The California Cure Picturing the Inside Story

by Shaun L. Samuels, M.D.

Don’t feel bad if you don’t have a thorough, or even passing, understanding of how the images from a diagnostic test you’ve received were produced. The dirty little secret is that few doctors know how they’re made, either. Most radiologists had it down long enough to pass the physics section of their board exam, after which the knowledge rapidly evaporated.

All are to be forgiven. It’s a mind-bogglingly complex world awash in physics, the kind of stuff where calculus is thrown around like most of us tweet or instant message. Preposterous amounts of data are acquired, processed and reconstructed in microseconds, and astonishing images pop up on a screen as if by magic.

It isn’t magic, of course, but the result of decades of bone-crushingly detailed work, done by those who have mastered the theoretical and engineering sides of producing imaging equipment. Lots of Nobel Prizes went out for this stuff, and probably a few dozen more were deserved for the select few who from the beginning believed these things could ever work in any practical way. The following is a ridiculously brief compendium of some of the imaging modalities in wide use, how they came about, what they are used for and some of their pitfalls. For those who love to pepper their conversation with buzzwords that convey a deeper knowledge than they possess, there’s something for you, too.

MRI

Brief History
Pioneered in the 1970s. Originally called NMRI, the N stands for nuclear, which gave people the heebie-jeebies. Paul Lauterbur and Sir Peter Mansfield shared the 2003 Nobel Prize in Physiology or Medicine for its development.

Physics
So dizzyingly complicated, this small space won’t do. Sad to say, we are but bags of water trapped in different tissues: kidney, brain, liver, etc. H2O contains hydrogen atoms, which act like magnets and align like iron fillings in a magnetic field, and they spin at a speed proportional to the strength of the magnetic field. Still with me? So, if you put someone into a magnet, and you vary its strength, you get the hydrogen spinning at different speeds in different parts of the body. If you then flatten these neatly aligned atoms with an electromagnetic punch, they each tend to stagger back to their feet at different rates, realigning with the magnetic field, depending on the tissue they are in. The result can be “encoded,” as it were, by that variable-magnetic-field principle. Astonishing, really. The tissue contrast produced is unparalleled and can be enhanced by giving the patient a contrast agent.

Particularly Good For...
• Central nervous system (brain, spinal cord)
• Musculoskeletal (knee, shoulder, spine)

Precautions
• Metallic foreign bodies (especially in the eye)
• Pacemakers/AICDs (MRI messes them up)
• Claustrophobics (that magnetic tube is snug)
• Poor kidney function (using contrast agent)

Terms that sound cool or oddly suggestive
K-space, Larmor frequency, spin echo, susceptibility, chemical shift, gradient coil, large bore, time-of-flight, wrap around

CT

Brief History
Another thing we can thank the Beatles for. Godfrey Hounsfield, who shared the 1979 Nobel Prize in Physiology or Medicine for his role in developing CT, had been a researcher at EMI, the record label for the Fab Four. With the money that came in from their world-altering success, EMI took a flyer and funded Hounsfield’s independent research.

Physics
Using the same electromagnetic waves that produce plain X-rays and mammograms, CT involves shooting those X-rays from a source rotating around the patient. (Those who’ve seen a CT scanner recognize the doughnut-shaped gantry which houses the source and detectors.) It’s no accident that the scanner is covered with an innocuous, putty-colored molding. Patients might freak watching these elements whirring around them at alarming speed. Just hearing it can be a bit unnerving. By sending out a fan-shaped beam and incorporating multiple detectors opposite it on the circular gantry, the scanner acquires a cross-sectional density map. A computer collects all the data and reconstructs image “slices” as the patient moves smoothly through the scanner. Many thought that MRI would sound the death knell for CT...but superfast acquisition times have kept CT very much in play.

Particularly Good For...
• Abdominal and pelvic imaging
• Imaging of coronary arteries
• Imaging of lungs
• Imaging of large blood vessels (especially with contrast enhancement)

Precautions
• Radiation dose (especially in younger patients who require frequent scans to follow a particular disease)
• Poor kidney function (using contrast agent)

Terms that sound cool or oddly suggestive
Multidetector row, beam hardening, slip ring, rotate-rotate, stationary-rotate, partial-volume averaging, maximum-intensity projection, multiplanar reconstruction

PET

Brief History
A repository of terminology right out of the existentialist handbook. The core principles are annihilation, decay and coincidence. The basics of positron emission were elaborated in the 1950s, primarily at the University of Pennsylvania and Washington University School of Medicine. The imaging has steadily improved in the decades since.

