MRI Physics and T2-weighted Imaging
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Questions and Answers

What primarily determines the contrast in a T2-weighted image?

  • Signal loss due to T1 effects
  • Density of protons in the tissue
  • Radiofrequency pulse duration
  • Signal differences among tissues at long TE (correct)

How does keeping a short TE affect the contrast in a Proton Density (PD) image?

  • Eliminates T1 effects (correct)
  • Reduces proton density signal
  • Enhances T1 effects
  • Enhances T2 effects

What is the effect of using a long TR in a Proton Density image?

  • Minimizes T1 effects (correct)
  • Increases T2 signal differences
  • Enhances T1 effects
  • Reduces T2 signal differences

In a T2-weighted image, why does tissue A maintain a good signal while tissue B loses most of its signal?

<p>Tissue A has longer T2 than B (C)</p> Signup and view all the answers

What happens to the signal intensity difference among tissues in a T2-weighted image at short TE?

<p>It becomes less significant (A)</p> Signup and view all the answers

What is the primary type of atom found in the human body that is crucial for MR imaging?

<p>Hydrogen (D)</p> Signup and view all the answers

What phenomenon occurs when protons align with an external magnetic field in MR imaging?

<p>Precession (C)</p> Signup and view all the answers

Which subatomic particle has a positive charge and participates in creating a magnetic field in MR imaging?

<p>Proton (D)</p> Signup and view all the answers

What occurs to protons when they are placed in the presence of an external magnetic field?

<p>They align parallel or anti-parallel to the magnetic field. (D)</p> Signup and view all the answers

What part of the atom contains most of its mass?

<p>The nucleus (B)</p> Signup and view all the answers

What is the term for the amount of times a proton rotates around its axis in MR imaging?

<p>Precession Frequency (B)</p> Signup and view all the answers

Which of these elements is NOT among the most abundant in the human body that contributes to MR imaging?

<p>Aluminum (A)</p> Signup and view all the answers

What describes the movement of the axis of rotation of a proton when it aligns with the magnetic field?

<p>Precession (D)</p> Signup and view all the answers

What phenomenon allows protons to exchange energy with the RF pulse?

<p>Resonance (C)</p> Signup and view all the answers

What is the primary effect of the RF pulse on protons?

<p>Cause precession and elevate energy level (D)</p> Signup and view all the answers

What kind of transformation is used to convert MR signals into MR images?

<p>Fourier Transformation (A)</p> Signup and view all the answers

Which gradient is responsible for determining the slice position in MRI?

<p>Slice selection gradient (C)</p> Signup and view all the answers

What does the strength of the MR signal depend on?

<p>The magnitude of the transverse magnetization (D)</p> Signup and view all the answers

How does slice thickness relate to the bandwidth of the RF pulse?

<p>Thicker slices correspond to wider bandwidth (D)</p> Signup and view all the answers

What term describes the combination of three magnetic fields aligned along the X, Y, and Z axes in MRI?

<p>Gradient fields (B)</p> Signup and view all the answers

Which gradient is applied to localize the point in a slice from where the signal originates?

<p>Phase encoding gradient (A)</p> Signup and view all the answers

What does TI in an Inversion Recovery (IR) sequence primarily influence?

<p>Image contrast (A)</p> Signup and view all the answers

How does T1 affect the brightness of tissues in T1 weighted images?

<p>Short T1 tissues regain maximum signal faster (A)</p> Signup and view all the answers

What is the significance of using a short TR in T1-weighted imaging?

<p>It maximizes the difference in signal intensity due to T1 differences (D)</p> Signup and view all the answers

What happens to TM immediately after the RF pulse is turned off in a T2 weighted image?

<p>It begins to decrease in magnitude (A)</p> Signup and view all the answers

Which tissue type will retain its signal for a longer time in T2 weighted images?

<p>Tissue with long T2 (C)</p> Signup and view all the answers

To make an image T2 weighted, what adjustment must be made?

<p>Increasing the TE duration (D)</p> Signup and view all the answers

What is observed in T1 weighted images when using a long TR?

