MRI Field of View (FOV), Matrix Size, Receiver Bandwidth, Dwell Time | MRI Physics Course #11

Introduction and Overview

In this section, the speaker introduces the topic of signal generation, localization, and data acquisition in MRI imaging. The focus is on understanding the pulse sequence and its impact on image resolution.

Signal Generation and Localization

• MRI signal generation, localization within a slice, and data storage for image generation have been discussed in previous talks.

• Specific slices are selected using frequency encoding gradients along the x-axis.

• Multiple phase encoding gradients are used to determine signal location along the y-axis.

Data Acquisition Period

• This section focuses on the time when analog signals are sampled and converted into digital signals.

• The strength of the frequency encoding gradient affects how many times the signal needs to be sampled.

• The number of samples determines the resolution along the x-axis.

Field of View and Resolution

This section explores how field of view (FOV) selection impacts image resolution. It discusses selecting a specific region of interest within a slice to improve resolution.

Defining Field of View

• The boundaries of a slice can be defined using distance values known as field of view (FOV).

• FOV dimensions can be adjusted during MRI machine operation.

• Unused regions within FOV do not provide useful information.

Impact on Resolution

• Along the y-axis, multiple phase encoding steps are needed to delineate signal.

• Along the x-axis, sampling analog signals determines pixel count and resolution.

• Reducing FOV while maintaining phase encoding steps improves resolution within a smaller region.

Matrix Size and Resolution

This section explains how matrix size affects image resolution. It demonstrates that even with similar matrix sizes, different FOVs can result in varying resolutions.

Matrix Size and Resolution

• Matrix size refers to the number of pixels within an image.

• A smaller FOV with the same matrix size can provide better resolution.

• Changing the matrix size can maintain the same resolution while reducing acquisition time.

Bandwidth and Frequency Encoding

This section discusses bandwidth and its relationship to frequency encoding. It explains how a range of frequencies is created along the x-axis during signal acquisition.

Bandwidth and Frequency Encoding

• Applying a frequency encoding gradient along the x-axis creates a range of frequencies based on tissue location.

• The bandwidth represents the range of frequencies across the selected FOV.

• Spins at the center resonate at the Larmor frequency, determined by magnetic field strength.

Conclusion

The speaker concludes by summarizing key points discussed in this talk regarding signal generation, localization, data acquisition, field of view, resolution, matrix size, bandwidth, and frequency encoding.

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Understanding Bandwidth and Field of View in MRI

In this section, the speaker explains the relationship between bandwidth and field of view in MRI imaging.

Bandwidth and Field of View

• The total bandwidth across an MRI slice is determined by adding or subtracting frequencies along the x-axis.

• The bandwidth is proportional to the field of view size. Increasing the field of view increases the bandwidth, while reducing it decreases the bandwidth.

• Decreasing the severity of the frequency encoding gradient also decreases the bandwidth.

• Bandwidth is a function of both gradient strength and field of view size. Increasing gradients and field of view will increase the bandwidth.

Measurement of Frequencies

• When measuring frequencies in MRI, relative changes in frequency are measured rather than absolute values.

• The analog signal from net magnetization vectors is converted into a digital signal, measuring changes in frequency along the x-axis slice.

• The measurement is done in a rotational frame of reference, where differences in processional frequency are observed. This allows for slower sampling rates compared to measuring absolute values.

Summary

Bandwidth in MRI imaging is influenced by both field of view size and gradient strength. Increasing either parameter will result in a larger bandwidth along the x-axis slice. Measurements are done based on relative changes in frequency rather than absolute values, allowing for more efficient sampling rates.

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Understanding Complex Signals in MRI

This section explains the concept of complex signals in MRI and how they allow for the measurement of both real and imaginary vectors.

Complex Signals in MRI

• A complex signal represents a particular period in time while measuring the net magnetization vector of the entire slice.

• The complex signal corresponds to the data point at that analog signal, which is a numerical value representing the net magnetization vector of the whole slice.

• Complex signals allow for the measurement of both real and imaginary vectors coming from the slice.

• Real and imaginary vectors represent positive and negative frequency changes along the x-axis of the slice.

• By measuring both components, it is possible to mathematically calculate whether a frequency comes from the left or right side of the slice.

Overlaying Frequencies onto MRI Image

This section discusses how frequencies obtained from an analog signal are overlaid onto an MRI image based on their x-axis location.

Overlaying Frequencies onto MRI Image

• The data acquisition time is used to determine each point where the analog signal is sampled.

• The frequency of the analog signal depends on its location along the x-axis of the slice.