Physics
Unlike the purely anatomic information of CT and MRI, PET actually maps out tissue metabolism. The common fuel of cells is sugar in the form of glucose. In a cunning bait-and-switch, unsuspecting cells, thinking they’re getting the straight stuff, take up a compound called 18FDG, fluoro-deoxyglucose. That fluoro part is radioactive and emits a positron, the positively charged antiparticle to the electron (don’t ask). That positron is inherently unstable, and when it meets the electron, both are annihilated, an altogether trippy event in which they cease to exist and emit in their place two gamma rays, super-high-energy waves. These waves shoot off in roughly opposite directions and can be recorded by gamma detectors. When these detectors get a simultaneous signal, the coincidence can be mapped and the origin of the event relatively closely pinpointed. Hence, tissues of high metabolic activity, such as many types of cancer, show up as hot spots on a scan. Similarly, areas of decreased metabolic activity, as in certain brain disorders, show up as cold spots.

Particularly Good For...
• Cancer detection
• Cancer surveillance, monitoring of treatment
• Brain diseases and disorders such as dementia

Precautions
• Radiation dose (especially combined with CT)

Terms that sound cool or oddly suggestive
Scintillation, avidity, radionuclide, backscatter, pick-off process, positronium, expectation maximization

Plain X-ray / Mammography

Brief History
The insight of Wilhelm Röntgen, winner of the first Nobel Prize in Physics in 1901, is legendary. He was passing electric current through a vacuum tube and noticed fluorescence coming from a square of coated cardboard attached to the tube. Röntgen was intrigued. He passed another current through the tube, only to notice a glow coming from a similarly coated piece of cardboard a few feet away. He concluded that an invisible ray, which he coined “X-ray,” must account for this light show and the science of roentgenology (radiology) was born.

Physics
X-ray is way out there on the electromagnetic spectrum, past visible, past ultraviolet—very high energy. These waves pass through solids and can interact in a variety of ways with them, as Röntgen discovered when he famously produced a film of his wife’s hand, bones and wedding ring visible. The upshot is that some get absorbed and some pass through, and this can be measured in a standard plain-film X-ray image. Parts of higher density absorb more X-rays and appear white; the less dense (for example, where there’s air) appear darker. While this used to be recorded on special film, it’s now typically detected digitally and stored electronically, to be viewed on a workstation. Don’t patronize it as quaint: It still boasts the best spatial resolution of any of its fancy-schmancy imaging descendants such as the CT. Hence, for mammography, it can pick up extremely tiny microcalcifications (about a tenth of a millimeter) that can be crucial in detecting early cancer. And in terms of sheer numbers, more plain X-rays are obtained than any other imaging study. So there.

Particularly Good For...
• Detecting fractures in all bones
• Assessing spine alignment and disk spaces
• Screening chest
• Mammography
• Detecting intestinal obstructions

Precautions
• Any single X-ray involves an extremely small amount of absorbed X-rays. Radiologists generally adhere to the ALARA concept with regard to radiation dose: As low as reasonably achievable.

Terms that sound cool or oddly suggestive
Compton scatter, Mach effect, Bremsstrahlung, rare earth screen

Ultrasound

Brief History
Sonar schmonar: You want really cool stuff, take a look at the current state of ultrasound, another technology to emerge, albeit indirectly, from the war effort. Sonar came out of World War I, and as early as the 1940s, researchers were demonstrating that in addition to detecting submarines many leagues under the sea, high-frequency sound waves could be used to ping tissues a few centimeters deep. The major advances in the technology were multinational in origin, coming from Europe, Asia and the U.S. All of this emerged from the discovery in the late 1800s (by Madame Curie’s husband and brother-in-law, Pierre and Jacques, no less) of an unusual property some substances displayed called piezoelectricity.

Physics
The wickedly high frequency used for ultrasound imaging is produced using a piezoelectric crystal (not something, despite the funky name, they were taking at Woodstock). The substance has the remarkable characteristic that when one applies voltage to it, it vibrates. Even crazier, the same substance, when vibrated, generates voltage. An exquisitely choreographed sequence of pulses of electricity is applied, sending ultrasound waves into the tissues. Just as sound waves echo off walls, ultrasound waves bounce off tissues, especially at interfaces between different tissues. As the reflected waves arrive back at the transducer, they vibrate the very crystal that produced the waves. The voltage created can be detected, and, depending on when the wave comes back and from what direction, a map can be created, which translates into phenomenal 2-D (even 3-D and 4-D) images. It all happens at such a fast rate that the images are seen in “real time,” creating a movie where a heartbeat can be seen or the motion of blood detected. And there are essentially no adverse biological effects. Nifty, to say the least.

Particularly Good For...
• Obstetrics, the progress of fetal development
• Abdominal imaging, especially solid organs and gallbladder
• Blood vessels, especially carotid arteries and veins of the leg, which can be interrogated for blockages

Precautions
• At diagnostic frequencies, ultrasound is safe, another plus for obstetric imaging

Terms that sound cool or oddly suggestive
Ring-down, reverberation, comet tail, defocusing, range ambiguity, posterior enhancement, probe