<p>Lower overall image brightness (C)</p> Signup and view all the answers

What effect does dephasing have on the signal received in T2 weighted sequences?

<p>Gradually reduces signal intensity (C)</p> Signup and view all the answers

What is the primary purpose of longitudinal relaxation time (T1)?

<p>To indicate the time taken for magnetization to recover to 63% of its original value (D)</p> Signup and view all the answers

Which term describes the process by which transverse magnetization decreases in magnitude?

<p>Transverse relaxation (C)</p> Signup and view all the answers

What distinguishes T2* relaxation from T2 relaxation?

<p>T2* is influenced by external magnetic field inhomogeneity. (A)</p> Signup and view all the answers

What does the Larmor’s equation express?

<p>The relationship between precession frequency and strength of external magnetic field. (C)</p> Signup and view all the answers

Which axis is the external magnetic field conventionally directed along?

<p>Z-axis (C)</p> Signup and view all the answers

What occurs to transverse magnetization immediately after a 90-degree RF pulse?

<p>It rotates out of phase at Larmor frequency. (A)</p> Signup and view all the answers

Why is longitudinal magnetization not directly measurable?

<p>It must be converted to transverse magnetization for measurement. (A)</p> Signup and view all the answers

What does the term Time to Repeat (TR) refer to in the context of RF pulses?

<p>The interval between the start of one RF pulse and the start of the next RF pulse (B)</p> Signup and view all the answers

In the absence of external magnetic field inhomogeneity, what happens to the transverse magnetization in a spin-echo sequence?

<p>It is preserved due to the application of 180-degree RF pulses. (B)</p> Signup and view all the answers

What happens to protons when they align under the influence of an external magnetic field?

<p>They align parallel and antiparallel to the magnetic field. (A)</p> Signup and view all the answers

What effect does the application of a radiofrequency (RF) pulse have on protons?

<p>It causes some protons to precess antiparallel to the magnetic field. (A)</p> Signup and view all the answers

What does the transverse relaxation time (T2) indicate?

<p>The time duration required for transverse magnetization to reduce to its original value (C)</p> Signup and view all the answers

What typically causes the decay of transverse magnetization (TM) in T2* relaxation?

<p>The combination of spin-spin relaxation and magnetic field inhomogeneity (B)</p> Signup and view all the answers

What is created when the forces of protons on positive and negative sides cancel each other?

<p>Longitudinal magnetization. (A)</p> Signup and view all the answers

How does stronger external magnetic field affect precession frequency?

<p>It increases the precession frequency. (C)</p> Signup and view all the answers

What is the result of tilting the magnetization into the transverse plane?

<p>It allows for the measurement of magnetic resonance signals. (D)</p> Signup and view all the answers

Flashcards

Atom

The smallest unit of matter, consisting of a central nucleus surrounded by orbiting electrons.

Molecule

Two or more atoms bonded together.

Proton

A positively charged particle found within the nucleus of an atom.

Proton Spin

The rotation of a proton around its own axis, creating a tiny magnetic field.

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Longitudinal Magnetization

The alignment of protons in a magnetic field, resulting in a net magnetic field that can be detected by MRI.

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Transverse Magnetization

The movement of protons perpendicular to the main magnetic field, causing a signal that can be measured by MRI.

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MR Signal

The signal that is detected by MRI and used to create images.

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Localization of MR Signal

The process of identifying the location of the MR signal within the body.

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Precession Frequency

The rate at which a nucleus spins around the external magnetic field, measured in Hertz (Hz).

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Precession Frequency and Magnetic Field Strength

The strength of the external magnetic field directly affects the precession frequency. A stronger magnetic field results in a higher precession frequency.

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Larmor's Equation

A mathematical equation that describes the relationship between precession frequency (w0), gyromagnetic ratio (γ), and external magnetic field strength (Bo): w0 = γ Bo

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Gyromagnetic Ratio

The ratio between a nucleus' magnetic moment and its angular momentum, a unique value for each nucleus.