• The MRI machine takes discrete points in time and calculates what it thinks that frequency is.

• Frequencies are plotted away from the midline in both positive and negative values, allowing for differentiation between left and right sides of the slice.

Aliasing Artifact

This section explains aliasing artifact, which occurs when frequencies outside of the field of view are falsely represented at lower frequencies within an image.

Aliasing Artifact

• Aliasing occurs when trying to measure a frequency that comes from outside of the field of view.

• Frequencies that exceed the bandwidth of the MRI machine result in aliasing, where the frequency is falsely represented at a lower value within the image.

• As frequencies increase beyond the machine's capability to sample accurately, the representation becomes less accurate.

• The bandwidth determines how quickly the analog signal needs to be sampled to accurately represent it.

Sampling Frequency Calculation

This section discusses how to calculate the sampling frequency required to accurately represent a specific frequency.

Sampling Frequency Calculation

• To accurately sample a specific frequency, it is necessary to consider its location on the edge of the slice.

• The accuracy of sampling depends on matching the sampling rate with the desired frequency.

• As frequencies increase beyond what can be accurately sampled, lower frequencies are calculated instead due to limitations in bandwidth and sampling speed.

The transcript does not provide further information on calculating sampling frequency.

New Section

This section discusses the calculation of the sampling rate needed to accurately represent a given frequency.

Calculation of Sampling Rate

• The maximum change in frequency from the center to the edge of the frequency encoding gradient is 25,000 Hertz.

• The Nyquist limit is equal to half of the sampling rate.

• In this case, the Nyquist limit is 25,000 Hertz, so the sampling rate needs to be 50,000 Hertz.

• The sampling rate equals the selected bandwidth. In this example, both are 50,000 Hertz.

• The sampling interval or dwell time is calculated by dividing one second by the number of samples taken per second.

• For a sampling rate of 50,000 Hertz, each sample has a dwell time of 20 microseconds.

New Section

This section explains how different parameters relate to each other in MRI imaging.

Relationship between Parameters

• The field of view determines the area of the slice that will be imaged accurately.

• The matrix size represents the resolution and determines how many phase encoding steps and samples are needed during the frequency encoding gradient.

• Bandwidth is set based on imaging requirements and affects image properties.

• The specific gradient needed for a given bandwidth across a certain distance can be calculated by an MRI machine.

• The Nyquist limit corresponds to half of the bandwidth and determines the maximum frequency that can be accurately sampled.

New Section

This section ties together all parameters discussed earlier and their relationship with each other.

Bringing it All Together

• Initially, define the field of view and determine desired resolution (matrix size) for accurate imaging.

• Set specific bandwidth based on imaging requirements.

• Calculate necessary gradient strength for achieving the desired bandwidth.

• Determine the sampling rate based on the Nyquist limit, which is half of the bandwidth.

• Calculate the dwell time or sampling interval, which is the time available for each sample during the frequency encoding gradient.

• With knowledge of dwell time and desired resolution, determine how many samples are needed to achieve the desired image quality.

New Section

This section clarifies the difference between bandwidth and sampling rate.

Bandwidth vs Sampling Rate

• Bandwidth represents the range of frequencies along the x-axis of a slice.

• Sampling rate refers to how frequently an analog signal needs to be sampled per second for accurate representation.

• Although they may have the same numerical value, bandwidth and sampling rate do not represent the same thing.

• Bandwidth determines frequency range, while sampling rate determines how often samples are taken.

New Section

This section explains how to calculate necessary samples based on desired resolution.

Calculating Samples

• The number of samples needed is determined by the desired resolution (e.g., 256 pixels along x-axis).

• The number of samples required corresponds to the number of pixels in an image dimension.

New Section

In this section, the speaker introduces the importance of understanding certain concepts in MRI. These concepts will help in understanding different MRI pulse sequences and artifacts that occur in MRI imaging.

Importance of Understanding Concepts

• Understanding these concepts is crucial for comprehending most of what's going on in MRI.

• The choice of bandwidth influences the gradient along the x-axis of the slice, which affects the type of image and signal-to-noise ratio.

• Two common artifacts in MRI imaging are aliasing and chemical shift artifact, and understanding these concepts is essential for explaining how they occur.

Exam Preparation

• Questions related to these concepts frequently appear in exams, especially physics exams or specifically an MRI Physics Exam.

• A question bank with curated questions from multiple past papers is provided in video form, explaining why certain answers are correct and others are incorrect.

• The question bank also demonstrates how examiners can ask questions in various ways.

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