| Have you ever wondered how computed | | | | MRIs image the rate at which spinning |
| tomographic (CT) and magnetic resonance | | | | hydrogen-atoms of water molecules within |
| imaging (MRI) scans are made? Admittedly, some | | | | different parts of the brain either line-up or fall out |
| of it occurs by magic, but the truth can be told | | | | or alignment with a strong magnetic field. These |
| about the rest of the process. | | | | differing rates of magnetization or |
| Before computed tomographic (CT) scans | | | | de-magnetization are fed into a computer. Then |
| became available in the 1970s, there was no good | | | | magic occurs yet again. A series of slice-like |
| method for imaging the brain. The available | | | | images is created and displayed on a |
| methods and technologies struck around the | | | | computer-screen or x-ray-type film in shades of |
| target without quite hitting the bull's-eye. We had | | | | gray. Abnormal structures, like brain-tumors or |
| skull x-rays which imaged the bony brain-case, but | | | | the plaques of multiple sclerosis, are displayed in |
| not the brain itself. We had arteriograms which | | | | their own shades of gray and are also |
| imaged the insides of blood-vessels supplying the | | | | recognizable by their shapes and locations. |
| brain. We had nuclear brain scans which imaged | | | | Obtaining another set of images after intravenous |
| chunks of brain that were recently damaged. We | | | | administration of gadolinium-the MRI equivalent of |
| had a particularly nasty test called a | | | | x-ray dye-also adds diagnostic information. |
| pneumoencephalogram (PEG) in which the doctor | | | | One of the virtues of MRI pictures is that they |
| squirted air through a spinal tap needle and | | | | are based on physical principles totally different |
| encouraged it to bubble around and inside the | | | | from those responsible for creating CT pictures. |
| brain by turning the patient every | | | | Thus, the MRI is good (or not so good) at |
| which-a-way-including upside-down-while x-ray | | | | showing different things than CTs. Another virtue |
| pictures showed where the air could and couldn't | | | | is that MRIs can slice and dice the brain at |
| go. Finally, the most accurate method was not a | | | | different angles, while CTs slices are limited to |
| physical picture at all, but a mind's-eye picture | | | | just the horizontal plane. Yet another virtue of |
| within the brain of an examining neurologist. Yet | | | | MRIs is that they are much better than CTs at |
| diagnoses still got made and patients did get | | | | imaging most diseases of the spine. Finally, MRIs |
| treated. | | | | are much more flexible than CTs: new bells, |
| CT scans revolutionized the practice of neurology. | | | | whistles and capabilities are being added all the |
| It's not that the other methods disappeared (well, | | | | time.To the patient, the experiences of having a |
| yes, PEGs thankfully did disappear) but that CT | | | | CT and of having an MRI greatly resemble each |
| scans vastly improved the accuracy of diagnosis | | | | other. In both cases the patient lies horizontally on |
| and treatment. Even when CT scans didn't show | | | | a flat table that moves into and out of an opening |
| the disease itself (e.g. multiple sclerosis or a fresh | | | | in the scanner that resembles a giant |
| stroke) they assisted the diagnostic process by | | | | doughnut-hole. The doughnut-hole in the MRI |
| proving the absence of a brain tumor, abscess or | | | | machine is narrower, so claustrophobic patients |
| hemorrhage that were also on the list of | | | | need to inform their doctors if this might be a |
| diagnostic possibilities. | | | | problem. The MRI machine is also noisier: a loud |
| CT scans did (and still do) this by sending x-ray | | | | sound is created each time its radio-frequency |
| beams through the head at various angles and | | | | coils turn on and off. For each kind of scan the |
| collecting the x-ray beams on the opposite side | | | | technologist might stick a needle in the patient's |
| that were not absorbed by the head. Then magic | | | | vein to administer contrast-material. |
| occurs. A series of images appear on a computer | | | | Both tests are otherwise painless and are very |
| monitor or on x-ray film as if the head had been | | | | safe with certain exceptions. Pregnant women |
| run through a giant salami-cutter and the slices | | | | who need a scan might have to do without one |
| were laid out flat and in sequence. | | | | for fear of exposing the fetus to excessive |
| On CT pictures the different parts of the head | | | | x-rays in the case of the CT scan or to an |
| are displayed in various shades of gray according | | | | excessive magnetic field in the case of the MRI. If |
| to how much they absorb x-rays. The skull-bone | | | | push comes to shove, the woman is more likely |
| absorbs x-rays the most and shows as the | | | | to receive a CT scan because her abdomen can |
| whitest component. At the other end of the | | | | be draped with a lead shield that blocks passage |
| gray-scale, the watery spaces in and around the | | | | of most x-rays, while there is no good method |
| brain absorb x-rays the least and show as the | | | | for blocking the magnetic field produced by an |
| blackest components. The brain itself is | | | | MRI machine.A circumstance in which MRIs are |
| somewhere in between, showing up in the | | | | simply not done is when the patient has a cardiac |
| mid-gray range. Abnormal components, like brain | | | | pacemaker. This is because the MRI machine's |
| tumors and blood-collections, are identified not just | | | | magnet might disrupt the pacemaker and stop |
| by appearing in their own shades of gray, but also | | | | the heart. No image is so necessary and valuable |
| by their locations and shapes. Creating a second | | | | that this risk would be worth taking. Another |
| set of slices after the patient receives an infusion | | | | circumstance in which an MRI is avoided is when |
| of intravenous dye provides an additional | | | | the patient is critically ill. An unstable patient can be |
| dimension to imaging not unlike that provided by | | | | adequately monitored and supported while |
| the older, nuclear scans. | | | | receiving a CT scan, but not while receiving an |
| Then in the 1980s magnetic resonance imaging | | | | MRI. |
| (MRI) scans burst upon the scene and astonished | | | | Depending on the nature of the patient's problem, |
| the medical community by not just imaging the | | | | the doctor will usually order just one of the two |
| brain itself, but by doing so in a brand-new way. | | | | types of scans and not the other, but in selected |
| Instead of imaging the extent to which the head's | | | | cases the magic of both kinds of scan might be |
| different components absorb x-rays, MRIs instead | | | | needed. |
| focus on water-molecules. To be more precise, | | | | |