Comprehensive MRI Technology, Physics, Sequence, and Artifact Review

Instructor Background & Course Structure

• Instructor Credentials & Experience:

  • 40 years of total radiology experience, with 30 years specializing in Magnetic Resonance Imaging (MRI) and 10 years in Computed Tomography (CT) and special procedures.

  • 14 years of academic teaching experience at a local community college.

  • 5 years of experience as an Application Specialist with GE Healthcare.

  • Faculty instructor for Pulse Radiology.

  • Lecturer for Advanced Health, a private continuing education (CEU) provider based out of Houston, Texas.

  • Author of two published textbook resources available through Barnes & Noble, Amazon, and Wiley-Blackwell Publishing (London, England):

    • MRI Tech to Tech Explanations

    • Registry Review Tech to Tech Questions and Answers

• Course Agenda & Topic Breakdown:

  • Module 1: Intrinsic Tissue Properties and Intrinsic Scan Parameters.

  • Module 2: Extrinsic Scan Parameters (Geometric Parameters) and Spatial Resolution.

  • Module 3: Pulse Sequences (Spin Echo, Gradient Echo, Fast Spin Echo, Inversion Recovery, MRA, Diffusion-Weighted Imaging).

  • Module 4: MRI Artifacts and Mitigation Strategies.

  • Integrated MRI Mathematics: Applied formulas (Larmor equation, pixel/voxel volume, signal-to-noise ratio, scan time calculations) are integrated sequentially throughout all four core topics.

Intrinsic Tissue Characteristics & Intrinsic Scan Parameters

• Intrinsic Tissue Parameters (Inherent Tissue Characteristics):

  • Intrinsic tissue parameters represent the non-adjustable physical properties inherent to biological tissues.

  • $T1$ Relaxation Time (Spin-Lattice Relaxation):

    • Definition: The time required for $63\%$ of the net magnetization vector ($\mathbf{NMV}$) to recover along the longitudinal axis (aligned with the main magnetic field, $B_0$).

    • Mechanism: Protons are tipped into the transverse plane ($XY$ plane) by a radiofrequency ($\text{RF}$) excitation pulse and subsequently undergo longitudinal regrowth back toward alignment with $B_0$.

    • Tissue Characteristics:

      • Short $T1$ Tissues (Fast $T1$ Recovery): Fat, gadolinium contrast agents, and proteinaceous fluids. These tissues recover longitudinal magnetization rapidly and appear bright (hyperintense) on $T1$-weighted images.

      • Long $T1$ Tissues (Slow $T1$ Recovery): Water, cerebrospinal fluid ($\text{CSF}$), urine, and edema. These recover slowly and appear dark (hypointense) on $T1$-weighted images.

  • $T2$ Relaxation Time (Spin-Spin Relaxation / Loss of Phase Coherence):

    • Definition: The time required for the transverse magnetization vector to decay/dephase to $37\%$ of its initial value.

    • Mathematical Relationship: Note that $63\% + 37\% = 100\%$. A tissue's $T2$ relaxation time is always shorter than its $T1$ relaxation time.

    • Mechanism: Loss of phase coherence among precessing protons in the transverse plane due to spin-spin interactions and local magnetic field inhomogeneities.

    • Tissue Characteristics:

      • Short $T2$ Tissues (Fast $T2$ Decay): Fat dephases rapidly.

      • Long $T2$ Tissues (Slow $T2$ Decay): Free water, $\text{CSF}$, and edema maintain phase coherence longer and appear bright (hyperintense) on $T2$-weighted images.

  • Proton Density ($\text{PD}$ / Spin Density / Hydrogen Density):

    • Definition: A tissue characteristic representing the relative concentration of mobile hydrogen nuclei (protons) per unit volume of tissue.

    • Mechanism: Not a relaxation process. Tissues with high proton concentrations (e.g., $\text{CSF}$, free water) yield higher signal amplitude when transverse magnetization is fully sampled compared to tissues with lower proton density (e.g., cortical bone, tendon).

• Relaxation Curves & Contrast Generation:

  • $T1$ Regrowth Curve:

    • Depicts longitudinal recovery over time ($\text{TR}$).

    • The vertical distance between two tissue curves at a given $TR$ represents $T1$ contrast.

    • If $TR$ is too short, insufficient recovery occurs, leading to low signal-to-noise ratio ($\text{SNR}$) and grainy images.

    • If $TR$ is excessively long, both short and long $T1$ tissues recover fully, eliminating $T1$ contrast differences.

  • $T2$ Decay Curve:

    • Depicts transverse decay over time ($\text{TE}$).

