Mri

Magnetic resonance imaging or Nuclear magnetic resonance imaging is primarily a medical imaging technique most commonly used in Radiology to visualize the structure and function of the body. It was started around 1946 by FELIX BLOCK , who got nobel price for MRI , now only it got popularised. MRI is a relatively new technology, which has been in use for less than 30 years the first study performed on a human took place on July 3, 1977.

ADVANTAGES SUPERIOR CONTRAST RESOLUTION.  HARMFULL RADIATIONS LIKE X rays, GAMMA rays are not produced since it is a non invasive technique. High accuracy. MRI is used to image every part of the body, but is particularly useful in neurological conditions, disorders of the muscles and joints, for evaluating tumors and showing

Magnetic Resonance Imaging  Elements of MRI

 Nuclear Magnetism  Hardware  Contrast

 Applications of MRI  Tumor Detection  Blood flow  Brain Imaging

NMR Hardware Control Room

Scanner

Console

Saturday Morning Physics: Nuclear Magnets

NMR Hardware Scanner Liquid Helium Cooled 1.5 Tesla Solenoid Magnet Radiofrequency Transmitter/Recieiver Coil Patient Platform

Saturday Morning Physics: Nuclear Magnets

Laser Polarized Gas Lung Imaging Chronic Obsructive Pulmonary Disease (COPD)

Healthy Volunteer

University of Virginia: used with permission

Magnetic Resonance Imaging

Functional MRI SCAN of Physics 290 Student (Winter 2000

How MRI works When a person lies in a scanner, the hydrogen nuclei (i.e., protons) found in abundance in the human body in water molecules, align with the strong main magnetic field. A second electromagnetic field, which oscillates at radiofrequencies and is perpendicular to the main field, is then pulsed to push a proportion of the protons out of alignment with the main field. These protons then drift back into alignment with the main field, emitting a detectable radiofrequency signal.

Since protons in different tissues of the body (e.g., fat vs. muscle) realign at different speeds, the different structures of the body can be revealed. Unlike CT scanning MRI uses no ionizing radiation and is generally a very safe procedure. Patients with some metal implants and cardiac pacemakers are prevented from having an MRI scan due to effects of the strong magnetic field and powerful radiofrequency pulses.

1.5 Tesla DC Current

1000 Amps

Solenoid Magnet Conventional Magnet: Many kilowatts (P=I2 R) Superconducting Magnet: Zero Electrical Resistance Saturday Morning Physics: Nuclear Magnets

PHYSICS PRINCIPLE Subatomic particles such as protons have the quantum mechanical property of spin.  Certain nuclei such as 1H (protons), 2H, 3He, 23 Na or 31 P, have a non–zero spin and therefore a magnetic moment. In the case of the so-called spin-1/2 nuclei, such as 1H, there are two spin states, sometimes referred to as "up&down"When these spins are placed in a strong external magnetic field they precess around an axis along the direction of the field. Protons align in two energy eigenstates one low-energy, and one high-energy, which are

RESONANCE & RELAXATION In the static magnetic fields commonly used in MRI, the energy difference between the nuclear spin states corresponds to a photon at radio frequency (rf) wavelengths. The net magnetization vector has two components. The longitudinal magnetization is due to a tiny excess of protons in the lower energy state. When the radio frequency pulse is turned off, the transverse vector component produces an oscillating magnetic field which induces a small

This signal is called the free induction decay. The transverse magnetization is due to coherences forming between the two proton energy states. The recovery of longitudinal magnetization is called longitudinal or T1 relaxation. The loss of phase coherence in the transverse plane is called transverse or T2 relaxation. T1 is thus associated with the enthalpy of the spin system while T2 is associated with its

IMAGING PROCESS A number of schemes have been devised for combining field gradients and radiofrequency excitation to create an image. One involves 2D or 3D reconstruction from projections, much as in Computed Tomography. Others involve building the image point-by-point or line-by-line. The majority of MR Images today are created either by the Two-Dimensional Fourier Transform (2DFT) technique with slice selection, or by the Three-Dimensional Fourier Transform (3DFT)

Another name for 2DFT is spin-warp. What follows here is a description of the 2DFT technique with slice selection. Slice selection is achieved by applying a magnetic gradient in addition to the external magnetic field during the radio frequency pulse. The relative change in the field strength depends on the electronic interaction with other atoms –CHEMICAL SHIFT.

Tomography: Pulse Sequence • Step 1. Slice Selection Pulse • Step 2: Apply Gradients (encode) 4

• Step 3: Measure 2

0

-2

-4 0

• Step 4: decodeM

x

20

40

60

80

100

120

t (kx) Time

4

y1

3

2

y4

1

0 0

20

40

60

80

100

120

x

Saturday Morning Physics: Nuclear Magnets

Fourier Transform

Encoding Position

Frequency (MHz)

Magnetic Field (Tesla) 1.51

64.2 64.0 1.50 63.8 63.6

1.49 POSITION

MAGENTIC FIELD GRADIENT Saturday Morning Physics: Nuclear Magnets