MRI Physics and Instrumentation

Chapter 9: Instrumentation

• Chapter 9 focuses on MRI instrumentation.

• The chapter includes a diagram of an MRI machine, showing a cross-section and inner workings.

MRI Machine Vendors and Software

• Various vendors manufacture MRI machines (Philips, Toshiba, Siemens, GE, etc.).

• MRI software is constantly evolving and changing.

• Different facilities may have varying software versions (e.g., one with the latest software, another with software 5-10 years old).

• Terminology differs across vendors.

• Even within the same vendor (e.g., Siemens), updated software versions will run differently from older versions.

• Implants are continuously being developed, requiring continuous learning.

Hardware Requirements

• MRI machines use a magnet with a bore size of 40-50 cm (though nowadays, 60-70 cm or even 80 cm bores exist).

• Modern MRI machines are typically short bore.

• Older MRI machines were longer with a 40-50 cm opening.

• The MRI system includes:

  • Magnet

  • Shim system (for homogeneity)

  • RF (radio frequency) pulses (transmission system)

  • Magnetic field gradient system (linear slopes)

  • Computer system

  • Coils

Coils and Signal Detection

• In MRI, a coil must be placed around the body part being imaged to send and receive signals, which is different from CT scans.

• Different coils are used for signal detection in different areas of the body.

• The computer subsystem converts the received signals into images.

Magnetism

• Magnetism is a fundamental property of matter; all substances interact with a magnetic field.

• Magnetic susceptibility: the way materials behave in the presence of an external magnetic field.

• Atomic magnetic dipole (magnetic moment): The degree of magnetism exhibited by a substance.

Electron Movement and Magnetic Fields

• Electron movement generates magnetic fields.

• Law of electromagnetic induction: Whenever a charged particle (electron) exhibits motion, a corresponding magnetic field is induced around it.

• The net magnetic moment of an atom results from the combined magnetic moments of all its electrons.

• Electrons in atoms' energy shells are described as spin up or spin down.

• Typically, filled electron shells have equal numbers of spin up and spin down electrons, canceling out the net magnetic moment.

• Atoms with partially filled shells have unpaired electrons, creating a net magnetic effect.

Magnetic Behavior of Atoms

• The magnetic behavior of an atom is determined by the configuration of its orbiting electrons.

• Elements are classified into four main categories based on electron configuration:

  • Diamagnetism

  • Paramagnetism

  • Superparamagnetism

  • Ferromagnetism

Diamagnetism

• Diamagnetic materials have paired electrons.

• In the absence of an external magnetic field, they do not show a magnetic moment.

• When exposed to an external magnetic field, they exhibit a small magnetic moment that opposes the applied field (repulsion).

• Diamagnetic substances are not attracted to magnetic fields but are slightly repelled by them.

• Copper tools are used in MRI because of diamagnetic properties.

• Diamagnetic substances have a low negative magnetic susceptibility and cause a slight decrease in magnetic field strength within the sample.

• Examples: Bismuth, carbon (diamond), carbon (graphite), copper, mercury, lead, water.

Paramagnetism

• Paramagnetic materials have unpaired electrons.

• Due to unpaired electrons, they possess a small magnetic moment.

• Without an external magnetic field, these magnetic moments are randomly oriented and cancel each other out.

• When an external magnetic field is applied, the magnetic moments align in the direction of the field.

• This alignment adds to the external magnetic field, resulting in a local increase in magnetic field.

• Gadolinium is used as a contrast agent in MRI (gadolinium-based contrast).

• Chelated gadolinium are diamagnetic materials that possess both diamagnetic and paramagnetic properties. If the positive paramagnetic effect is greater than the negative diamagnetic effect, the substance will appear paramagnetic.

Superparamagnetism

• Superparamagnetic materials are similar to ferromagnetic materials but exhibit different behavior at the macroscopic level.

Ferromagnetism

• Ferromagnetic materials exhibit a strong attraction to magnetic fields.

• They can become permanently magnetized after being removed from the magnetic field.

• Permanent magnets can have a magnetic field hundreds or thousands of times greater than the applied external field.

Chapter 9: Instrumentation

• Chapter 9 focuses on MRI instrumentation.

• The chapter includes a diagram of an MRI machine, showing a cross-section and inner workings.

MRI Machine Vendors and Software

• Various vendors manufacture MRI machines (Philips, Toshiba, Siemens, GE, etc.).

• MRI software is constantly evolving and changing.

• Different facilities may have varying software versions (e.g., one with the latest software, another with software 5-10 years old).

• Terminology differs across vendors.

• Even within the same vendor (e.g., Siemens), updated software versions will run differently from older versions.

• Implants are continuously being developed, requiring continuous learning.

