How to see the inner man (or woman)

There is a major event happening in the life of my family right now, and I haven’t written about it yet but have been waiting for the opportunity. My husband and I are expecting our first child! There are so many things that I’ve thought about writing with respect to the science of pregnancy – what causes morning sickness, how amazing the pattern of development of the human body really is, how statistically unlikely it was that we would have twins (though lots and lots of people teased us about the possibility), and how much I hope our child grows up loving science as much as we do. But I held off, waiting for the perfect topic. And today, I think I’ve found it – I want to write about ultrasounds.

Actually, I’d like to write about some of the various ways that medicine has come up with to look at what’s going on inside the human body – short of surgery, that is. Three big techniques come to my mind, and I’d like to take a few minutes to discuss what each one does, how they are different from each other, and what their advantages are. These three are ultrasounds, x-rays, and MRIs.

I’ll start with the ultrasound (particularly near and dear to us at the moment). The word “ultrasound” actually means sound waves that are above the range of human hearing (20,000 hertz), so when we talk about ultrasounds in a medical sense, we are actually talking about ultrasonography. Ultrasonography has been around for about 50 years, and is extremely widely used in diagnostic procedures to visualize soft tissues, muscles, tendons, and some internal organs (including the heart, liver, gallbladder, kidneys and bladder). It is also commonly used to look at a developing fetus within a mother’s uterus. During the process, ultrasound waves are produced by a small wand, or transducer, which radiate out into the body to focus at the specified depth. This sound wave is partially reflected from the layers between different tissues – specifically, where there is a change in tissue density. The sound waves that get bounced back towards the transducer are detected by a sensitive microphone, which are then translated into an image on a computer screen. There are several big advantages to using sonography as a diagnostic tool. For one thing, it does not use ionizing radiation (as do x-rays), making it safe to use for developing babies. For another, it is relatively cheap compared to its high-power brothers like the MRI. However, it is limited in its ability to see certain structures within the body – it is not good at visualizing bones or the brain, for example.

So let’s go now to the next imaging technique on my list – the x-ray. The medical use of x-rays manipulates the physical properties of – you got it – x-rays. (Clever, huh?) An x-ray is a high energy type of light wave. The energy of a light wave can be measured by its wavelength – the shorter the wavelength, the higher the energy the wave has. In the visible spectrum, red light has lower energy (and longer wavelengths) and blue light has higher energy (and shorter wavelengths). Past the visible spectrum comes ultraviolet light, followed by x-rays. While visible light does not have enough energy to pass through your skin, x-rays have considerably more energy, and thus can pass right through your skin and muscle. However, they are not strong enough to pass through bone. So when you undergo a medical x-ray (for example, to see whether you’ve broken a bone or when you are at the dentist), the doctor will put you in front of an x-ray emitter, which sends x-rays through your body and picked up by a detector (usually a piece of film) on the other side of you. Places of your body where the x-rays pass through (eg muscles and soft tissue) show up as black, while pieces of your body where the x-rays were absorbed (eg bone and teeth) show up white. The film is developed, and the doctor can tell whether your bones are all as they should be – whole and unbroken (hopefully). X-rays are more powerful than sonograms, especially for diagnosing problems specific to the skeleton. However, their major drawback is that they use ionizing radiation in the process. Too much ionizing radiation can cause all kinds of problems for your cells and tissues; however, the exposure any of us will be likely to receive from medical x-rays over the course of our lives is minimal and of low risk.

What about the fancier techniques, like MRI? MRI stands for magnetic resonance imaging, and it uses an entirely different basic principle to visualize the interior of the human body. Instead of sonography (which uses sound) or x-rays (which uses high-energy light), MRIs use magnetic fields. When a person is subjected to an MRI, their body is immersed in a strong magnetic field, which has an effect on the hydrogen atoms throughout their body. The human body can be upwards of 75% water; in each molecule of water, there are 2 hydrogen atoms. Thus, the amount of hydrogen in your body from water alone is really high. And these hydrogen ions will all align with the magnetic field when you are in the MRI machine. So you sit there, with all your hydrogens aligned, and then your body is pulsed with a radio wave. This pushes some of the hydrogen atoms out of alignment with the magnetic field. The radio wave stops, and the hydrogens all slowly snap back into alignment. However, depending on what tissue they happen to be sitting in, they will snap back into place at different speeds. And the speed at which the hydrogens align themselves with is detected by the machine, then calculated to determine what tissue is what. An MRI is a very powerful technique, and can be used to diagnose a number of different medical conditions, including multiple sclerosis, brain tumors, torn ligaments, spinal hernias, tendonitis, and even strokes in the early stages. Another advantage is that they, like sonograms, do not use any form of ionizing radiation. And yet another advantage is that MRIs can be used to look at any plane of the human body – sideways, top-to-bottom, or any other way you can think of. There are some disadvantages, though. Certain people cannot receive MRIs, because the strong magnetic field would be dangerous for them (for example, people with pacemakers). MRIs take a very long time to do, as well, and they are extremely expensive – much more so than either x-rays or a sonogram.

All three of these techniques are powerful in their own right. They can be used to look at different parts of the body – soft tissue, organs, bones or ligaments – with different resolutions. Each one uses a different major method of visualization – magnetic fields, sound waves or electromagnetic radiation. Each one has different costs, risks and benefits. And all in all, I’m glad to live in a day and age where all three are used as a part of everyday medicine. Each one is so much safer than having to cut the body open to see what’s going on inside!

Oh, and by the way, we have had our ultrasound to check on our developing baby. All looks good – 2 arms, 2 legs, and all pieces where they should be! Now we just have to wait to see the little one in person!