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Radiofrequency (RF) Pulse

A pulse of energy focused on the transverse plane delivered by the MRI machine. This pulse causes some protons to flip their spin and creates transverse magnetization.

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Excitation

The process of using a radiofrequency pulse to flip the protons from longitudinal magnetization to transverse magnetization.

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Larmor Frequency

The frequency at which protons precess in a magnetic field. It's determined by the strength of the magnetic field.

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Inversion Time (TI)

The time interval between the 180-degree pulse and the 90-degree pulse in an Inversion Recovery (IR) sequence.

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T1 Relaxation Time

The time it takes for the longitudinal magnetization (LM) to return to 63% of its original value after a 90-degree pulse.

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Resonance

When a proton absorbs energy from an RF pulse and transitions to a higher energy level.

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Inversion Recovery (IR) Sequence

A type of MRI sequence that uses a 180-degree pulse to invert the longitudinal magnetization and then a 90-degree to excite the tissue.

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T1-weighted Image

An image created by a sequence that is sensitive to the T1 relaxation time of tissues.

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Gradient Magnetic Field

A magnetic field that varies in strength across a specific region, used to locate the source of a signal in MRI.

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T2 Relaxation Time

The time it takes for the transverse magnetization to decay to 37% of its original value after a 90-degree pulse.

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Slice Selection Gradient

A gradient field that determines the slice thickness in MRI.

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T2-weighted Image

An image created by a sequence that is sensitive to the T2 relaxation time of tissues.

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Echo Time (TE)

The time elapsed between the 90-degree pulse and the acquisition of the MR signal.

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Phase Encoding and Frequency Encoding Gradients

A gradient field used to pinpoint the exact location of the MR signal within a selected slice in MRI.

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Longitudinal relaxation (T1)

The process by which protons in a magnetic field lose energy and return to their original alignment along the magnetic field.

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Longitudinal relaxation time (T1)

The time it takes for the longitudinal magnetization (LM) to recover 63% of its original value after an RF pulse is applied.

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Transverse relaxation (T2)

The process by which protons lose their synchronized precession and the transverse magnetization (TM) decreases in magnitude.

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Transverse relaxation time (T2)

The time it takes for the transverse magnetization (TM) to decrease to 37% of its original value after an RF pulse has been applied.

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T2* relaxation

The decay of the transverse magnetization (TM) caused by both spin-spin relaxation and inhomogeneity of the external magnetic field.

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Repetition Time (TR)

Time interval between the start of one RF pulse and the start of the next RF pulse.

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What is a T2-weighted image?

T2-weighted images show contrast based on the time it takes for protons to return to their original alignment after being disturbed by radio waves. Tissues with a short T2 value lose signal quickly and appear darker, while tissues with a long T2 value retain signal longer and appear brighter.

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What is a Proton Density (PD) image?

Proton density (PD) images show contrast primarily based on the number of protons present in a tissue. This means tissues with a higher concentration of protons will appear brighter.

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How does echo time (TE) affect a T2-weighted image?

In a T2-weighted image, the signal difference between tissues is more pronounced at longer echo times (TE). This happens because tissues with a short T2 value lose their signal more rapidly with increasing TE.

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How is a T2-weighted image created?

A T2-weighted image is created by using a long echo time (TE) to allow for the signal differences between tissues to be magnified.

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How is a proton density (PD) image created?

A proton density (PD) image is created by using a long repetition time (TR) to minimize the effect of T1 relaxation and a short echo time (TE) to minimize the effect of T2 relaxation. This way, the signal primarily reflects the number of protons within the tissue.

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Study Notes

Principles of MRI

  • MRI uses a strong magnetic field and radio waves to create detailed images of the body's internal structures.
  • Atoms in the body, particularly hydrogen atoms, have tiny magnets (protons).
  • These protons can be aligned by a strong magnetic field.
  • A radio frequency (RF) pulse is used to tilt the protons from their aligned state.

Atomic Structure

  • All matter is composed of atoms.
  • Atoms are made up of a nucleus (containing protons and neutrons) and orbiting electrons.
  • Hydrogen is the most abundant atom in the human body, found in water and fat.
  • Protons have positive electrical charges.
  • Neutrons have no electrical charge.

