Presentation on theme: "MRI Imaging By: Scott Hayes. MRI measures the movement of hydrogen atoms: Why hydrogen atoms? Hydrogen is abundant in the water molecules in human tissue."— Presentation transcript:
MRI measures the movement of hydrogen atoms: Why hydrogen atoms? Hydrogen is abundant in the water molecules in human tissue. The nuclei of Hydrogen can act like a “compass needle” in a magnetic field. Some types of tissues prevent hydrogen from spinning as freely. For example, bone is more restrictive to movement than fat. These differences can be detected to distinguish tissue type. http://www.cs.sfu.ca/~stella/papers/blairthesis/main/_4056_figure87.gif
RF Pulse knocks H out of alignment Hydrogen are knocked out of alignment with a radio frequency (RF) pulse and relax back into alignment with magnetic field. H BoBo 90 o RF Pulse Applied Hydrogen Relaxes and Realigns in Magnetic field However, hydrogen does not simply pivot back into alignment. It precesses!
Physics Review Precession – Gyroscope Example Animation by Dr. Michael R. Gallis Penn State Schuylkill Creative Commons Lisence
Precession of Hydrogen Atoms Larmor frequency (ω) is proportional to magnetic field strength: ω = γ B Animation from http://www.e-mri.orghttp://www.e-mri.org Hydrogen are knocked out of alignment with a radio frequency pulse and process until they are again aligned with the magnetic field. Movement is analogous to gyroscope movement.
Physics Review Lenz’s Law NS Increasing B through coil = CW Current “Right Hand Rule” Decreasing B through coil = CCW Current If no change in B NO CURRENT
Why is Lenz’s Law important to MRI? Each Hydrogen acts like a mini-magnet. Procession of Hydrogen can produce a measurable electric current along a pickup coil. As hydrogen precesses, current oscillates until hydrogen aligns with magnetic field. Current Time Pickup Coil Hydrogen is relaxing back into alignment of magnetic field.
MRI Basic Layout The magnetic field of an MRI machine is typically 3 Tesla! The Earth’s magnetic field is less that 30 microtesla (0.00003 Ts). http://www.magnet.fsu.edu/education/tutorials/magnetacademy/mri/index.html
MRI Machinery Images: http://www.magnet.fsu.edu/education/tutorials/magnetacademy/mri/page5.html Coming up next: How do we tell hydrogen along each axis apart?
Gradient Slice Selection HHHHHHHHHHHHHHHH B applied w L = γB o w L : Lamor Freq. γ : Gyration Const. B o : Mag. Field Applied Perpendicular to desired plane. Spin speed represents processional frequency.
Why is a Slice Selection Gradient used? Magnetic Field applied perpendicular to desired slice, because we can now “focus” on a layer with a specific processional frequency. Hydrogen atoms to either side of desired layer are either too fast or too slow. H HHH B applied H HHH H HHH H HHH Lets select this slice
H H H H H H H H H H H H Phase Encoding – Resolves image in second dimension. Apply a magnetic gradient, but only briefly. Goal: Get hydrogen atoms out of sync with each other so they can be distinguished along another axis. H H H H H H H H HH HH HH HH B First resolve the first dimension with an applied gradient. Next, select one slice based on precessional frequency. Apply a gradient along another axis. Turn off gradient! Notice the precession is “out of sync” B Current Time
Phase Encoding Animation from http://www.e-mri.orghttp://www.e-mri.org
Resolving the Third Dimension Frequency Encoding H H H H H H H H H H H H H H H H x y z Slice plane Review of Spatial Resolution: 1.Apply slice selection gradient and choose a slice based on precession frequency. Consider plane your image! 2.Apply and turn off phase encoding gradient. This gets hydrogen in the x- axis out of sync. 3.Apply a third gradient, now we can distinguish hydrogen in the y-axis based on the precessional speeds. We have now resolved all three dimensions! But now what do we do with all this info…. B
Fourier’s Transform The pick up coil receives many different frequency oscillations. Use Fourier’s Transform to process the data. Time [s] Signal Strength 1 4 Freqency [Hz] Signal Strength 1 0.25 f = 1/T = ¼ =.25 1.5 0.5 1.5 -1.5 f = 1/T = ½ =.5 f = 1/T = 1/1 = 1 1.0 Transform
Fourier Transform (cont.) The pickup coil does not distinguish between the input of each hydrogen. They are all read together, and constructively and destructively interfere. Fourier’s allows us to determine which frequencies are along the axis. For instance, if there are two hydrogen at different frequencies along an axis: Signal Strength Time [s] 1 4 Time [s] 1 4 Time [s] 1 4 + = Current Fourier Frequency [Hz] 1 0.25 Signal Strength 0.25 Frequency [Hz] 1 0.251 Frequency [Hz] 1 0.25 1
Image formation Animation from http://e-mri.org
2D Fourier Transform Recall that the second axis is resolved with a phase encoding gradient. These hydrogen have the same frequency, but interfere with each other due to phase shift. Signal Strength 1 -1 4 Time [s] 1 -1 4 1 -1 4 + = A 1D Fourier Transform cannot distinguish between shifted phases. But if we take the Fourier Transform again, orthogonal to the first access the phase encoding gradient can be distinguished! The resulting data is known as a K-Space.
K-Space A 2D Fourier transform is conducted by performing two Fourier transforms orthogonal to each other. This yields a “K-Space” An example is seen on the right. The “K-Space” undergoes an Inverse Fourier Transform. Following this mathematical step, we finally have an image. http://www.revisemri.com/tutorials/what_is_k_space/what_is_k_space_files/fullscreen.htm
K-Space General spatial information is concentrated towards the center of “K-Space” In the figure to the right we see an image formed taking only the Inverse Fourier Transform of the center of the K-Space. As seen on the right, the peripheral regions of the K-Space encode for the edges of the image. http://www.revisemri.com/tutorials/what_is_k_space/what_is_k_space_files/fullscreen.htm
Why does an MRI machine make so much noise? When gradients are applied, the strong magnetic field causes the coils to stretch. Examples of sounds resulting from standard pulse sequence – LinkLink Pulses can be “tuned” (Wilson 2001) Just for fun…some MRI music. http://www.adarngooddog.com/Man_Covering_His_Ears%20cartoon.gif
MRI Modifications- Open MRI Claustrophobic patients can’t tolerate the confined enclosure of an MRI machine. Even mildly claustrophobic patients have trouble due to the very loud noise produced by the machine. Open MRI machine works the same way, but with a weaker magnetic field and less resolution. New 1 Tesla open MRI machines offer adequate resolution A standard MRI machine has a 3 Ts magnetic field. A 1 Tesla open MRI machine manufactured by Phillips.
MRI Modifications- Functional MRI (fMRI) Hemoglobin has different magnetic properties when bound to oxygen, that can be distinguished by fMRI. Areas of brain activity have a surge of oxygenated blood. fMRI can identify areas of the brain with high oxyhemoglobin content, which correlates to areas of heightened brain activity. http://www.unmc.edu/dept/alliedhealth/rste/ctmri/
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