MRI Registry Review WEEK 9 Special MRI Procedures

Objectives

  • Overview of MRI Angiography

  • Flow Characteristics

  • Time-of-Flight MR Angiography

  • Phase Contrast Angiography

  • Contrast Enhanced Angiography

  • Introduction to Functional MRI

  • Diffusion Weighted Imaging

  • Perfusion Imaging

  • fMRI

  • Spectroscopy

  • Conclusion

Overview of MRI Angiography

  • MRI Angiography (MRA) is categorized as a special procedure that facilitates the study of moving blood, as well as the function and composition of tissues.

  • Advantages of MRA include:

    • Non-invasive.

    • Does not require the use of a contrast agent.

    • Typically faster and less expensive than other angiographic techniques.

  • There are three primary types of MRA:

    1. Time-of-Flight (TOF)

    2. Phase Contrast (PCA)

    3. Contrast Enhanced Angiography (CEMRA)

Basic Principles

  • A flow-sensitive set of images must be acquired during the image acquisition process.

  • Signal suppression from stationary background tissues is essential for maximizing contrast.

  • The goal is to achieve the maximum contrast between stationary protons and flowing protons, allowing clear differentiation in the images.

  • The process involves acquiring base images versus projected images for enhanced visualization of blood vessels.

Flow Characteristics

Laminar Flow

  • Defined as blood flow exhibiting different but consistent velocities across a vessel.

  • Characteristics include:

    • The flow at the center of the lumen is faster compared to the flow near the walls, where resistance slows it down.

    • The velocity difference across the vessel remains constant.

    • Commonly found in veins and small arteries.

    • Exhibits a parabolic profile in terms of velocity distribution.

    • Varieties of MRA effectively represent laminar flow.

Turbulent Flow

  • Described as flow with varying velocities that exhibit random fluctuations.

  • Characteristics include:

    • Change in velocity across the vessel.

    • Blood can move in multiple directions.

    • Present in tortuous sections of vessels and distal to bifurcations or stenoses.

    • MRA may not accurately represent turbulent flow due to signal loss caused by significant dephasing of protons.

    • Hemodynamics will vary based on the location in the circulatory system, influencing the choice of scan technique.

Phase Effects of Flow

Stationary Protons

  • When exposed to balanced gradients (dephasing and rephasing), stationary protons rephase during MR echo collection.

  • Result: Signals from stationary tissues yield no net phase.

Flowing Protons

  • Flowing protons also encounter balanced gradients; however, they end up with a net phase after their exposure.

  • During echo collection, flowing protons exhibit a phase shift.

  • Result: Signals from flowing nuclei will have an accumulated net phase, aiding differentiation from stationary tissues.

Flow Void

  • Flow Void: Occurs when there's a lack of signal from flowing nuclei due to phase shifts, leading to mismatch and misplacement along the phase-encoded direction of the image.

Inflow Enhancement

  1. Gradient Moment Nulling (GMN): A technique utilized to eliminate flow voids.

    • GMN aligns the shifted phase correctly for accurate detection.

    • Creates an appearance of increased brightness of blood within vessels compared to the surrounding tissue.

  2. When a pulse sequence is executed, repetitive RF is applied which excites stationary nuclei. If the repetition time (TR) is short, longitudinal magnetization may not significantly recover.

    • Inflowing nuclei entering the scan slice present with full longitudinal magnetization as they haven't been exposed to previous RF pulses, influencing signal intensity between flowing and stationary nuclei.

Time-of-Flight Angiography (TOF)

  • Performed as either 2D or 3D, utilizing GRE (Gradient Echo) sequences:

    • A set of base images needs acquisition, and GMN is employed to eliminate phase shifts.

    • Appropriate MRI parameters must be chosen to achieve optimal contrast between flowing blood and non-moving tissues.

Processing Images

  • Stationary nuclei yield no signal, while inflowing blood produces a bright signal.

  • Base images are processed to generate projection images for full vascular visualization.

  • Maximum Intensity Projection (MIP): Utilizes the brightest pixels from 2D or 3D anatomical images to project onto a plane, creating a visual representation of the vessels of interest.

Image Manipulation

  • Technologists can rotate base images around a defined axis, repeating the MIP process to produce additional final images from various angles.

  • This rotation can be repeated multiple times to reduce the volume of anatomical data impacting projections, thus preventing vessel overlap and improving contrast while simultaneously reducing scan time.

2D Time-of-Flight MRA

  • Achieved with GRE pulse sequences:

    • It effectively suppresses stationary tissue signals thereby maximizing blood inflow impact.

    • Any flow within the slice plane is also saturated and remains unseen in the resulting projection images.

    • Collected 2D images are stacked together, with MIP applied for the final projection image.

Applications of 2D TOF Angiography

  • 2D TOF angiography is effective in situations where blood flow is slower or a greater distance of coverage is needed, such as for cardiac triggering in pulsatile blood flow or imaging great and peripheral blood vessels.

3D Time-of-Flight MRA

  • Conducted with 3D GRE sequences:

    • Signal from stationary nuclei is suppressed, and GMN is applied to optimize inflow while minimizing stationary signal.

    • Initial RF pulses (90° or 180°) must be timely to achieve brightness.

    • Maximum intensity algorithm applied after collection and stacking of 3D images.

Applications of 3D TOF Angiography

  • Best suited for vessels exhibiting faster flow, limited coverage scenarios, tortuous vessels, and intracranial vessels.

