Magnetic Resonance Imaging (MRI)

The Components: A magnet which produces a very powerful uniform magnetic field. A magnet which produces a very powerful uniform magnetic field. Gradient Magnets which are much lower in strength. Gradient Magnets which are much lower in strength. Equipment to transmit radio frequency (RF). Equipment to transmit radio frequency (RF). A very powerful computer system, which translates the signals transmitted by the coils. A very powerful computer system, which translates the signals transmitted by the coils.

The Magnet The most important component of the MRI scanner is the magnet: The magnets currently used in scanners today are in the.5-tesla to 2.0-tesla range (5,000 to 20,000-gauss). Higher values are used for research. Earth magnetic field: 0.5-gauss

The Magnet (cont.) There are three types of magnets used in MRI systems: Resistive magnets Permanent magnets Super conducting magnets (the most commonly used type in MRI scanners). The most commonly used imaging planes in MRI are axial, sagittal and coronal. The most commonly used imaging planes in MRI are axial, sagittal and coronal.

The main uses of MRI NEUROLOGY NEUROLOGY Brain (early detection of cerebral white matter disease, posterior fossa and brain stem pathology, intracranial arterial and venous abnormalities ) Brain (early detection of cerebral white matter disease, posterior fossa and brain stem pathology, intracranial arterial and venous abnormalities ) Spinal cord (spinal cord lesions) Spinal cord (spinal cord lesions) Brachial plexus Brachial plexus ORTHOPAEDICS ORTHOPAEDICS Lumber spine Lumber spine Knee Knee Shoulder Shoulder

Advantages of MRI No ionizing radiation. No ionizing radiation. Superior soft tissue contrast than CT. Superior soft tissue contrast than CT. Can select any plane Can select any plane e.g coronal,sagittal,oblique. e.g coronal,sagittal,oblique. More sensitive to tissue changes More sensitive to tissue changes No bone artifacts No bone artifacts No iodinated contrast media used. No iodinated contrast media used. IV contrast (gadolinium) has a lower risk of death compared with iodinated contrast used with CT.

Disadvantages Limited slice thickness 3mm(CT-1mm) Limited slice thickness 3mm(CT-1mm) Bone imaging limited to display of marrow Bone imaging limited to display of marrow Claustrophobia(due to the confined space in the scanner). Claustrophobia(due to the confined space in the scanner). Cannot use with pacemaker or ferromagnetic implant Cannot use with pacemaker or ferromagnetic implant Longer imaging times mins Longer imaging times mins Cardiac scan upto 1hr. Cardiac scan upto 1hr. MRI equipment is expensive to purchase, maintain, and operate MRI equipment is expensive to purchase, maintain, and operate

Contraindication to MRI Cardiac pacemaker Cardiac pacemaker Implanted cardiac defibrillators Implanted cardiac defibrillators Cochlear implants Cochlear implants Pregnancy Pregnancy marked obesity marked obesity

Tissue contrast The relaxation times T1 and T2 are times related to the nuclei returning to their original alignment in the of the magnetic field. The relaxation times T1 and T2 are times related to the nuclei returning to their original alignment in the longitudinal axis (T1) and in the transverse axis (T2) of the magnetic field. There are 2 basic types of relaxation T1 and T2. There are 2 basic types of relaxation T1 and T2. To define a tissue or pathology accurately a radiologist often compare its appearance on both T1 and T2. To define a tissue or pathology accurately a radiologist often compare its appearance on both T1 and T2.

T1 weighted images T1 MRI is known as T1 MRI is known as Spin-lattice relaxation time. T1 shows anatomy very well. T1 shows anatomy very well. Water is black on T1.-low signal Water is black on T1.-low signal Fat subacute blood melanin proteinaceous fluid and contrast media- high signal Fat subacute blood melanin proteinaceous fluid and contrast media- high signal

T2 weighted images T2 MRI is known as T2 MRI is known as spin-spin relaxation time T2shows pathology or inflammation well T2shows pathology or inflammation well Water and inflammation are white-high signal Water and inflammation are white-high signal Fat is high signal on T2. Fat is high signal on T2.

Low signal or no signal tissues Cortical bone Cortical bone Calcification Calcification Tendons/ligaments Tendons/ligaments Metal Metal Gas Gas