CT and US Anatomy - ACR

Non-Interpretive SkillsMRI Safety

Before You Begin

This module is intended primarily for clinical medical students or interns intending to learn or review non-interpretive radiology skills.

Please note that while not integral, this module series assumes some familiarity with basic imaging techniques and interpretive skills. If you wish to learn or review these concepts, please see our “Interpretive Skills” module series.

If material is repeated from another module, it will be outlined as this text is so that you are aware

Objectives

By the end of this module, you will be able to:

• State the four main categories of risk for patients undergoing MRI

• Discuss strategies for decreasing the risks of MRI

• Name the person(s) who is responsible for MRI safety

Introduction

• Safety issues in medical imaging are often discussed “in general,” with

• For MRI safety in particular, this method doesn’t work nearly as well as actual examples…

Introduction

• MRI offers an amazing level of diagnostic ability and involves no ionizing radiation

• For these and many other reasons, MRI is often viewed as “better” or “safer” than CT

• But, the mechanism for achieving MRI’s benefits creates several other safety issues

Format

• We’ll go through four common cases that we may encounter in inpatient and outpatient MRI patients - first as unknown cases, and then with answers and descriptions

• Please note: this module does NOT address all elements of MRI safety, but focuses on the four that you are most likely to see day to day

Case 1

• A patient is being rushed to the MRI suite for imaging of a suspected cerebrovascular accident (CVA). He was intubated for respiratory distress and arrives on a ventilator. The ICU team wants bring him directly into the scanner room. Is there anything you would like to check beforehand?

Case 1

Case 2

• A patient arrives for an MRI for evaluation of “possible focal nodular hyperplasia vs. adenoma” of the liver. They have no information in the electronic record, only a CT from an outside institution without a report.

• Should this patient under go MRI?

Coronal CT – IV + Oral Contrast

Case 2

Case 3

• A patient arrives for an MRI for evaluation of headaches. She is a veteran, and has served two tours in Afghanistan. Her only imaging is a sinus-protocol CT done earlier that day (no report yet).

• Should this patient undergo MRI?

Case 3

Case 4

• Two patients arrive for their MRI evaluations (one for a shoulder rotator-cuff protocol, and the other for a brachial plexus protocol).

• Should these patients undergo MRI?

Case 4

Or, from the point-of-view of our patients…

• How safe does this feel now? ☺

• FYI, another patient issue we won’t talk about

is claustrophobia – which is easy to minimize

until you’ve been in the bore of a scanner

• See let’s get in there …

THE “BIG MAGNET”

• You’ve heard about the “big” MRI magnet – but how big is “big?” First, some units.

• 1 Tesla (T) = 10,000 Gauss

• Earth’s magnetic field = 0.5 Gauss Okay, so...?

• 1.5 T = 30,000x Earth’s magnetic field

• 3 T = 60,000x Earth’s magnetic field

The “lower strength” in practice

Most MRIs in practice

THE “BIG MAGNET”

• We put our patients – knowingly – into a magnetic field that is 60,000x stronger than Earth’s – WHY?

• We’ll skip most of the physics – but we actually needthat strength to be able to send and receive enough signal from the hydrogen nuclei that we “see”

• Thus, our patient needs to go into that magnetic field

• What does NOT need to go into that magnetic field..?

Leaving these “outside” may seem

obvious

FERRO-MAGNETIC = NOT OKAY

Placed in 1.5T or 3T these “everyday objects” become

flying projectiles

The results of taking a non-MRI compatible

O2 tank into the scanner can be DISASTROUS…

But these may not be obvious – especially

when you are dealing with an incredibly sick patient that needs a scan “RIGHT NOW”

Case 1

First category of MRI risk:

Projectile

Coronal CT – IV + Oral Contrast

Anything problematic

here?

Case 2

Coronal CT – IV + Oral Contrast

What are THESE?

(and what study can

show them more

clearly?)

Case 2

Case 2

• We are good at remembering the “big magnet”

• But we are NOT good at remembering that the magnet only gets the hydrogen nuclei organized so that the radiofrequency (RF) pulses can interact with them

• Those RF pulses involve a HUGE amount of electricity – and PACEMAKERS and other implanted electronic devices can malfunction due to the induction of current!

• What “other implanted devices?”

Case 2

For example: ????????

• In recent years, companies have begun making MRI-safe pacemakers. BUT…

• If no product info is available, or the pacemaker was placed long ago – assume it is UN-safe to go into the MRI!

COCHLEAR IMPLANT

DEEP BRAIN STIMULATOR

Case 2

COCHLEAR IMPLANT

DEEP BRAIN STIMULATOR

Second category of MRI risk:

Induction of Current

Case 2

Anything problematic

here?

Case 3

On further questioning-

Case 3

The patient had a “blast injury” where she had a facial contusion and mild corneal

injury but no chronic symptoms…

… from this metallic debris in her orbit!

Case 3

• So the big magnet – the big “STATIC” magnet – organizes the hydrogens

• The RF pulses create the signal

• But how do we LOCALIZE that signal in 3D space? Smaller “gradient” magnets that CHANGE during the scan

Case 3

The torque on a ferromagnetic object caused by the STATIC and CHANGING

magnetic fields create TORSIONAL force

Those objects can rotate or translate, injuring adjacent structures – the most

vulnerable area for this being the ORBIT!

ANY possible metal exposure to eyes / face? Get:

Pre-MR Screening Orbit Radiograph

Another example - A “possible” cerebral aneurysm clip/ unspecified brain surgery?

Head CT

Third category of MRI risk:

Torsional Force

Case 3

Recap…

3. Induction of Current

1. Projectile

2. Torsional Force What’s left?

Case 4

So what exactly is wrong with a little body art?

Nothing – unless you are in an MRI with a big static magnet, smaller changing/gradient magnets, and a

huge amount of electricity…

Question – what makes black tattoo ink black?

Case 4

Question – what makes black tattoo ink black?

Ferrous Oxide

= Millions of tiny magnets …

Case 4

Resistive heating occurs in these magnets, proportional to the strength of the

magnet and time of the scan

Burns have been reported (usually in unconscious or sedated patients – like patients

being sedated due to claustrophobia ! )

FYI – the only way we avoid resistive heating with the “big magnet” – to achieve 1.5T or 3T field strength – is to cool it to the point of being a “superconductor” free of resistance

– with a whole ton of pressurized Helium and Nitrogen

Case 4

• Tattoos are not an absolute contraindication

• But, the risk increases if they are at the level of interest for imaging (= where the greatest amount of energy is directed)

• What to do? Consent, frequent checks during the scan, and in some cases, packing with ice

Fourth category of MRI risk:

Resistive Heating

Case 4

SUMMARY – MRI Safety

KEY = SCREENING:

• EQUIPMENT

• DEVICES

• Exposure to metal (anywhere)

• ANY possible craniofacial metal exposure? ORBIT RADIOGRAPH / CT

SUMMARY – MRI Safety

•You’ve heard this before, but now more than ever:

•SEE SOMETHING = SAYSOMETHING

•MRI safety is

EVERYONE’sresponsibility!

References

•www.MRI Safety.com (excellent free site with

information about MRI safety of a

variety of medical devices)

•AAPM “Physics of MR Safety:” (https://www.aapm.org/meetings/am

os2/pdf/59-17207-59975-979.pdf)

END