Magnetic Resonance Imaging - MRI King Saud University College of Dentistry Magnetic Resonance Imaging - MRI Asma’a Al-Ekrish BDS Demonstrator ( OMF Radiology )

Sources Source of illustrations: Understanding MRI- An interactive guide to MRI principles and applications Philips Medical Systems Best, The Netherlands Theory of MRI is based on the magnetic properties of an atom and its response to radiofrequency stimulation

Some MRI images reproduced from: Sources Some MRI images reproduced from: Som, P.M. and H.D. Curtin (2003). Head and neck imaging. St. Louis, Mosby. White, S. C. and M. J. Pharoah (2004). Oral radiology.Principles and interpretation. St. Louis, Mosby. Theory of MRI is based on the magnetic properties of an atom and its response to radiofrequency stimulation

Introduction RF pulse Theory of MRI is based on the magnetic properties of an atom and its response to radiofrequency stimulation

Lecture Contents Magnetic Resonance Image Production Diagnostic Applications

Magnetic Resonance

Electric charge + spinning Magnetic Nuclei Electric charge + spinning Tiny magnetic field

Strength and direction of magnetic field represented by a vector: Magnetic Nuclei magnetic moment Strength and direction of magnetic field represented by a vector: MAGNETIC MOMENT

Magnetic Nuclei Uneven number of protons which causes them to have a net “spin” Spinning nuclei are MR active

Abundant in the human body and have a large magnetic moment Magnetic Nuclei 1H Abundant in the human body and have a large magnetic moment

Magnetic Nuclei Moments are constantly changing their alignments making this a dynamic system

Precession

Precession Magnetic nucleus placed in a scanner’s magnetic field rotates in a cone around the main field’s direction

Precession Larmor Frequency: speed or frequency of precession Proportional to the strength of the magnetic field

strength of magnetic field Precession LARMOR EQUATION Larmor frequency (MHz) gyromagnetic ratio (MHz/T) strength of magnetic field (T) = x Every element has a specific gyromagnetic ratio

Magnetic Resonance Magnetic resonance induced by exposing nuclei to a second magnetic field B1 and a radiofrequency pulse Resonance only occurs if the radiofrequency applied matches the Larmor frequency of the nuclei

Magnetic Resonance MR signal is emitted After removal of the RF signal, nuclei gradually return to their position relative to main magnetic field and the MR signal “decays”

Magnetic Resonance VOLUME COILS TMJ COILS Head coil Body coil ( fixed inside magnet ) Head coil VOLUME COILS TMJ COILS

Individual magnetic moments cannot be measured Net Magnetization Individual magnetic moments cannot be measured Signal produced in MRI is produced by the sum of all the magnetic moments: the NET MAGNETIZATION (represented by the magnetic vector)

Net Magnetization NET MAGNETIZATION points in the same direction as the main magnetic field

Net Magnetization LONGITUDINAL MAGNETIZATION Net magnetization is situated on the z-axis of a 3-dimensional coordinate system  the “static frame” The z-axis indicates the direction of the scanner’s main field

Net Magnetization LONGITUDINAL MAGNETIZATION Net magnetization precesses at the larmour frequency At equilibrium, precession is not detectable and MR signal cannot be measured

Net Magnetization TRANSVERSE MAGNETIZATION To detect precession, the magnetization vector must be tipped into the horizontal plane Basis of MR signals

Net Magnetization TRANSVERSE MAGNETIZATION Larger net magnetization Larger transverse magnetization Stronger MR signal

The MR Signal Nuclei at equilibrium with main magnetic field  only longitudinal magnetization which cannot be measured RF pulse applied  transverse magnetization Net magnetization spirals down towards the horizontal plane

The MR Signal Strength and duration of the RF pulse determine the degree of tilt or flip angle (a) Pulses are named after the flip angle they induce

The MR Signal Receiver coil As net magnetization precesses in the transverse plane, an oscillating electrical signal is generated which is detected by a receiver coil

This current is the MR signal measured in MRI Current in receiver coil s t This current is the MR signal measured in MRI

The MR Signal Free Induction Decay s t The MR signal immediately after an RF signal is The Free Induction Decay

The MR Signal Free Induction Decay Properties: = amplitude Free Induction Decay s t Properties: The signal strength or amplitude of the signal is the largest value in one oscillation Dictated by size of transverse magnetization vector

The MR Signal Free Induction Decay Properties: Measured in mVs = amplitude Free Induction Decay s t Properties: Measured in mVs The amplitude gradually decays as net magnetization gradually returns to equilibrium

The MR Signal Free Induction Decay Frequency = cycles per second (MHz)

The MR Signal Free Induction Decay The signal’s phase is the position in the signal’s cycle of oscillation Measured in degrees

RF Pulse Flip Angles 90o pulse 180o pulse Tips net magnetization to the horizontal plane 180o pulse Tips net magnetization to the negative z-axis

Time interval between individual RF pulses Repetition Time ( TR ) Time interval between individual RF pulses

Time interval between the RF pulse and detection of the image Echo Time Time interval between the RF pulse and detection of the image

Image Production

Image Production Tissues and lesions are differentiated from eachother when they have different signal intensities  tissue contrast Strong signal  white areas Weak signal  dark areas Intermediate signal  gray areas

Image Production Signal intensity depends on: Proton Density (PD) Contrast: Number of hydrogen nuclei in tissue ­ H  high signal intensity ¯ H  low signal intensity Determines size of the equilibrium magnetization of a tissue

Image Production Signal intensity depends on: Differences in T1 and T2 relaxation rates between tissues Stronger source of contrast Flow Susceptibility Diffusion Perfusion

