CT (computerized tomography) scanning revolutionized medicine when it arrived in the mid-1970’s. Suddenly, organs and bones were visible with the clarity of an anatomy textbook. Its popularity shows no signs of abating as faster, more powerful scanners capture 3-dimensional movies of the heart and show the inside of the colon or bronchial tree. And no wonder it’s a ubiquitous device: it’s quiet, easy to run, takes up little floor space and gives answers in seconds. In many Emergency Rooms it’s “The Truth Machine”. But this truth has a hidden price.
A CT scanner is a relatively new and very sophisticated way to use an old-fashioned medical stand-by: x-rays. Just as a standard x-ray machine has a tube that shoots x-rays (with the aid of lots of electricity) in the direction of a detecting surface (once a silver-coated film, now a digital plate), every CT scanner has an array of x-ray emitters opposite an array of detectors. The patient is interposed between them, so that the xrays pass through. The CT scanner adds circular movement to the emitter-detector pairs so that a slice of the patient can be examined. The patient moves as well, rolling smoothly on a padded table through the circling emitters and detectors, with continuous acquisition of data that’s over in a few seconds. Called helical scanning, it’s like getting a spiral-sliced ham, with slices thinner than tissue paper. However, it takes a lot of radiation to get detail like this.
There is radiation all around us, all the time, passing through us, bouncing off us: cosmic rays, solar radiation, radiation from the radon in our basements, radio waves, microwaves from cell phone towers. Medical radiation has become a bigger and bigger part of everyone’s radiation burden due to the popularity of CT.
Radiation damages the DNA in your cells faster than it can repair itself and x-rays are very serious offenders. Damaged DNA has been implicated as the start of many cancers. As the burden of damage accumulates during your life, your chance of developing radiation-related cancer goes up. Children are particularly susceptible to radiation damage, both because of the immaturity of their tissues and because they have longer lives ahead of them for the cancers to show up.
Luckily, there is an excellent alternative. MRI (magnetic resonance imaging) scanning has completely different capabilities that put it far ahead of CT as a way of making images of what’s inside you, the way a turbo jet helicopter outclasses a mini van as a means of transportation. It gives fantastically detailed images without poisoning us with the x-rays that are the language of the CT scanner.
Dr. Thierry Huisman, the director of Pediatric Radiology at The Johns Hopkins Hospital in Baltimore, is very enthusiastic about MRI for children. “There is no radiation, you get high resolution anatomical images, and it allows the study of functional processes in the body.”
But why do MRI scans have to be so uncomfortable – so lengthy, so noisy, and so tightly confining? The answer lies in the terrific strengths and quirky weaknesses of the technology, which dictate how the scanners are built.
A CT scan is something done TO you, but an MRI scan is something done WITH you: it’s interactive, but don’t worry—there’s no skill required on your part. It’s all about your hydrogen atoms and they know exactly what to do. Hydrogen is the most plentiful element in the human body. It’s more concentrated in some tissues, like fat, and less so in others, like water and blood. In bone, there is the least amount of all. So, if you can figure out how to get hydrogen atoms to show you where they are, you can tell one tissue from another.
Hydrogen atoms have two properties that allow an MRI to happen. The first is the ability to line up with a magnetic field (that’s the “M” in MRI). The other is the ability to absorb radio waves (that’s resonance, the R). The magnetic field sets the stage for the hydrogen atoms and the radio waves sent in from outside the scanner give them the script. But the magnet that makes a magnetic field strong enough to make every hydrogen atom in your body sit up and take notice is not like the one that sticks your shopping list on your refrigerator.
From the outside, an MRI scanner resembles the business-end of a very clean cement truck: an open tunnel, just big enough for a person to lie down, in the middle of a room-size machine covered with smooth plastic and a few blinking lights. The only visible moving part is the padded table that rolls you inside the tunnel. Behind the plastic facing, most of the machine’s bulk is the magnet that encircles the tunnel: thousands of feet of wire housed in a refrigerating jacket that freezes the wire down to -452F, four times colder than Antarctica. (Inside the tunnel, though, it feels just like normal air-conditioning.) At that temperature the wires become superconductors: electric current flows through them much faster than usual, creating a stronger magnetic field. This superconducting magnet has a powerful, stable magnetic field about 3000 times stronger than your refrigerator magnet.
In this magnetic field, all your hydrogen atoms face the same direction. But that doesn’t mean everything stops, like a crowd of people on the street all looking up, or that your tissues are pulled out of shape. Hydrogen atoms are great multi-taskers and go right on being blood or fat or whatever. When a radio wave hits them, they absorb enough energy to be momentarily distracted and turn away from the magnetic field direction. After giving off the energy again in an answering radio wave, they realign themselves with the main magnetic field.