Motion of Atoms

  • Protons, when spinning, also have a magnetic dipole moment. This means they behave like tiny magnets.
  • The spinning axis, or the spin, can be aligned "up" or "down". It is important to note here that a spin is a quantized quantity
  • The axis of rotation (spin) precesses (moves in a cone-like fashion) around the axis of the external magnetic field.

Protons in MRI Imaging

  • Protons have a positive electrical charge.
  • Protons spin and have a tiny magnetic field (magnetic dipole moment).
  • When an external magnetic field is applied, protons align either parallel or anti-parallel to the field.
  • The spinning axis of a proton precesses around the external magnetic field's direction.

Longitudinal Magnetization

  • When placed in an external magnetic field, protons tend to align parallel to the field.
  • More protons align with the magnetic field than against it resulting in a net magnetization vector known as longitudinal magnetization
  • The net magnetization vector forms along the direction of the main magnetic field.

Transverse Magnetization

  • A radiofrequency (RF) pulse is used to tip the aligned protons to a transverse plane (perpendicular to the main magnetic field). These are not technically transverse magnetization; the result is rather a tipping vector of longitudinal magnetization components
  • This tipping creates transverse magnetization.
  • This transverse magnetization rotates, constantly creating an electromagnetic current that the MRI machine detects.

MR Signal

  • Transverse magnetization produces an electric current detectable by the MRI machine.
  • The strength of the electric current is related to the magnitude of the transverse magnetization.
  • The rotating precessing protons produce an electromagnetic signal for the MRI device to measure.

Localization of the Signal

  • Three extra magnetic fields (gradient fields) are used to pinpoint the source within the body.
  • These additional fields are carefully shaped and alter the magnetic field strength in specific locations.
  • Each area of body has its own magnetic field
  • Gradient fields help to localize signals from specific locations.

Slice Selection Gradient

  • Determines which region of the body to image
  • Varying magnetic field strength creates distinct layers (i.e slices) along the body.
  • This is a crucial component for making cross-sectional or slice images as required for MRI procedures.

Phase Encoding & Frequency Encoding Gradients

  • Fine-tuning the process, the location of the part of the image/slice to be obtained
  • Accurately pinpoint a spot in the given slice/layer.

Basic Four Steps of MRI Imaging

  1. Patient placement in the magnet, aligning protons in body
  2. Application of an RF pulse to tip the protons, creating transverse magnetization.
  3. Detection of the electric current produced by the tipped protons.
  4. Computer processing of the signals to create precise images.

T1, T2 Relaxations and Image Weighting

  • Relaxation times (T1 and T2) indicate how quickly protons return to their initial equilibrium state after an RF pulse.
  • T1 relaxation relates to how quickly the longitudinal magnetization recovers.
  • T2 relaxation describes how quickly the transverse magnetization decays (decreases).
  • These time values impact image generation and help distinguish between different tissues.

T1 Weighted Image

  • Images that emphasize differences in T1 relaxation times.
  • T1 values vary between tissue types (fat, water, cartilage, etc.)
  • Short T1 values translate into brighter signals.

T2 Weighted Image

  • Images showing differences in T2 relaxation time.
  • T2 values change among tissue types (fat, water, cartilage, etc).
  • Long T2 times usually appear brighter on the image.

Proton Density (PD) Image

  • Images focus on differences in the concentration of hydrogen protons in different tissues.
  • Used for differentiating tissues without relying too much on T1 or T2 differences.

TR and TE Values

  • TR: Time to repeat (time between RF pulse sequences).
  • TE: Time to echo (time it takes for the signal to return to the receiver after an RF pulse).
  • These values are adjusted to obtain optimized images.

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Principles of MRI PDF

Description

Test your understanding of MRI physics, focusing on T2-weighted imaging and proton density concepts. This quiz covers topics such as signal differences, TE, TR effects, and the role of protons in MR imaging. Ideal for students learning about medical imaging techniques.

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