Relaxation T1 relaxation T2 relaxation Is the gradual return of net magnetization to the longitudinal axis after excitation with an RF pulse Two independent components: T1 and T2 relaxation rates

Relaxation Different tissues have different T1 and T2 relaxation rates  their signal intensities appear different Most MRI acquisition techniques are influenced by T1 or T2 contrast

T2- “Spin-spin” Relaxation T2 Relaxation T2- “Spin-spin” Relaxation Decay of transverse magnetization after RF pulse Fast

T2 Relaxation Occurs as magnetic moments interact with eachother They have different precession frequencies so they de-phase  decay of transverse magnetization

T2 Relaxation T2 relaxation time: the time needed for a 63% reduction of transverse magnetization

T2 Relaxation Water and abnormal tissues: long T2 relaxation time  bright Normal tissues: intermediate T2 relaxation time  gray Normal Abnormal 63% reduction

T2 Relaxation T2 weighted images are favored when searching for pathological conditions

T2* Relaxation Dephasing caused by spin-spin relaxation accelerated by de-phasing caused by imperfections in the main magnetic field Cumulative effect leads to “effective T2 ” or “ T2* ” relaxation

T1- “Spin lattice” Relaxation T1 Relaxation T1- “Spin lattice” Relaxation Recovery of longitudinal magnetization Occurs due to interaction of hydrogen nuclei with their surroundings Slow Affected by flip angle (a) of RF pulse Small (a)  faster net magnetization returns to z-axis

T1- “Spin lattice” Relaxation T1 Relaxation T1- “Spin lattice” Relaxation Time required for recovery of 63% of longitudinal magnetization

T1- “Spin lattice” Relaxation T1 Relaxation T1- “Spin lattice” Relaxation 63% recovery Fat: short T1 relaxation time  high signal intensity Good image contrast High anatomic detail

T1 Relaxation T1 weighted images are useful for demonstration of anatomy, especially of small regions where high spatial resolution is needed (eg TMJ)

Pulse Sequences Pulse sequences are carefully coordinated and timed sequence of RF pulses, gradient applications, and intervening time periods which generate a particular type of image contrast

Pulse Sequences 2 main categories: Spin Echo sequences Gradient Echo sequences

Pulse Sequences Spin Echo Contrast can be adjusted by variations in: Repetition time ( TR ) Echo time ( TE ) Transverse magnetization characterized by T2 because extra relaxation due to field inhomogeneities is counteracted

Pulse Sequences Gradient Echo Contrast may be adjusted by variations in: Relaxation time (TR ) Echo time ( TE ) Flip angle ( a ) Decay of transverse magnetization characterized by T2* ( T1 and PD also ??)

Pulse Sequences

Obtained by manipulating the parameters of a sequence of RF pulses Image Production T1 weighted T2* weighted T2 weighted PD weighted Obtained by manipulating the parameters of a sequence of RF pulses

Gradient Coils 3 sets of gradient coils set at right angles to eachother Each coil produces a magnetic gradient in a particular direction ( x-y-z axes) When all three used together, a gradient can be produced in any direction and images may be acquired from any plane. Provide 15 mT/meter

Slice Selection Strength of secondary magnetic field varies linearly along the length of the field to produce a magnetic gradient Therefore, Larmour frequency of nuclei varies along length also Select slice to be imaged by applying an RF pulse whose frequency matches the Larmour frequency of the nuclei in that area

Slice Selection Orientation of the slice is perpendicular to the field gradient

Slice Thickness Steepness of field gradient Bandwidth of RF pulse May be selected by manipulating: Steepness of field gradient Bandwidth of RF pulse

Spatial Encoding Signals from individual voxels must be distinguished from eachother Achieved by 2 gradient fields at right angles: First, the Phase gradient Second, the Frequency gradient

Spatial Encoding Each voxel: unique phase and frequency

Multi-slice and Volumetric Imaging Multi-slice scanning Volume ( 3D) scanning

Multi-slice and Volumetric Imaging

Diagnostic Applications

Diagnostic Applications Axial Acquisition of images in any plane Coronal Sagittal

Diagnostic Applications Obtaining corrected views of TMJ Oblique sagittal views Oblique coronal views

Diagnostic Applications Evaluation of articular disc position and morphology Detection of joint effusion

Diagnostic Applications High Resolution MRI Evaluation of articular disc integrity

Diagnostic Applications CT CT MRI MRI More accurate evaluation of internal structure and extent of soft tissue lesions

Diagnostic Applications Separation of pathological and normal soft tissues Evaluation of effect of lesions on adjacent soft tissues

Diagnostic Applications MR sialography

Diagnostic Applications Functional Imaging Techniques Utilize ultra-fast imaging sequences in order to assess function and physiology ( eg of TMJ when opening and closing)

Diagnostic Applications ADVANTAGES OF MRI Superior anatomic and pathological details in soft tissues No ionizing radiation Non-invasive Imaging possible in several planes without moving the patient Fewer artifacts

Diagnostic Applications DISADVANTAGES OF MRI High cost Special site planning and shielding Patient claustrophobia Inferior images of bone Long scanning times

Diagnostic Applications Contraindications to MR Imaging Cerebral aneurysm clips Cardiac pacemakers Ferromagnetic implants Metallic prosthetic heart valves Claustrophobic or uncooperative patients First trimester of pregnancy ( ?? )

Conclusion

Image characteristics in MRI are dependent on several factors These factors may be manipulated to achieve the required quality and contrast according to the specific diagnostic need Advances in MRI technology are allowing the use of this modality an increasingly versatile ways

Thank You Questions?