effects of the other electromagnetic fields of the MR system to support or allow unnecessary exposures. This recommendation was influenced by a government legal decision concerning the rights of pregnant workers in hazardous environments.

As an additional precaution, some facilities recommend that the employee stay out of the magnetic field entirely during the first trimester of pregnancy.

ANCILLARY EQUIPMENT AND IMPLANTS

To deem ancillary equipment safe for use in MRI, the FDA recommends one of three criteria: manufacturer declaration, FDA approval, and prior testing. Manufacturer declaration simply means that the manufacturer has tested the equipment and assures its safety. FDA approval means that the FDA has tested the material and determined that it is safe for use in MRI. The third requirement means that the instrument has been subjected to prior testing, which means that someone has tested the instrument before its use in MRI.

Implants and Prostheses

As we consider metallic implants and their safety profile in the MR environment, three serious effects become clear: torque, heating, and artifactual results on MR images.

Therefore before we consider imaging patients using MR, be aware of surgical procedures that the patient has undergone before the MR examination. For a complete list of MR-compatible implants and prosthesis, refer to “MR Imaging and Biomedical Implants, Materials, and Devices: An Updated Review” in Radiology magazine (1991).

Torque and Heating

Some metallic implants have shown considerable torque when placed in the presence of a magnetic field.

The force or torque exerted on small and large metallic implants can cause serious effects since unanchored implants have the potential of unpredictable movement within the body.

The type of metal used in these implants is one factor that determines the force exerted on them in magnetic fields. Although nonferrous, metallic implants may show little or no deflection to the field, they can cause significant heating as a result of their inability to dissipate heat from RF absorption.

Artifacts from Metallic Implants

Although artifacts cannot be considered as biologic effects of the MR process, misinterpretation of MR images can yield devastating consequences. It should be noted that the type of metal and the size of the metallic implant determines the size of the artifact noted on the MR image. Therefore when a metal artifact is noted on the MR image and no metal is present within the patient, the presence of blood products suggestive of a hemorrhagic lesion may be indicated.

The presence of an aneurysm clip in a patient referred for an MR procedure represents a situation that requires the utmost consideration because of the associated risks (Figure 1-1). Certain types of intracranial aneurysm clips (i.e., clips made from martensitic stainless steels, such as 17-7PH or 405 stainless steel) are an absolute contraindication to the use of MR procedures because excessive magnetically induced forces can displace these clips and cause serious injury or death. By comparison, aneurysm clips classified as nonferromagnetic or weakly ferromagnetic (i.e., those made from Phynox, Elgiloy, austenitic stainless steels, titanium alloy, commercially pure titanium) are safe for patients undergoing MR procedures. For the sake of discussion, the term “weakly ferromagnetic” refers to metal that may demonstrate some extremely low ferromagnetic qualities using highly sensitive measurements techniques (e.g., vibrating sample magnetometer, superconducting quantum interference device, SQUID magnetometer), and, as such, may not be technically referred to as being nonferromagnetic. It is further recognized that all metals possess some degree of magnetism, such that no metal is considered to be totally nonmagnetic or nonferromagnetic.

Figure 1-1 This axial image of the brain demonstrates a patient who has had two aneurysm clips implanted. Note that the metal artifacts are different sizes (smaller toward midline and larger toward the right side just behind the orbit), which probably indicates that these clips are made from different trace metals. (Both clips were deemed safe for MRI.)

It is not uncommon to use MR procedures to evaluate patients with certain types of aneurysm clips. Becker et al, in “using MR systems that ranged from 0.35 to 0.60 Tesla (T), studied three patients with nonferromagnetic aneurysm clips (one patient, Yasargil, 316 LVM stainless steel; two patients, Vari-Angle McFadden, MP35N; 316 LVM) and one patient with a ferromagnetic aneurysm clip (Heifetz aneurysm clip, 17-7PH) without incident.” Similarly, Dujovny et al have reported no adverse effects in patients with nonferromagnetic aneurysm clips who have undergone procedures using 1.5 T MR systems.

It is therefore recommended that MR imaging in patients with aneurysm clips be delayed until such time that the type of clip is emphatically identified as nonferrous.

Hemostatic Vascular Clips

Hemostatic clips should be evaluated ex vivo before the MR exam, although none of the six hemostatic vascular clips evaluated has shown deflection by the static magnetic field.

To date, none of the various hemostatic vascular clips evaluated was attracted by static magnetic fields up to 1.5 T. These hemostatic clips are made from nonferromagnetic materials such as tantalum and nonferromagnetic forms of stainless steel.

There has never been a report of an injury to a patient in association with the presence of a hemostatic vascular clip in the MR environment.

Intravascular Coils, Filters, and Stents

Although they have shown deflection in the magnetic field, these devices usually become imbedded in the vessel wall after several weeks and are unlikely to become dislodged.

Therefore it is considered to be safe to perform MR imaging on most patients with intravascular devices, provided a reasonable period after implantation has elapsed.

Carotid Artery Vascular Clips

Each of five carotid artery vascular clamps displayed deflection in the magnetic field.

The deflection was mild when compared with pulsatile vascular motion within the carotid arteries.

Only the Poppen-Blaylock carotid artery clamp is contraindicated for MRI because of its large attractive response to the magnetic field.

Vascular Access Ports

Vascular access ports and catheters are bioimplants that are commonly used to provide long-term vascular administration of chemotherapeutic agents, antibiotics, analgesics, and other medications.

Vascular access ports are implanted typically in a subcutaneous pocket over the upper chest wall with the catheters inserted in the jugular, subclavian, or cephalic veins. Smaller vascular access ports, which are less obtrusive and tend to be tolerated better, have also been designed for implantation in the arms of children or adults, with vascular access via an antecubital vein.

Vascular access ports have a variety of inherent features (e.g., a reservoir, central septum, catheter) and are constructed from various types of materials including stainless steel, titanium, silicone, and various forms of plastic. Because of the widespread use of vascular access ports and associated catheters and the high probability that patients with these devices may require MR procedures, it was important to determine the MRI-compatibility of these bioimplants.

Only three of the implanted vascular access ports tested had measurable deflection in the magnetic field. These deflections were thought to be insignificant to the applications of these ports. Therefore it is probably safe to image patients with implanted vascular access ports.

Artifacts from Implanted Vascular Access Ports

Even the MRI-compatible or MRI-ports made entirely from non-metallic materials are, in fact, observed on the MR images because they contain silicone. The septum portion of each of the vascular access ports typically is made from silicone. Using MRI, the Larmor precessional frequency of fat is similar to that of silicone (i.e., 100 Hz below fat at 1.5 T).

Therefore silicone used in the construction of vascular access ports may be observed on MR images with varying degrees of signal intensity depending on the pulse sequence selected for imaging. Manufacturers of non-metallic vascular access ports have not addressed this finding during the advertising and marketing of their products.

Vascular access ports made from nonmetallic materials are claimed to be MRI-compatible and invisible on MR images. However, if a radiologist did not realize that this type of vascular access port was present in a patient, the MR signal produced by the silicone component of the device could be considered an abnormality, or at the least, present a confusing image. This confusion may present a diagnostic problem for a patient being evaluated for a rupture of a silicone breast implant, because silicone from the vascular access port may be misread as an “extracapsular silicone implant rupture.”