Now we’ve figured out how to make hydrogen atoms talk to us, but the answering radio waves are like a big undifferentiated chorus. We need to know each hydrogen atom’s location in the body. Another part of the MRI scanner does this.
Just outside the plastic wall of the tunnel, a few inches from your body as you lie inside, is a network of small magnets, each about 20 times the strength of your refrigerator magnet. These are used to make the main magnetic field stronger in one part and tapering to less strong in another. From right to left across your body and from your head to your feet, these magnets create two gradients in the strength of the field, like the intersection of two sloping streets that gives each atom a unique address. This will affect the quality of the radio waves the hydrogen atoms send back: individual voices can be identified in the big chorus.
The radio waves that the hydrogen atoms respond to are just below the first station on the dial of your FM radio. The answering radio waves are very weak and the apparatus that detects them works best if it’s placed on or very near your body. These devices are called coils and are usually flat and enclosed in flexible plastic. There are special ones for the brain that fit around your head like a bucket.
A specially-programmed computer takes all the returning radio signals and constructs a map of what signals came from where, a census of atoms and where they live. Put enough atoms together and you have a tissue. Compare your MRI map – your image – with what you know about how the body usually looks inside and you can give the tissues their names, right down to layers in the brain just a few cells thick.
That’s why it’s such a tight fit in the tunnel—the machine’s magnets have to be close to you to bathe your body in the best magnetic field and the detectors have to be even closer to you to hear the faintest signals from your hydrogen atoms. But aren’t there Open MR Scanners, where you lie in the middle of a magnet-sandwich? And what about the ones you can stand up in?
Dr. Huisman explained that these scanners use resistive magnets (like a 500 pound refrigerator magnet), not superconducting magnets. Their strength is only about 1/10 that of a superconducting magnet. A magnet like this can’t push your hydrogen atoms around as much, so they don’t talk back as much. Therefore the images lack the fine detail that you can get with superconducting magnets. Stand-up scanners certainly have their uses, as when pathology in your back only reveals itself with weight-bearing or bending, but the images suffer from the same coarseness. “This might be OK for a shoulder or knee,” he said, “but not for your brain.” Still, for a very claustrophobic person, an open MRI will give results adequate to answer many questions while avoiding the radiation of CT.
So, what’s all that noisy racket? The MRI technologist at the controls has several choices that can be combined into a sequence all designed to make your hydrogen atoms give up their secrets. The gradient magnets can be switched on and off slowly or rapidly, the radio waves can be given singly or in bursts, the time at which the hydrogen atom’s response radio wave is recorded can be short or long. The loud rhythmic knocking or groaning noise that you hear comes from the gradient magnets as they turn off and on, causing vibration in the machine’s frame. The faster the sequence, the louder the sound. Some ultrafast sequences reach 100 decibels, akin to mowing your lawn with a gas-powered mower or standing 10 feet away from a subway train as it goes by.
One sequence combination can take from 1/50 of a second to 3 seconds. A scan sequence is repeated many times over a period of 30 seconds to 10 minutes to gather enough radio wave data to make an image. It’s important to lie quietly and not to move at all during that time. If you do, the machine will misinterpret the location of the answering hydrogen atoms and the image will be blurred.
But if the scanner can make an image in 30 seconds, then why does a study take 30, 60 or even 90 minutes? The answer gets to the heart of what makes MRI scanning such an exciting and versatile technology and sets it apart from CT.
To a CT scanner, you are inert, like a landscape that varies only in density. X-rays travel through you like tourists: one trip, one report, one image. But to an MRI scanner, you are a huge vibrant on-line community of atoms who can blog endlessly about what’s happening in their tiny part of inner space. The scanner interviews your atoms, and every sequence is like a question. An in-depth interview has many pointed questions; the best MRI scan has many sequences, each designed to find out a specific physiologic fact. Depending on the question, liver atoms might have a different opinion than pancreas atoms. Another scan sequence might have them agreeing while the tumor atoms are tricked into an indiscretion that unmasks them.
There are (or soon will be) 7 billion people in the world – 7 followed by 9 zeros. The average person is composed of 7-followed-by-27-zeros of atoms. Every one of them has a voice and something important to say about you, something that might save your life someday. But it will take an MRI scanner to hear it.
Top image credit: Flickr / Avulsionist
Guest Blogger Profile: JANE BENSON MD has been a radiologist at the Johns Hopkins Children’s Center in Baltimore, Maryland for over 20 years, performing and reading radiography, fluoroscopy, ultrasound, CT and MRI . She teaches medical students, residents and fellows in radiology and in all the pediatric medical and surgical subspecialties. Her happiest moments are when she’s explaining something.
MRI vs CT: Can good medicine be bad for your health? by PLOS Blogs Network, unless otherwise expressly stated, is licensed under a Creative Commons Attribution 4.0 International License.