    • The vertical distance between tissue curves at a specific $TE$ represents $T2$ contrast.

    • If $TE$ is excessively long, signal decays below detectable thresholds for most tissues, leaving only free fluid bright and destroying overall contrast.

• Intrinsic Scan Parameters (Operator-Selectable Parameters):

  • Repetition Time ($\text{TR}$):

    • Definition: The time, measured in milliseconds ($ms$), between the application of the initial $90^\circ$ $RF$ excitation pulse and the subsequent $90^\circ$ $RF$ excitation pulse applied to the same slice.

    • Function: Controls $T1$ contrast weighting by limiting or allowing longitudinal relaxation between pulse cycles.

    • Short $TR$: Restricts longitudinal recovery time, allowing only short-$T1$ tissues (fat, protein, gadolinium) to recover sufficient longitudinal magnetization to generate high signal amplitude on the subsequent excitation pulse ("image fast, see fast").

    • Long $TR$: Allows full longitudinal recovery for both short and long $T1$ tissues, thereby eliminating $T1$ contrast differences.

    • Field Strength Dependencies: At higher main magnetic field strengths (e.g., $3.0\,T$ vs. $1.5\,T$), tissue $T1$ relaxation times lengthen; therefore, longer $TR$ values are required at $3.0\,T$ to achieve optimal $T1$ contrast.

  • Echo Time ($\text{TE}$ / Two-Tau / $2\tau$):

    • Definition: The time, measured in milliseconds ($ms$), from the center of the $RF$ excitation pulse to the center of the peak echo signal peak.

    • Alternative Terminology ($2\tau$): Tau ($\tau$) represents the time interval from the center of the $90^\circ$ pulse to the center of the $180^\circ$ refocusing pulse ($1\tau$). The time from the $180^\circ$ refocusing pulse to the echo peak is also $1\tau$. Thus, $\text{TE} = 2\tau$. If $\text{TE} = 90\,ms$, the $180^\circ$ pulse is applied at $\tau = 45\,ms$.

    • Function: Controls $T2$ contrast weighting by providing time for transverse dephasing.

    • Short $TE$: Minimizes dephasing differences among tissues, suppressing $T2$ contrast.

    • Long $TE$: Provides time for short-$T2$ tissues to dephase, leaving long-$T2$ tissues (water, edema) bright ("image slow, see slow").

  • Echo Train Length ($\text{ETL}$ / Turbo Factor):

    • Definition: The number of $180^\circ$ refocusing pulses and resulting echoes acquired per $TR$ interval in Fast Spin Echo ($\text{FSE}$) / Turbo Spin Echo ($\text{TSE}$) pulse sequences.

    • Effect on Scan Time: Scan time is reduced directly proportional to the $\text{ETL}$.

    • Effective $TE$: The specific echo in the echo train assigned to fill the central, low-order lines of k-space, dictating the overall image contrast weighting.

    • Trade-offs:

      • Blurring: Abnormally long echo trains lead to $T2$ attenuation across late echoes, causing spatial blurring (especially at tissue boundaries). If an image appears blurry in the absence of patient motion, $\text{ETL}$ must be reduced.

      • Specific Absorption Rate ($\text{SAR}$ / $B_1^{+,\text{RMS}}$): High $\text{ETL}$ increases total $\text{RF}$ energy deposition ($B_1^{+,\text{RMS}}$ exposure), increasing patient tissue heating.

  • Inversion Time ($\text{TI}$):

    • Definition: The time interval between a $180^\circ$ inverting $RF$ pulse and the subsequent $90^\circ$ excitation pulse in Inversion Recovery ($\text{IR}$) sequences.

    • Null Point Principle: When tissue magnetization passes through the transverse plane (the null point) during $T1$ recovery following inversion, applying a $90^\circ$ excitation pulse yields zero net transverse signal for that specific tissue.

    • Short $TI$: Suppresses short-$T1$ tissue (fat) in Short $TI$ Inversion Recovery ($\text{STIR}$).

    • Long $TI$: Suppresses long-$T1$ tissue ($\text{CSF}$) in Fluid Attenuated Inversion Recovery ($\text{FLAIR}$).

  • Flip Angle (Nutation Angle):

    • Definition: The angular displacement of the net magnetization vector from the longitudinal axis ($B_0$) into the transverse plane following an $RF$ excitation pulse.

    • Spin Echo Standard: Uses a $90^\circ$ flip angle.

    • Gradient Echo Standard: Uses variable flip angles, typically $\le 90^\circ$ (shallow flip angles: $10^\circ\text{--}20^\circ$; steep flip angles: $60^\circ\text{--}70^\circ$).

