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Medical Devices 04- Magnetic Resonance Imaging (MRI)

Dalton H Bermudez, B.Eng

Medical Physics PhD student

Some slice gotten from Thomas Lilieholm

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MRI 3D printed model

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History of MRI

Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT)

Raymond Damadian, an Armenian-American physician, scientist: worlds first MRI machine 1972

Paul Lauterbur- technique to generate images from MRI

Peter Mansfield – mathematical technique to generate images from MRI.

Nobel Prize in Physiology or Medicine for their ”discoveries concerning magnetic resonance imaging”

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What is MRI?

Magnetic resonance imaging (MRI) is a spectroscopic imaging technique used in medical settings to produce images of the inside of the human body.

MRI is based on the principles of nuclear magnetic resonance (NMR), which is a spectroscopic technique used to obtain microscopic chemical and physical data about molecules.

In 1977 the first MRI exam was performed on a human being. It took 5 hours to produce one image.

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MRI – Big Magnet

1 Tesla (T) = 10,000 Gauss
Earth’s magnetic field = 0.5 Gauss
4 Tesla = 4*10,000/0.5 = 80,000*Earth’s magnetic field

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MRI-System Components

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MRI - Nuclear Spin in Magnetic Field

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MRI- Nuclear Spin

Properties on nuclei found at high abundance in the body:

MRI can be performed with odd atomic mass (non-zero spin)

¹H, ¹³C, ¹⁹F, ²³Na, ³¹P

Most frequent medical imaging is performed with ¹H (proton)

Abundant: high concentration in human body
High sensitivity: yields large signals

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Generate images using NMR

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Free-induction decay

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MRI-Nuclear Magnetic Resonance

The decay constants T₁ and T₂ depends on physical properties of the resonating sample. By measuring the decay constants one can therefore deduce what is in the sample.

In the 70’ is was realized that this may be used for medical application

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Block equations and solution at steady state

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Given the value of T1, what is the optimum tip angle that maximizes Mxy -Ernst angle to Maximize SNE

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MRI acquisition parameters

source:https://practicalfmri.blogspot.com/2011/08/physics-for-understanding-fmri_15.htm

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Pulse Sequence and k-space

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In class Assignment:

Draw the corresponding Gx and Gy pulse sequence for the corresponding radial trajectory

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K-space

Each row of k-space contains the raw data received under a particular phase gradient, where the order in which the rows are recorded depends on the image sequence used.

Once all the k-space has been assembled, it is Fourier transformed (2D FFT) to obtain the image.

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FFT importance in MRI

Answer: To relate k-space to image space

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In Class Assignment:

Find the relative change in FOVx, FOVy, resolution in x, resolution in y, and SNR of the second scan compared to the first scan

Sketch the pulse sequence on the reference scan and the pulse sequence of the second scan.

Assume the readout gradient amplitude and the sampling interval Ts ( time to sample one data point in k-space) along the readout directions in Scan 2 does not change compared to the Reference scan

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MRI Phase encoding

Phase encoding Gy in the y direction and frequency encoding Gx in the x direction. The resulting echo signal is:

It can be re-written as a Fourier transform after doing the following substitutions:

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MRI- Slice selection

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MRI image plane

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How to run a GE MRI Scanner

https://youtu.be/UicWgGXER8M

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Converting PNG MRA images to AFNI Volumes for Rendering

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Magnetic Resonance Angiography

MR angiography (MRA) uses a powerful magnetic field, radio waves and a computer to evaluate blood vessels and help identify abnormalities.
Exam does not use radiation and may require an injection of contrast material
Contrast material used for MRA is less likely to cause an allergic reaction than the contrast material used for computed tomography (CT)