MRI Slice Selection | Signal Localisation | MRI Physics Course #7

Introduction and MRI Signal Generation

In this section, the speaker introduces the topic of MRI signal generation and explains how an MRI signal is generated by placing an element with a non-zero spin within an external magnetic field.

Generating an MRI Signal

• An element with a non-zero spin placed in an external magnetic field processes at a frequency known as the Llama frequency. This processional frequency depends on the gyromagnetic ratio of the element and the strength of the external magnetic field.

• Applying a radio frequency pulse perpendicular to the main magnetic field causes nuclear magnetic resonance, where spins resonate in phase with each other and fan out into the transverse plane. This results in transverse magnetization and loss of longitudinal magnetization.

• Transverse magnetization can be measured as a signal within an MRI machine. After gaining transverse magnetization, two independent processes occur - T2 relaxation (loss of transverse magnetization due to dephasing of spins) and T1 relaxation (gain of longitudinal magnetization). Different tissues have different rates of T2 and T1 relaxation, which provide contrast in images.

Image Contrast and Manipulating Image Acquisition

In this section, the speaker discusses how image contrast is achieved in MRI by manipulating time to Echo (TE) and time to Repetition (TR).

Image Contrast

• Differences in T2 and T1 relaxation rates among tissues create contrast in images. By manipulating TE or TR times during image acquisition, we can emphasize either T1 or T2 contrast differences in the resulting image.

• The choice of TE and TR times allows us to control the contrast in the image, but we cannot determine the exact location of the signal within the image. Unlike other imaging modalities, such as x-ray or ultrasound, where signals are generated externally and provide depth information, MRI signals come from within the patient.

Spatial Localization in MRI

In this section, the speaker explains how spatial localization is achieved in MRI by separating different signals on a Cartesian plane.

Spatial Localization

• To determine the exact location of an MRI signal within space, we need to separate different signals on a Cartesian plane using three coordinate values (x-axis, y-axis, and z-axis). This process is known as spatial localization.

• By knowing the three coordinates on a Cartesian plane as a frame of reference, we can pinpoint where a signal is coming from within the patient's body.

Slice Selection

In this section, the speaker introduces slice selection and explains how it determines where the signal is coming from along the z-axis.

Slice Selection

• When scrolling through an MRI image, we are looking at different slices stacked upon one another along the z-axis. Each slice represents some width in 3D space.

• Slice selection involves choosing a specific slice along the z-axis to examine. A gradient magnetic field called slice selection gradient is applied across the z-axis to cause differential magnetic field strength along that axis. This results in different processional frequencies along the z-axis and allows for selecting a specific slice within the patient.

• The selected slice represents a z-axis value, and further separation into x-axis and y-axis values is needed to determine the exact location of the signal within that slice.

Frequency Encoding Gradient

In this section, the speaker discusses frequency encoding gradient, which helps differentiate the x-axis components of the signal.

Frequency Encoding Gradient

• To separate the signal coming from a specific x-coordinate within a selected slice, we use the frequency encoding gradient. This gradient allows for differentiating frequencies along the x-axis.

Phase Encoding Gradient

In this section, the speaker introduces phase encoding gradients, which help differentiate the y-axis components of the signal.

Phase Encoding Gradient

• After using frequency encoding gradient to differentiate x-axis components, we need to differentiate y-axis components as well. This is achieved through phase encoding gradients.

Slice Selection and Processional Frequencies

In this section, the speaker explains how processional frequencies are changed along the z-axis during slice selection.

Changing Processional Frequencies

• To select a specific slice within a patient's body, processional frequencies of spins need to be different along the z-axis. This is accomplished by applying a gradient magnetic field along that axis using gradient coils. The differential magnetic field strength causes different processional frequencies based on location along the z-axis.

Conclusion

The transcript provides an overview of MRI signal generation and explains how spatial localization, slice selection, frequency encoding gradient, and phase encoding gradient are used to determine the location of signals within an MRI image. Understanding these concepts is crucial for interpreting MRI images accurately.

Nuclear Magnetic Resonance and Slice Selection

This section explains the concept of nuclear magnetic resonance (NMR) and how it is used in slice selection for imaging.

Nuclear Magnetic Resonance (NMR)

• When a radio frequency pulse matching the precessional frequency is applied, nuclear magnetic resonance occurs, causing spins to flip into the transverse plane.

• Only spins with matching frequencies exhibit NMR, while others at different frequencies do not flip.

• The radio frequency pulse has a slight range known as the bandwidth, within which it is applied to the patient.

