Presentation on theme: "Principles of the MRI Signal Contrast Mechanisms MR Image Formation John VanMeter, Ph.D. Center for Functional and Molecular Imaging Georgetown University."— Presentation transcript:
Principles of the MRI Signal Contrast Mechanisms MR Image Formation John VanMeter, Ph.D. Center for Functional and Molecular Imaging Georgetown University Medical Center
Outline Physics behind MRI Basis of the MRI signal Tissue Contrast Examples Spatial Localization
90 o Radiofrequency Pulse used to “tip” protons into X-Y plane. x y z Flip Angle - Degree of Deflection from Z-axis
Following an RF pulse the protons precess in the x-y plane B o M o Magnetic Moment Measurable After RF Pulse
The MRI Measurement (Up to this point) In the presence of the static magnetic field –Protons align with the field –Protons precess about the magnetic Briefly turn on RF pulse –Provides energy to tip the protons at least partially into the imaging plane What happens to the protons next?
Types of Relaxation Longitudinal – precessing protons are pulled back into alignment with main magnetic field of the scanner (B o ) reducing size of the magnetic moment vector in the x-y plane Transverse – precessing protons become out of phase leading to a drop in the net magnetic moment vector (M o ) Transverse relaxation occurs much faster than Longitudinal relaxation Tissue contrast is determined by differences in these two types of relaxation
The MRI Measurement (Sans Spatial Localization) RF time Voltage (Signal) time MoMo t x y z x y z x y z M o 90° V(t) BoBo Mo
Main Tissue Contrast Controls Echo Time (TE) – time after 90 o RF pulse until readout. Determines how much transverse relaxation will occur before reading one row of the image. Repetition Time (TR) – time between successive 90 o RF pulses. Determines how much longitudinal relaxation will occur before constructing the next row of the image.
T1 Curve T2 Curve Intensity Time Tissue Contrast Every tissue has a different affect on longitudinal (T1) and transverse (T2) relaxation.
3000 200010000 0.0 0.2 0.4 0.6 0.8 1.0 TR (milliseconds) Signal gray matter T1 = 1000 CSF T1 = 3000 white matter T1 = 600 Contrast in MRI: T1-Weighting
But Wait How do you set TE to generate a T1 weighted image? How do you set TR to generate a T2 weighted image? How do you set TR & TE to generate a proton density weighted image?
Mixing T1 & T2 Contrast What do you get if you use the optimal TR setting for T1 contrast and the optimal TE setting for T2 contrast? T3 contrast? No contrast!!
Tissue Contrast Dependence on TR, TE TR Long Short Long TE PDW T1 poor! T2 (time in 10’s of ms) ( time in 1000’s of ms )
Damadian’s Discovery Differential longitudinal relaxation between healthy and tumorous tissue in the rat Walker sarcoma had longer T1 relaxation time than healthy brain Novikoff Hepatoma had shorter T2 relaxation time than healthy liver
Two Main Classes of Pulse Sequence Spin Echo (SE) - uses a second RF- pulse to refocus spins –TR & TE control T1 and T2 contrast Gradient Echo (GE) - uses a gradient to refocus spins –Flip Angle & TE control T1 and T2* contrast –Used in EPI (fMRI) sequences
T2*-Weighting (GE) Refer to T2-weighting in a gradient echo sequence as T2*-weighting Because of inhomogeneities in the B 0 magnetic field T2 relaxation occurs faster using a gradient echo sequence than ‘true T2 relaxation’ as measured with a spin-echo sequence The greater the inhomogeneity the faster T2 decay occurs
The MRI Experiment x y z RF time x y z Voltage (Signal) time MoMo t x y z M o 90° V(t) BoBo Mo
The MRI Sequence (Sans Spatial Localization) 1)Equilibrium (magnetization points along Bo) 2)RF Excitation (tip magnetization away from equilibrium) 3)Precession produces signal, dephasing starts 4)Readout signal from precession of the magnetization vector (TE) 5)Return to equilibrium and reapply RF Excitation ( TR )
Spatial Localization Gradients, linear change in magnetic field, will provide additional information needed to localize signal Makes imaging possible/practical –Remember the Indomitable? –Couldn’t spatially localize MRI signal instead moved subject to get each voxel Nobel prize awarded for this idea!