    • Function in Gradient Echo: Flip angle acts as the volume control for $T1$ weighting. Higher flip angles produce greater $T1$ contrast; lower flip angles suppress $T1$ contrast.

    • Ernst Angle: The specific flip angle that maximizes signal intensity ($\text{SNR}$) for a given tissue at a chosen $TR$. Exceeding the Ernst angle causes saturation and SNR loss.

• Image Contrast & Weighting Triangles:

  • $T1$-Weighted Combination:

    • Parameter Settings: Short $TR$ (maximizes $T1$ differences) + Short $TE$ (minimizes $T2$ differences).

    • Appearance: Fat is bright (hyperintense); free water/$\text{CSF}$ is dark (hypointense).

  • $T2$-Weighted Combination:

    • Parameter Settings: Long $TR$ (minimizes $T1$ differences) + Long $TE$ (maximizes $T2$ differences).

    • Appearance: Fat is intermediate/dark; free water/$\text{CSF}$ is bright (hyperintense).

  • Proton Density ($\text{PD}$)-Weighted Combination:

    • Parameter Settings: Long $TR$ (minimizes $T1$ differences) + Short $TE$ (minimizes $T2$ differences).

    • Appearance: Contrast is governed strictly by relative proton concentration. Provides the highest overall $SNR$ among all contrast weightings because no tissue signal is intentionally suppressed.

  • Invalid Parameter Combination (Short $TR$ + Long $TE$):

    • Outcome: Maximizes both $T1$ and $T2$ contrast simultaneously in opposing directions, resulting in extremely poor contrast and compromised signal ("diagnostic garbage").

• The Image Quality Triangle:

  • Image quality depends on three interdependent factors: Contrast, Signal-to-Noise Ratio ($\text{SNR}$), and Spatial Resolution.

  • Contrast is the most critical pillar: without distinct brightness differences between normal and pathologic tissue, spatial resolution and high SNR are clinically ineffective.

Extrinsic Scan Parameters & Spatial Resolution Geometry

• Signal-to-Noise Ratio ($\text{SNR}$) Fundamentals:

  • $\text{SNR}$ is the ratio of true diagnostic signal generated from patient voxels relative to ambient background thermal/electronic noise.

  • Signal is generated by precessing hydrogen nuclei within voxel volumes and remains constant for a fixed parameter set. Noise is generated by thermal motion within patient tissues and electronic system components.

• Field of View ($\text{FOV}$):

  • Source: Derived from the amplitude of the frequency encoding (readout) gradient.

    • Shallow frequency encoding gradient = Large $\text{FOV}$ (e.g., $400\,mm$).

    • Steep frequency encoding gradient = Small $\text{FOV}$ (e.g., $200\,mm$).

  • Relationship to $\text{SNR}$: $\text{SNR}$ is directly proportional to the square of $\text{FOV}$ alterations across two dimensions:

    • Doubling the $\text{FOV}$ (e.g., $200\,mm \rightarrow 400\,mm$) increases pixel area and $\text{SNR}$ by a factor of 4 ($2 \times 2 = 4$).

    • Halving the $\text{FOV}$ (e.g., $400\,mm \rightarrow 200\,mm$) reduces $\text{SNR}$ to one-quarter ($\frac{1}{4}$) of its original value.

  • Relationship to Resolution: Larger $\text{FOV}$ creates larger pixels, reducing spatial resolution and increasing partial volume averaging.

• Slice Thickness:

  • Adjusted by changing the amplitude of the Slice Select Gradient ($\text{SSG}$) or the transmitter $RF$ bandwidth.

  • Slice Select Gradient Amplitude (Most Time-Efficient Method):

    • Steep $\text{SSG}$ = Thin slice thickness (crams precessional frequencies across a narrow spatial dimension).

    • Shallow $\text{SSG}$ = Thick slice thickness.

  • Trade-Offs:

    • Thicker slices increase voxel volume, incorporating more hydrogen protons, thereby increasing $\text{SNR}$ at the expense of increased partial volume averaging and reduced axial/spatial resolution.

    • Thinner slices improve spatial resolution and decrease partial volume averaging but severely reduce $\text{SNR}$.

  • Partial Volume Averaging Definition: The signal intensity of a pixel/voxel represents the mathematical average of all diverse tissue signals contained within that 3D volume element. If bright and dark structures occupy the same voxel, the resulting pixel displays an intermediate gray tone.

• Matrix, Pixel, and Voxel Geometry:

  • Scan Matrix: The total number of phase encoding and frequency encoding divisions across the $\text{FOV}$.