• The gradient represents different precessional frequencies proportional to the magnetic field strength along the z-axis.

• By matching the radio frequency pulse with specific precessional frequencies, NMR can be induced in a particular slice.

Slice Selection

Moving the Slice Along Z-Axis

• Changing the radio frequency pulse frequency shifts the slices along the z-axis by selecting higher precessional frequencies.

• Changing the gradient field strength alters proton precessional frequencies, allowing selection of different slices at constant radio frequency pulse frequency.

• Moving the patient along the z-axis can also change which part of them is imaged while keeping the selected slice region constant.

Changing Slice Thickness

• Increasing bandwidth of radio frequency pulses increases slice thickness by covering a wider range of precessional frequencies.

• Increasing gradient field steepness narrows slice thickness by reducing coverage of precessional frequencies within a smaller range.

Challenges with Slice Thickness

• Slices have some width, resulting in protons not perfectly aligned in the z-axis.

The transcript is already in English, so no language conversion is required.

New Section

This section explains the concept of spins resonating in phase with each other and how the frequency is dependent on the radio frequency pulse and gradient field.

Spins Resonating in Phase

• Spins resonate in phase with each other but at a frequency dependent on the radio frequency pulse and gradient field.

• The spins are out of phase with each other due to this differential.

New Section

This section discusses the application of a re-phasing gradient after slice selection to allow spins to resonate in phase with each other.

Re-phasing Gradient

• After applying the slice selection gradient, a re-phasing gradient is applied along the z-axis to make spins resonate in phase with each other.

• The re-phasing gradient creates an equal and opposite magnetic field that allows all spins within the selected slice to experience the same external magnetic field.

New Section

This section recaps the process of selecting a specific slice along the z-axis using MRI techniques.

Recap of Slice Selection Process

• The MRI machine constantly has a main magnetic field (B naught) along the z-axis. A radio frequency pulse is applied to generate signal within the MRI image.

• By applying a 90-degree radio frequency pulse and a slice selection gradient, only certain spins with matching precessional frequencies flip into the transverse plane, allowing for selection of a specific slice.

• A re-phasing gradient is then applied to account for differences in spin frequencies within that thickness of the selected slice, ensuring all spins resonate in phase with each other.

New Section

This section explains how the signal from the selected slice is measured and how the MRI machine distinguishes it from other spins.

Measuring Signal from Selected Slice

• The signal measured in the MRI machine comes only from the specific slice that has been tipped into the transverse plane.

• The slice selection gradient and radio frequency pulse are switched off before measuring the signal, but only spins selected during that period exhibit transverse magnetization.

• The differences in T2 relaxation at the time of echo are sampled to measure the signal.

New Section

This section discusses the challenge of determining where along the selected slice a signal is coming from.

Determining Signal Location within Slice

• Currently, all spins within the selected slice contribute to a single long trace of signal without specifying their location along the x-axis or y-axis.

• Further techniques are needed to tease out where exactly within the slice a particular signal is coming from along both axes.

New Section

In this section, the speaker discusses adding another line to the pulse sequence in order to differentiate where the signals are coming from along the x-axis of a slice. This process is separate from differentiating signals along the y-axis.

Adding Line for X-Axis Differentiation

• The speaker explains that by adding another line to the pulse sequence, they can determine where the signal is coming from along the x-axis of a slice.

• This step is crucial in understanding where specific signals are originating within a patient.

New Section

In this section, the speaker emphasizes that differentiating signals along the y-axis of a slice is a separate process from differentiating signals along the x-axis. They mention that these concepts may be confusing and require revisiting multiple times.

Separate Processes for Y-Axis Differentiation

• The speaker highlights that differentiating signals along the y-axis of a slice requires a separate line added to the pulse sequence.

• Understanding how an MRI machine precisely identifies where specific signals are coming from within a patient involves comprehending both x-axis and y-axis differentiation processes.

• It is important to spend ample time grasping these concepts before moving on to subsequent talks or sections.

New Section

The speaker acknowledges that these three sections may be challenging and advises viewers to revisit them multiple times if needed. They also provide additional resources for studying physics exams.

Revisiting Confusing Sections

• The speaker encourages viewers to dedicate sufficient time to understand the concepts presented in these three sections.

• It is normal to find these topics confusing and may require revisiting previous talks or sections.

• Additional resources, such as question banks curated from past papers, are provided in the video description for self-testing and identifying knowledge gaps.

The transcript is already in English, so there is no need to respond in a different language.