Magnetic Resonance Imaging PDF

Magnetic Resonance Imaging Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation ✓ Indications. ✓ Advantages of MRI. ✓ Disadvantages of MRI. ✓ The Components of the MRI System Indications Magnetic Resonance Imaging (MRI) : is a medical imaging technique used in radio...

Magnetic Resonance Imaging Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation ✓ Indications. ✓ Advantages of MRI. ✓ Disadvantages of MRI. ✓ The Components of the MRI System Indications Magnetic Resonance Imaging (MRI) : is a medical imaging technique used in radiology to visualize detailed internal structures. MRI makes use of the property of nuclear magnetic resonance (NMR) to image nuclei of atoms inside the body. Indications The concept of the magnetic resonance image is a display of radio frequency signal intensities that are emitted by magnetized tissue during the imaging process. The tissue becomes magnetized because it contains protons that are the magnetic nuclei of hydrogen atoms. When placed in the strong magnetic field, some of the protons align with the field producing the tissue magnetization. ADVANTAGES OF MRI No ionizing radiation & no short/long-term effects demonstrated. Variable thickness, any plane Better contrast resolution & tissue discrimination Various sequences to play with to characterize the abnormal tissue. Many details without I.V contrast. DISADVANTAGES Very expensive Dangerous for patients with metallic devices placed within the body. Difficult to be performed on claustrophobic patients.( fear of closed space) Movement during scanning may cause blurry images. RF transmitters can cause severe burns if mishandled. Not easily available  The Components of the MRI System 1. The magnet which is a key element of the MRI system. It is integrated into the system which also includes; (i) the RF and (ii) Gradient system. 2. Power supplies 3. A computer system 4. A documentation system 5. Cooling system 6. Monitoring camera The Components of the MRI System 1.Superconducting magnet A superconducting magnet is the heart and most expensive part of an MRI scanner. The magnetic field requirement in the present day MRI scanners for clinical use is limited to 3T only. The main magnet is superconducting, cooled to LHe temperature and mounted in an efficient cryostat with a horizontal bore to accommodate the patient. The Components of the MRI System 2.Gradient coils Gradient coils are used to produce deliberate variations in the main magnetic field. There are usually three sets of gradient coils, one for each direction. The variation in the magnetic field permits localization of image slices as well as phase encoding and frequency encoding. The set of gradient coils for the z axis are Helmholtz pairs, and for the x and y axis paired saddle coils.  The Components of the MRI System 2.Gradient coils It generates secondary magnetic field with in primary magnetic field, they are located in bore of primary magnet. They are arranged in opposition to each other to +ve and –ve pulse. Gradient coils are set of magnetization coils, which cause of variation in magnetic field. They must be able to cause spatial variation along the direction of man magnetic field. They are along with RF pulse are responsible for slice and voxel formation. Gradient is extra magnetic field which is added to the magnetic field.  The Components of the MRI System 2.Gradient coils X coil – create a varying magnetic field from left to right. Y coil- create a varying magnetic field from top to bottom. Z coil- create a varying Magnetic field from head to toe. The Components of the MRI System 3.RF Coils Same as Radio waves – high wavelength, low energy electromagnetic waves. RF coils are the "antenna" of the MRI system That transmit the RF signal and receives the return signal. They are simply a loop of wire either circular or rectangular. Inside the gradient coils are the RF coils producing the field B for rotating the spin by an angle dictated by the pulse sequence. These coils also detect the signal emitted by the spins inside the body. At the centre is a patient table which is computer controlled.  The Components of the MRI System 3. RF Coils Coils Start RF pulses (Excitation- Protons jump to higher energy state by absorbing radiation) RF coils Stop RF pulses (Relaxation- Protons return to their original state emitting radiation) The Components of the MRI System RF used to transmit RF pulses receiving signals in MRI produce best possible images. It can make magnetization of hydrogen nuclei , turn it 90 degree away from magnetic field. Some low energy (parallel protons) flip to a high energy (anti parallel) state decreasing longitudinal magnetization. Protons process in phase, at a result net magnetization vector turns towards the transverse plane, i.e. right angles to the primary magnetic field = transverse magnetization. Each proton is rotating around its axis 63,000,000 rotation per second. The 63MHz rotation is in the frequency range called Radio frequency. Rotation speed α magnetic field strength The Components of the MRI System Receives RF signal and performs analog to digital conversion. Digital signal representing image of body part is stored in temporary image space or case space. It store digital signal during data acquisition, digital signal then sent to an image processor were a mathematical formula called Fourier transformation is applied to image of MRI scan is displayed on a monitor.