  • This requires that people continuously keep learning about new implants and how they affect the MRI results

Hardware Requirements

• MRI machines use a magnet with a bore size of 40-50 cm (though nowadays, 60-70 cm or even 80 cm bores exist).

  • Bore size affects factors like patient comfort and the ability to image larger patients.

• Modern MRI machines are typically short bore.

  • Short bore machines are easier to use and usually better for claustrophobic patients

• Older MRI machines were longer with a 40-50 cm opening.

• The MRI system includes:

  • Magnet

  • Main component which generates the strong static magnetic field.

  • Shim system (for homogeneity)

  • Ensures the magnetic field is uniform across the imaging area.

  • RF (radio frequency) pulses (transmission system)

  • Transmits radio waves into the patient to excite the hydrogen nuclei.

  • Magnetic field gradient system (linear slopes)

  • Creates spatial encoding of the MR signal.

  • Computer system

  • Processes the received signals and constructs the MR images.

  • Coils

Coils and Signal Detection

• In MRI, a coil must be placed around the body part being imaged to send and receive signals, which is different from CT scans.

  • Coils act as antennas to transmit and receive RF signals.

• Different coils are used for signal detection in different areas of the body.

  • Head coil, knee coil, body coil, etc.

• The computer subsystem converts the received signals into images.

  • Raw data is transformed using Fourier transforms to create images.

Magnetism

• Magnetism is a fundamental property of matter; all substances interact with a magnetic field.

  • Atomic level interactions determine macroscopic magnetic behavior.

• Magnetic susceptibility: the way materials behave in the presence of an external magnetic field.

  • Materials are classified based on their magnetic susceptibility.

• Atomic magnetic dipole (magnetic moment): The degree of magnetism exhibited by a substance.

  • Arises from the spin and orbital motion of electrons.

Electron Movement and Magnetic Fields

• Electron movement generates magnetic fields.

  • Moving charges create magnetic fields.

• Law of electromagnetic induction: Whenever a charged particle (electron) exhibits motion, a corresponding magnetic field is induced around it.

  • This principle is fundamental to MRI signal generation.

• The net magnetic moment of an atom results from the combined magnetic moments of all its electrons.

  • Electron configuration determines overall magnetic properties.

• Electrons in atoms' energy shells are described as spin up or spin down.

  • Spin is a quantum mechanical property of electrons.

• Typically, filled electron shells have equal numbers of spin up and spin down electrons, canceling out the net magnetic moment.

  • Paired electrons result in no net magnetic moment.

• Atoms with partially filled shells have unpaired electrons, creating a net magnetic effect.

  • Unpaired electrons contribute to paramagnetism and ferromagnetism.

Magnetic Behavior of Atoms

• The magnetic behavior of an atom is determined by the configuration of its orbiting electrons.

  • Electron configuration dictates how atoms interact with magnetic fields.

• Elements are classified into four main categories based on electron configuration:

  • Diamagnetism

  • Paramagnetism

  • Superparamagnetism

  • Ferromagnetism

Diamagnetism

• Diamagnetic materials have paired electrons.

  • Paired electrons cancel out magnetic moments.

• In the absence of an external magnetic field, they do not show a magnetic moment.

  • No spontaneous magnetization.

• When exposed to an external magnetic field, they exhibit a small magnetic moment that opposes the applied field (repulsion).

  • Lenz's Law explains this opposition.

• Diamagnetic substances are not attracted to magnetic fields but are slightly repelled by them.

  • Repulsion is weak but measurable.

• Copper tools are used in MRI because of diamagnetic properties.

  • Non-interference with the MRI's magnetic field.

• Diamagnetic substances have a low negative magnetic susceptibility and cause a slight decrease in magnetic field strength within the sample.

  • Susceptibility values are small and negative.

• Examples: Bismuth, carbon (diamond), carbon (graphite), copper, mercury, lead, water.

Paramagnetism

• Paramagnetic materials have unpaired electrons.

  • Unpaired electrons create atomic magnetic moments.

• Due to unpaired electrons, they possess a small magnetic moment.

  • Atomic moments are randomly oriented.

• Without an external magnetic field, these magnetic moments are randomly oriented and cancel each other out.

  • No macroscopic magnetization without an external field.

• When an external magnetic field is applied, the magnetic moments align in the direction of the field.

  • Alignment is proportional to the field strength.

• This alignment adds to the external magnetic field, resulting in a local increase in magnetic field.

  • Enhancement of magnetic field strength.

• Gadolinium is used as a contrast agent in MRI (gadolinium-based contrast).

  • Enhances the visibility of tissues and structures.

• Chelated gadolinium are diamagnetic materials that possess both diamagnetic and paramagnetic properties. If the positive paramagnetic effect is greater than the negative diamagnetic effect, the substance will appear paramagnetic.

  • Che