十万个为什么·科学启蒙30篇（4）
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Why MRI Scans Avoid Ionizing Radiation
为什么核磁共振不用电离辐射
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MRI machines use strong magnetic fields and radio waves instead of X-rays or gamma rays to create body images.
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Unlike ionizing radiation, radiofrequency pulses do not carry enough energy to remove electrons from atoms or damage DNA.
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The magnets align hydrogen nuclei in water-rich tissues, and radio waves temporarily disturb that alignment.
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When nuclei snap back, they emit faint signals detected by sensitive receivers around the body.
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Computers translate those signals into detailed cross-sectional pictures of organs and soft tissues.
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Because no ionizing radiation is involved, MRI is safer for repeated use and for children or pregnant women.
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Patients must remove metal objects since the magnet can move or heat them dangerously.
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Contrast agents sometimes used in MRI are also non-radioactive and cleared safely by the kidneys.
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Doctors choose MRI when they need high-resolution views of the brain, nerves, or ligaments.
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This technology shows how physics principles like nuclear spin enable safe, precise medical diagnostics.