3D Multi-volume TOF MRA

  • Addressed issues of slow flow by using smaller multiple volumes to capture adequate signal from each.

  • All images from the acquired volumes are combined, with MIP applied to the set of base images.

Saturation Pulses in Multi-volume MRA

  • To prevent over-clustered vessels in the Field of View (FOV), saturation pulses are applied to eliminate signals from unwanted vessels:

    • Inferiorly applied to target arterial signals.

    • Superiorly applied for venous signals.

  • Positioning of saturation bands depends on the anatomical features and regions.

Phase Contrast Angiography (PCA)

  • PCA relies on phase shifts induced by flowing blood through equal and opposite gradients.

  • There is a proportional relationship between flow velocity and signal intensity:

    • Increased flow velocity results in significant phase shifts, yielding brighter blood appearance.

  • The velocity encoding value corresponds to gradient amplitude, determining the flow velocity yielding maximum signal intensity. For instance, a high velocity-encoding value (e.g., 90 cm/sec) optimally visualizes vessels with similar flow rates.

Sensitivity of PCA

  • PCA can detect even very small vessels or slow flows effectively due to comprehensive background suppression.

  • It is more sensitive to motion compared to TOF angiography, as it utilizes phase accumulation from moving protons leading to net phases throughout the echo collection time.

Procedure for PCA

  1. Obtain 2D or 3D GRE velocity-sensitive images.

  2. Acquire multiple images per slice for accurate flow sensitivity.

  3. For unidirectional flow, one acquisition suffices. For three-direction flow, three acquisitions are necessary.

  4. An additional acquisition must create a flow-compensated image with GMN to remove patient-induced phase changes.

  5. The subtracted flow-compensated background image allows the visualization of flows within the images.

  6. Images are combined to create 3D composite images leading to the final PCA image.

Applications of PCA

  • Fast scout images for heightened 3D PCA studies.

  • 3D PCA facilitates inspection of vessels from any angle and captures small vessels, areas with slow or multi-directional flow with efficiency, often requiring fewer slices for ROI coverage.

Contrast Enhanced Angiography (CEMRA)

  • CEMRA stands for Contrast Enhanced MRI Angiography.

  • This approach is dependent on timing the image acquisition while paramagnetic contrast media flows through targeted vessels, resulting in brightly illuminated vessels on T1W MRA.

Procedure for CEMRA

  • Two sets of images are captured at identical slice positions, one before and the other after contrast injection:

    • The stationary tissues will appear unchanged between images, but vessels illuminated during contrast flow become visible on the second set.

  • Post-study, the non-contrast images are subtracted from those captured during contrast passage.

Efficiency of CEMRA

  • CEMRA can reduce scan times, as fewer slices cover the ROI, allowing slices to align with the plane of the vessels for improved accuracy regardless of flow type.

Applications of CEMRA

  • Useful in abdominal studies, single breath-hold imaging of carotid arteries and large vessels, as well as abdominal vasculature.

Challenges in CEMRA

  • Critical aspects include properly timing the imaging of gadolinium bolus through the vessels. Two notable methods for this are:

    1. Test Bolus Administration: A low-concentration test bolus determines the duration from injection site to the vessel.

    2. Automated Methods: Many manufacturers provide methods for timing optimization.

Introduction to Functional MRI

  • MRI enables morphological assessments of patients through three methods:

    1. Diffusion Weighted Imaging (DWI).

    2. Perfusion Weighted Imaging.

    3. Functional MRI (fMRI).

Overview of Functional MRI

  • Functional MRI evaluates brain activity based on blood flow, marking changes in oxygen concentration as tasks are performed.

  • Blood Oxygen Level Dependent (BOLD) MRI relates to local variations in blood oxygenation levels due to cerebral activation.

  • Notably:

    • Oxyhemoglobin is diamagnetic, whereas deoxyhemoglobin is paramagnetic, resulting in heightened susceptibility and significant signal loss on T2*W images.

Identifying Active Brain Regions in fMRI

  1. Collect data aligning points for accurate image localization.

  2. Generate cross-correlation maps.

  3. Register functional maps with anatomical images.

  4. Identify brain areas with increased oxygenation linked to task performance.

Applications of fMRI

  • Primarily involved in investigations related to the visual cortex, motor center, and speech centers.

  • Critical in pre-surgical evaluations for tumors, arteriovenous malformations (AVMs), and epilepsy research, as well as in scientific studies.

Spectroscopy

  • MR Spectroscopy serves as a non-invasive imaging technique to assess the chemical composition of tissue.

  • Conducted using B0 and RF energy fields.

Free Induction Decay (FID)

  • Notably focuses on the voxel’s composition, represented via a Fourier Transform that reveals single peaks correlating to pure sine wave frequencies.

  • The sine wave width reflects T1 and T2 characteristics;

    • NMR spectrum delineates signal intensity against frequency:

    • Measured in parts per million (ppm).

  • Data from various field strengths can be compared for analysis.

Techniques in MR Spectroscopy
Single Voxel Technique

  • Localized within the patient’s brain utilizing three orthogonal RF pulses and a voxel size no less than 15 mm³.

  • Yields NMR spectrum results.

Multi-Voxel Technique

  • Also termed as chemical shift imaging, it offers a metabolite map superimposed on anatomical images.

  • Presence or absence of metabolites indicates the relative health of the tissue sample.

  • Preparation steps include water suppression and shimming for effective results.

Conclusion

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