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Space Faring: The Radiation Challenge
Magnetic Resonance Imaging
Not all medical imaging tools use X-rays or gamma rays. Magnetic Resonance Imaging
(MRI) scans use low energy radio waves and strong magnetic elds to excite magnetic
resonance in tissue atoms. MRI scans are commonly used to dierentiate subtle dierences
within soft tissue regions of the body (dierences in concentrations of water and fat give
rise to dierent MRI signals in these regions). As in CT scan, all or part of the patient’s
body is placed inside a large cylinder during the MRI. A strong magnetic eld is applied,
which causes some of the molecules in the patient to align themselves along the direction
of the eld. is causes the hydrogen-containing compounds within the body to resonate
at radio-frequency signals, which are picked up by a detector. A computer converts it to an
image, which can be color-coded. In some cases, it is useful to combine MRI imagery with
SPECT scintigrams to compare anatomical structures with their function (see Figure 8).
Radiation Therapy
When surgery alone cannot entirely remove a cancerous tumor, radiation therapy
is sometimes used in conjunction with the surgery. Intraoperative radiation therapy
(IORT) delivers a high dose of radiation to cancerous tumors while they are exposed
during surgery. Other techniques like three-dimensional conformal radiation therapy
(3-D CRT) use radiation only. In 3-D CRT (also known as gamma-knife surgery),
computers with specialized software use the information from CT Scans or MRIs to
create beams of radiation that conform to the shape of a tumor. Once the exact loca-
tion, size, and shape of the tumor is known, the computer instructs the linear accelera-
tor to bombard the tumor with the conformal radiation. is technique is particularly
useful in treating prostate cancer, lung cancer and certain brain tumors. Another
example of conformal radiation therapy is Intensity-Modulated Radiation erapy
(IMRT), which precisely varies the intensity of the radiation beams used during treat-
ment. Greater radiation intensity is directed at larger areas of the tumor, while weaker beams are directed to smaller areas of the
tumor. is helps to reduce the amount of radiation used and limits the amount of radiation exposure to healthy tissue. In some
cases, radiation is also combined with heat in treatments to kill cancer.
Brachytherapy
In each of the previously discussed examples, radiation is delivered from a source exter-
nal to the patient’s body. However, there are internal forms of radiation therapy like
Brachytherapy, which is designed to deliver a high dose radiation from inside the body.
Brachytherapy involves placing a protected source of radiation (such as Iridium-192 or
Cesium-131 that has been encased in tubes, wires, or capsules, see Figure 11) directly
within the tumor or very near to it.
Side Effects of Radiation Therapy
Exposure to radiation can cause negative side eects, ranging from dry mouth, diculty
swallowing, changes in taste, nausea, vomiting, diarrhea, irritated skin, hair loss, chest
tightness, cough, shortness of breath, fatigue, or ear aches. In addition, physiological
complications like bone marrow suppression may result. is can cause anemia, low
white blood cell count, and low platelet count. Secondary malignancies or treatment-
associated cancers can sometimes occur in patients years after radiation therapy. Some
patients experience damage to healthy tissues that leads to cognitive impairment, or the
loss of the ability to remember, learn, and complete certain tasks. As a result, a great deal of research focuses on maximizing the
benets of tumor killing radiation while minimizing its eects on healthy tissue surrounding the tumor. It is important to note
that the location and intensity of radiation exposure aects the nature of negative side eects.
Figure 9: An MRI scanner.
Figure 10: Radiation therapy uses radiation to
treat disease. Image Credit: Mayo Clinic.
Figure 11: Tiny capsules lled with Cesium-131
are implanted in or near a tumor. X-rays emitted
by the cesium kill the cancer cells. Image
Credit: Pacic Northwest National Lab.