  • Pixel Size Calculation:         $\text{Pixel Dimension} = \frac{\text{FOV}}{\text{Matrix Size}}$

    • Example: For a $250\,mm$ $\text{FOV}$ and a $224 \times 224$ matrix:             $\text{Pixel Size} = \frac{250\,mm}{224} = 1.11\,mm \times 1.11\,mm$

  • Voxel Volume Calculation:         $\text{Voxel Volume} = \text{Pixel Height} \times \text{Pixel Width} \times \text{Slice Thickness}$

    • Example: For a $180\,mm$ $\text{FOV}$, $256 \times 256$ matrix, and $4\,mm$ slice thickness:             $\text{Pixel Dimensions} = \frac{180}{256} = 0.703\,mm$             $\text{Voxel Volume} = 0.703\,mm \times 0.703\,mm \times 4.0\,mm = 1.97\,mm^3$

  • Isotropic vs. Anisotropic Voxels:

    • Isotropic Voxel: A voxel possessing equal dimensions across all three planes (e.g., $1.0\,mm \times 1.0\,mm \times 1.0\,mm$). Essential for high-resolution 3D volumetric reformats (e.g., MRA, internal auditory canals, cisternograms) to prevent directional distortion/blurring.

    • Anisotropic Voxel: A voxel with unequal dimensions across its planes (e.g., $0.7\,mm \times 0.7\,mm \times 4.0\,mm$ or generated via non-square matrix grids such as $192 \times 256$).

  • Consequences of Reducing Phase Matrix:

    • Decreasing phase encoding steps reduces scan time directly.

    • Increases pixel dimensions, which increases $\text{SNR}$.

    • Decreases spatial resolution.

• Receiver Bandwidth ($\text{rBW}$):

  • Definition: The range of frequencies collected by the receiver coil during readout across the frequency encoding axis, expressed in kilohertz ($kHz$) or hertz per pixel ($\text{Hz/pixel}$).

  • Wide Receiver Bandwidth:

    • Fast sampling rate of the echo.

    • Reduces minimum $\text{TE}$ requirements.

    • Decreases chemical shift artifact.

    • Trade-off: Decreases overall $\text{SNR}$ because a broader frequency spectrum captures more background electronic noise across the echo sampling period.

  • Narrow Receiver Bandwidth:

    • Slow sampling rate of the echo.

    • Increases minimum $\text{TE}$.

    • Trade-off: Increases overall $\text{SNR}$ by filtering out peripheral noise frequencies, but exacerbates chemical shift artifacts.

• Number of Excitations ($\text{NEX}$ / $\text{NSA}$ / Acquisitions):

  • Definition: The number of times each individual line of k-space is sampled on successive $TR$ cycles.

  • Effect on Scan Time: Scan time scales linearly with $\text{NEX}$. Doubling $\text{NEX}$ doubles total scan time.

  • Effect on $\text{SNR}$: $\text{SNR}$ increases by the square root of the factor of $\text{NEX}$ alteration ($\sqrt{\text{NEX}}$):

    • $\text{NEX} = 1 \rightarrow \text{Relative SNR} = 1.0$

    • $\text{NEX} = 2 \rightarrow \text{Relative SNR} = \sqrt{2} \approx 1.41$ ($40\%$ increase in $\text{SNR}$ for $100\%$ increase in time).

    • $\text{NEX} = 3 \rightarrow \text{Relative SNR} = \sqrt{3} \approx 1.73$ ($70\%$ increase in $\text{SNR}$).

    • $\text{NEX} = 4 \rightarrow \text{Relative SNR} = \sqrt{4} = 2.0$ ($100\%$ increase in $\text{SNR}$ requiring $400\%$ scan time).

• Concatenations:

  • The practice of splitting total slice acquisitions into two or more separate interleaved sub-groups across consecutive $TR$ periods.

  • Used to preserve short $TR$ values required for strict $T1$ contrast or fat saturation parameters when requested slice coverage exceeds the physical capacity allowed within a single $TR$ loop.

• Fundamental Equations & Field Constants:

  • Larmor Equation:         $\omega_0 = B_0 \times \gamma$

    • Where $\omega_0$ is precessional/Larmor frequency in megahertz ($MHz$), $B_0$ is main magnetic field strength in Tesla ($T$), and $\gamma$ is the gyromagnetic ratio constant of hydrogen ($42.57\,MHz/T$).

