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An MRI scanner uses a powerful magnet and pulses of radio waves to make hydrogen atoms in the body give off faint signals, which a computer turns into detailed pictures of the brain, muscles and organs. Unlike X-rays and CT scans, it uses no ionising radiation, the kind that can damage cells.

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Most of the body is water and fat, both rich in hydrogen, and the nucleus of each hydrogen atom behaves like a tiny magnet. The scanner's magnet is usually tens of thousands of times stronger than Earth's magnetic field, and inside it these nuclei line up. A radio pulse knocks them out of line; as they settle back they send out weak radio signals. Different tissues settle at different speeds, which gives the images their contrast. The method grew out of nuclear magnetic resonance, a laboratory technique, and 'nuclear' was later dropped from the name because it worried patients, though no radioactivity is involved. Paul Lauterbur and Peter Mansfield, who worked out in the 1970s how to turn the signals into pictures, shared the 2003 Nobel Prize in Physiology or Medicine. The loud knocking during a scan comes from rapidly switched magnetic coils.

Türkçe oku
MR cihazı, güçlü bir mıknatıs ve kısa süreli radyo dalgalarıyla vücuttaki hidrojen atomlarının zayıf sinyaller yaymasını sağlar. Bilgisayar bu sinyallerden beynin, kasların ve organların ayrıntılı görüntülerini oluşturur. Röntgenden ve tomografiden farklı olarak, hücrelere zarar verebilen iyonlaştırıcı radyasyon kullanmaz.

Vücudun büyük bölümü su ve yağdan oluşur; ikisi de hidrojen bakımından zengindir. Her hidrojen atomunun çekirdeği küçücük bir mıknatıs gibi davranır. MR cihazının mıknatısı, Dünya'nın manyetik alanından genellikle on binlerce kat güçlüdür; bu alanın içinde çekirdekler aynı yöne dizilir. Bir radyo dalgası onları bu düzenden çıkarır; çekirdekler eski hâllerine dönerken zayıf radyo sinyalleri yayar. Her doku bu dönüşü farklı hızda yapar; görüntülerdeki açık ve koyu tonlar bu farktan doğar. Yöntem, laboratuvarlarda kullanılan nükleer manyetik rezonans tekniğinden geliştirildi. “Nükleer” sözcüğü hastaları kaygılandırdığı için sonradan addan çıkarıldı; oysa işlemde radyoaktivite yoktur. Sinyalleri görüntüye dönüştürmenin yolunu 1970'lerde bulan Paul Lauterbur ile Peter Mansfield, 2003 Nobel Fizyoloji veya Tıp Ödülü'nü paylaştı. Çekim sırasında duyulan yüksek sesli tıkırtılar, hızla açılıp kapanan manyetik bobinlerden gelir.

Sources and further reading

Background reading: Magnetic resonance imaging
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