Larmor Equation Frequency (rate) of precession is proportional to the strength of magnetic field = * B
Dissecting Larmor Equation = * B Gyromagnetic Constant Rate of precession Magnetic field
Center Frequency Center frequency is the frequency (i.e. rate) at which protons spin (precess) with just the static magnetic field If the center frequency of a 1.5T scanner is 63MHz what it the center frequency of our 3.0T scanner?
Center Frequency B 63MHzIf B = 1.5T 2 * 63MHzIf B = 3.0T 126MHz
Gradients A gradient is simply a deliberate change in the magnetic field Gradients are used in MRI to linearly modify the magnetic field from one point in space to another Gradients are applied along an axis (i.e. G x along the x-axis, G y along the y-axis, G z along the z-axis) What happens to the frequency at which the precess when we turn on a gradient?
B B= B 0 + B 1 +r0-r 1 2 3 4 5 6 7 8 9 Effect of Gradient on Rate of Precession
From Proton Signal to Pixel Intensities Amplitude of the sinusoidal wave at a pixel used to determine the brightness of the pixel (i.e. color)
Net Signal at Coil Signal from Multiple Pixels Pixel 1. Pixel n +
Decomposing Received Signal Left unchanged the signal received cannot be broken down by location of individual pixels Need method for efficiently pulling out the signal from many pixels at once Gradients used to relate where a particular signal is coming from
Frequency Encoding Use a gradient to modify the rate at which the protons spin based on location of the proton Requires the gradient to remain on
Prior to Gradient Col 1 Col 2 Col 3 Uniform Field
Gradient Applied Col 1 Col 2 Col 3 Lower Field Higher Field
Frequency Encoding Apply gradient in one direction and leave it on Result: Protons that experience a decrease in the net magnetic field precess slower Protons that experience an increase in the net magnetic field precess faster
Side-Effect of Gradient Gradient also causes phase of the protons to change Application of a second gradient of opposite polarity will undo this
Frequency Encode Gradient The area under the second gradient must be equal to that of the first gradient
Phase Encoding Turn gradient on briefly then turn it off Turning on the gradient will cause some protons to spin faster others to spin slower depending on where they are located Turning off the gradient will make them all spin at the same rate again BUT they will be out of ‘phase’ with one another based on where they are located
Prior to Gradient Row 1 Row 2 Row 3 Uniform Field
Gradient Applied Row 1 Row 2 Row 3 Lower Field Higher Field
Gradient Turned Off Row 1 Row 2 Row 3 Uniform Field
Phase Encoding Apply gradient in one direction briefly and then turn off Result: Protons initially decrease or increase their rate of precession After the gradient is turned off all of the protons will again precess at the same rate Difference is that they will be out phase with one another
Combining Phase & Frequency Encoding Row 1, Col 1 Row 2, Col 2 Row 3, Col 3
Sum Corresponds to Received Signal ++++ Row 1, Col 1 Row 2, Col 2 Row 3, Col 3
Converting Received Signal into an Image Signal produced using both frequency and phase encoding can be decomposed using a mathematical technique called the Inverse Fourier Transform Result is the signal (sinusoidal squiggles) produced at each individual pixel
From Signal to Image Row 1, Col 1 Row 2, Col 2 Row 3, Col 3 Inv FFT Pixels
Lauterbur’s Insight Use of gradients to provide spatial encoding Frequency and Phase - was Lauterbur’s contribution Awarded Nobel prize for this work
Components of Frequency Domain Three components to a signal in the frequency domain: –Amplitudecomes from contrast –Frequencyrate at which protons spin –Phasedirection of proton’s spin Inverse Fourier Transform (IFT) is a mathematical tool for converting data from frequency domain to ‘image’ domain
k-space Frequency increases from the center out in all directions Phase varies by angle
Images From k-space K-space is turned into an image using a Fourier Transformation 2D-IFT
Selecting a Slice Again use gradient to modify frequency of the proton’s spin Slice select gradient is positive on one side of the slice and negative on the other side At the desired slice location the slice select gradient is zero Thus, protons in this slice and only this slice will be spinning at the center frequency of the scanner! If this gradient is on when we apply RF pulse only protons in the slice will be tipped into x- y plane and thus measurable