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Magnetic Resonance Magnetic Resonance Imaging (MRI)Imaging (MRI)
The Components:The Components: A magnet which produces a very powerful uniform A magnet which produces a very powerful uniform
magnetic field.magnetic field.
Gradient Magnets which are much lower in Gradient Magnets which are much lower in strength.strength.
Equipment to transmit radio frequency (RF).Equipment to transmit radio frequency (RF).
A very powerful computer system, which A very powerful computer system, which translates the signals transmitted by the coilstranslates the signals transmitted by the coils. .
The MagnetThe 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.)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, The most commonly used imaging planes in MRI are axial, sagittal and coronal.sagittal and coronal.
The main uses of MRI The main uses of MRI NEUROLOGYNEUROLOGY Brain (early detection of cerebral white matter Brain (early detection of cerebral white matter
disease, posterior fossa and brain stem disease, posterior fossa and brain stem pathology, intracranial arterial and venous pathology, intracranial arterial and venous abnormalities )abnormalities )
Spinal cord (spinal cord lesions)Spinal cord (spinal cord lesions) Brachial plexusBrachial plexus ORTHOPAEDICSORTHOPAEDICS Lumber spineLumber spine KneeKnee ShoulderShoulder
Advantages of MRIAdvantages 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 changesMore 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 IV contrast (gadolinium) has a lower
risk of death compared with risk of death compared with iodinated contrast used with CT.iodinated contrast used with CT.
DisadvantagesDisadvantages Limited slice thickness 3mm(CT-1mm)Limited slice thickness 3mm(CT-1mm) Bone imaging limited to display of marrowBone imaging limited to display of marrow Claustrophobia(due to the confined space in Claustrophobia(due to the confined space in
the scanner).the scanner). Cannot use with pacemaker or ferromagnetic Cannot use with pacemaker or ferromagnetic
implantimplant Longer imaging times 20-30 minsLonger imaging times 20-30 mins Cardiac scan upto 1hr.Cardiac scan upto 1hr. MRI equipment is expensive to purchase, MRI equipment is expensive to purchase,
maintain, and operatemaintain, and operate
Contraindication to MRIContraindication to MRI Cardiac pacemakerCardiac pacemaker Implanted cardiac defibrillatorsImplanted cardiac defibrillators Cochlear implantsCochlear implants PregnancyPregnancy marked obesity marked obesity
Tissue contrast Tissue contrast The relaxation times T1 and T2 are times The relaxation times T1 and T2 are times
related to the nuclei returning to their related to the nuclei returning to their original alignment in the original alignment in the longitudinal axis (T1) and in the transverse axis (T2) of the of the magnetic field.magnetic field.
There are 2 basic types of relaxation T1 There are 2 basic types of relaxation T1 and T2.and T2.
To define a tissue or pathology accurately To define a tissue or pathology accurately a radiologist often compare its a radiologist often compare its appearance on both T1 and T2. appearance on both T1 and T2.
T1 weighted images 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 signalWater is black on T1.-low signal Fat subacute blood melanin Fat subacute blood melanin
proteinaceous fluid and contrast proteinaceous fluid and contrast media- high signalmedia- high signal
T2 weighted images T2 weighted images T2 MRI is known as T2 MRI is known as spin-spin
relaxation time T2shows pathology or inflammation T2shows pathology or inflammation
wellwell Water and inflammation are white-Water and inflammation are white-
high signalhigh signal Fat is high signal on T2.Fat is high signal on T2.
Low signal or no signal Low signal or no signal tissuestissues
Cortical boneCortical bone CalcificationCalcification Tendons/ligamentsTendons/ligaments MetalMetal GasGas