We were greeted by Lei Zhou, a Chinese postdoc on duty that evening. Lei had received his PhD in physics and was intimately familiar with the technical wizardry behind MRI. His English, however, had a ways to go. I could only hope that we understood each other during this unusual procedure.
Lei and Andrew preparing to scan the lamb’s head.
(Gregory Berns)
Andrew unloaded our cargo, and we proceeded to arrange it in the head coil of the scanner. With foam pads propping up the body parts, Lei snapped on the top of the coil and sent the whole mess into the center of the scanner.
When you place something in the MRI, the magnetic field tugs on the atoms inside the object. In living tissue or, as in the case of the lamb’s head, formerly living tissue, hydrogen is the most common atom. There are two hydrogen atoms in every water molecule, and water accounts for 60 percent of body weight in humans. Hydrogen is also abundant in the brain. The outer membranes of neurons and their supporting cells, called glia, are rich in fat and cholesterol, which have large numbers of hydrogen atoms.
A hydrogen atom has one proton and one electron. The proton is like a spinning top. Normally, the protons spin in random directions, but inside the MRI they line up with the magnetic field. Like spinning tops, the protons also wobble a little bit. The stronger the magnetic field, the faster they wobble. If you hit the protons with radio waves exactly in sync with their wobbling, the protons jump into a higher energy state. This is called magnetic resonance. Different types of atoms resonate at different frequencies. For the strength of scanner we use, hydrogen resonates at 127 MHz, which falls in the range of radio waves—just beyond the FM dial. Carbon, another common element in the body, resonates at 32 MHz. MRI works by sending in a blast of radio waves that excite the atom of interest—in most cases hydrogen because of its abundance and superior sensitivity to magnetic fields. When the radio waves are turned off, the protons relax back to their original state and, in the process, cause an oscillating magnetic field that can be picked up by an antenna. The head coil is nothing more than a fancy FM radio antenna that picks up these signals from the protons in the brain.
Not all protons behave the same way. The protons in a water molecule wobble slightly different from the protons in a fat molecule. These slight differences can be detected by the MRI and, with the help of a computer, be used to construct a visual image representing the types and locations of these different molecules.
We would need to do three types of scans on each subject. The localizer, which lasts only a few seconds, gives a snapshot of the location and orientation of the head in the magnet. The localizer scan of the lamb’s head came out well. We could clearly make out the brain. The human settings for the localizer seemed to work. Next up was the structural image. For humans, we like as much anatomical detail as possible, but this has to be weighed against the time it takes to get high-resolution images. Images clear enough to resolve features as small as one millimeter take six minutes to complete. Humans have no problem holding still for that long, but there was no way our dogs would. I told Lei that we needed to come up with a structural sequence that would take no more than thirty seconds. I figured that would be the limit for most dogs.
This turned out to be somewhat difficult. The normal structural scans couldn’t be completed that quickly, so we had to switch to a different type of scan. This one didn’t show as much detail, but we were able to find a combination of parameters that produced a usable image in under thirty seconds.
We spent an awful lot of time figuring out the best orientation of the brain. If you think of the MRI as being a digital bread slicer, we had to decide which way to cut the slices: left to right, top to bottom, or front to back. Since the human head is pretty close to a sphere, it doesn’t make a whole lot of difference which way you slice it. But a dog’s head, like the lamb’s, is elongated front to back and generally pretty flat from top to bottom.
As the images of the lamb’s head came up on the screen, we saw how little of the head was actually occupied by brain. Most of it was nose and muscle. Those air pockets in the nose can wreak havoc with the MRI images too. Abrupt transitions in tissue density, such as going from air to skull, cause distortions in the magnetic field, which result in warped images. By carefully selecting the orientation of the slices, you can minimize this effect. Slicing from front to back seemed to give us the best results.
Anatomical images of the lamb’s head. The slices go from front to back. The eyeballs are visible in the top row, while the brain appears prominently in the middle and lower rows. The large black cavities are nasal sinuses.
(Gregory Berns)
Finally, it was time to attempt some functional scans, which are two-second glimpses of the brain in action. By continuously acquiring these functional scans while the subject does something, we can measure changes in brain activity. Think of the functional scans as the individual frames of a movie. Even though each one takes only two seconds, the subject might be in the scanner for half an hour during functional scanning. During such a session, we would acquire nine hundred functional images, at a rate of thirty scans a minute for thirty minutes.
Of course, the lamb was dead, so we didn’t expect to see much “activation.” But we only needed to figure out how many slices it took to cover the brain and how to orient the brain for the most efficient coverage. Once we worked that out, Andrew and I recorded the sounds of the scanner running this sequence.
We could now introduce Callie and McKenzie to the actual noise they would experience in the scanner and gradually let them get used to it.
11
The Carrot or the Stick?
THE CHALLENGE OF ENTERING the head coil and placing her chin on the boogie board chin rest had long been overcome. As soon as Callie heard the rustling of the plastic baggie containing bits of chopped-up hot dog, she knew. She would bound into the kitchen, wagging her whole rear end, and look at me with excitement and anticipation.
“Wanna do some training?” I would ask in a high-pitched voice.
Our training regimen had outgrown the basement. The only room in the house big enough to contain what was now a full-blown MRI simulator was the living room. Kat eyed the monstrosity in her living room, a space formerly occupied by an elegant sofa set and coffee table now pushed off to the side.
“There isn’t any other place for this?” she asked.
“It’s too heavy to move down in the basement,” I replied. “And I don’t think it will fit through the door.”
“You mean you constructed this in the living room without a way to get it out?”
“No, no,” I reassured her. “It comes apart.”
I had dusted off a PA system left over from my guitar-swinging days in a garage band. As I set the speakers on a stand facing the tube, Helen came into the living room.
“What’s that for?”