Magnetic resonance imaging MRI

Transcription

1 Magnetic resonance imaging MRI

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3 Introduction What is MRI MRI is an imaging technique used primarily in medical settings that uses a strong magnetic field and radio waves to produce very clear and detailed computerized images of the structures inside the body. MRI based on the principles of nuclear magnetic resonance (NMR),

4 MRI Equipment components

5 Main Components Static Magnetic. Gradient Coils. Magnetic shim coils. Radiofrequency Coil. system control computer. Data transfer and storage computers. display

6 MRI Concept MRI images are usually based on the signal from protons. A Proton is the nucleus of the hydrogen atom. Hydrogen is the most common element in tissue. Essentially all MRI is hydrogen (proton) imaging.

7 Hydrogen atoms

8 Magnetic Principles There is electric charge on the surface of the proton, thus creating a small current loop and generating magnetic moment µ. The proton also has mass which generates an angular momentum J when it is spinning.

9 Magnetic Principles Moving (spinning) charged particle generates its own little magnetic field Such particles will tend to line up with external magnetic field lines(b ). o

10 Magnetic Principles A nucleus is exposed to an external field that it an radio frequency (RF), The application of an RF pulse that causes resonance to occur is termed excitation as it results in the nuclei absorbing energy. This energy absorption causes the magnetic moment

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13 Magnetic Principles The stationary conductor (Gradient Coils) in a moving magnetic field then an electromotive force (e m f) is produced across its ends. These (e m f) may be readily detected by the potential difference signal

14 Magnetic Principles Signals collected with multiple gradients are processed by computer to produce an image, typically of a section through the body.

15 Steps of MRI Image Formation Put subject in big magnetic field. Transmit radio waves into subject. Turn off radio wave transmitter. Receive radio waves re-transmitted by subject... Convert measured RF data to image.

16 MRI Equipment components

17 MRI Basic Layout The magnetic field of an MRI machine is typically 3 Tesla! The Earth s magnetic field is less that 30 microtesla ( Ts).

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19 Imaging Hardware A schematic representation of the major systems on a magnetic resonance imager

20 Features of MRI No ionizing radiation (safe). Effective for soft tissues. Tomography at arbitrary angle. Many imaging modes. Very good image quality. Many contrast mechanisms: contrast between tissues is determined by pulse sequences. Can produce sectional as well as projection images. Slow. Expensive.

21 MRI Applications MR Spectroscopy measures the distribution of chemicals in the brain. Functional MRI allows to assess brain activity during certain tasks. valuable for brain functional studies, but also for surgery planning and diagnosis.

22 MRI Applications MR Microscopy use for small animal experiments Diagnosis: brain tumors, abdomen and organs.

23 MRI vs. CT MRI is similar to a (CT) scanner in that it produces cross-sectional images of the body. MRI has advanced beyond a tomography imaging technique to a volume imaging technique. CT: utilizes only X-ray attenuation to generate image contrast. MRI: a variety of properties that may generate image contrast.

24 MRI vs. CT CT: usually more available, faster, much less expensive. MRI: slower, expensive. MRI: generally superior for tumor detection and. MRI: best if patient is to undergo examination several times. CT: if repeated, may expose the patient to excessive ionizing radiation.

25 MRI CT