  • Precessional Frequencies of Hydrogen across Field Strengths:

    • At $1.0\,T$: $42.57\,MHz$

    • At $1.5\,T$: $63.85\,MHz$

    • At $3.0\,T$: $127.70\,MHz$

  • Gauss Conversion Standards:

    • $1.0\,T = 10,000\,\text{Gauss}$

    • $5.0\,\text{Gauss} = 0.5\,mT$ (FDA safety boundary line for unshielded cardiac pacemakers).

    • $1.0\,\text{Gauss} = 0.1\,mT$

Pulse Sequences & Advanced Applications

• Classification of Pulse Sequences:

  • All MRI pulse sequences belong to one of two foundational families:

    1. Spin Echo ($\text{SE}$)-Based Sequences: Utilize a $90^\circ$ $RF$ excitation pulse followed by one or more $180^\circ$ $RF$ refocusing pulses.

    2. Gradient Echo ($\text{GRE}$)-Based Sequences: Utilize variable flip angles ($\le 90^\circ$) and gradient magnetic field reversals rather than $180^\circ$ $RF$ refocusing pulses.

• Spin Echo ($\text{SE}$) vs. Gradient Echo ($\text{GRE}$):

  • Spin Echo Mechanism: The $180^\circ$ $RF$ refocusing pulse systematically reverses phase errors caused by static magnetic field inhomogeneities, local magnetic field variations, and tissue magnetic susceptibility differences (collectively termed "The Big Three").

  • Gradient Echo Mechanism: Lacks a $180^\circ$ refocusing pulse. Uses gradient reversals to rephase spins. Consequently, $\text{GRE}$ sequences are highly susceptible to "The Big Three", making them $T2^*$ ("T2-star") weighted rather than true $T2$ weighted, yielding lower baseline $\text{SNR}$ and high sensitivity to metallic hemorrhages/implants.

• Fast Spin Echo ($\text{FSE}$ / Turbo Spin Echo - $\text{TSE}$ / $\text{RARE}$):

  • Historical Origin: Originally designated $\text{RARE}$ (Rapid Acquisition with Relaxation Enhancement).

  • Mechanism: Employs a series of multiple $180^\circ$ refocusing pulses following a single $90^\circ$ pulse, generating an echo train. Each echo is encoded with a unique phase encoding gradient amplitude to fill multiple lines of k-space per $TR$ period.

  • K-Space Allocation Strategy:

    • Weak Phase Encoding Gradients: Encodes echoes near the effective $TE$ and places them in central k-space lines (governing primary contrast and $\text{SNR}$).

    • Steep/Strong Phase Encoding Gradients: Places echoes in outer k-space lines (governing edge resolution and spatial detail).

• Inversion Recovery ($\text{IR}$) Family:

  • Pulse Scheme: $180^\circ \text{ (Inversion Pulse)} \rightarrow \text{TI} \rightarrow 90^\circ \text{ (Excitation Pulse)} \rightarrow 180^\circ \text{ (Refocusing Pulse)} \rightarrow \text{Echo}$.

  • Short $TI$ Inversion Recovery ($\text{STIR}$):

    • Purpose: Fat suppression.

    • Timing: $\text{TI}$ is timed to match fat's null point ($\text{TI} = \text{T1 of fat} \times 0.69$).

    • Field Settings: $\approx 150\,ms$ at $1.5\,T$; $\approx 190\text{--}200\,ms$ at $3.0\,T$

    • Contraindication: Cannot be utilized post-gadolinium contrast administration because gadolinium shortens the $T1$ of pathology to match fat, causing enhancement to be suppressed.

  • Fluid Attenuated Inversion Recovery ($\text{FLAIR}$):

    • Purpose: Suppresses signal from free water ($\text{CSF}$) to highlight periventricular lesions (e.g., multiple sclerosis plaques).

    • Timing: $\text{TI}$ is timed to $\text{CSF}$'s null point ($\approx 2000\text{--}2500\,ms$ at $1.5\,T$).

    • Types:

      • $T2$ FLAIR: Long $TR$ + Long $TE$ + Long $\text{TI}$.

      • $T1$ FLAIR: Long $TR$ + Short $TE$ + Moderate/Short $\text{TI}$.

• Chemical Shift Fat Saturation (Fat Sat):

  • Mechanism: Delivers a frequency-selective $\text{RF}$ presaturation pulse tuned precisely to the precessional frequency of fat, driving fat protons into saturation immediately prior to slice excitation.

  • Chemical Shift Constant: Fat precesses slower than water by $3.5\,\text{ppm}$ (parts per million).

  • Frequency Separations by Field Strength:

    • At $1.5\,T$: $63.85\,MHz \times 3.5\,\text{ppm} \approx 220\text{--}223\,Hz$ difference.

    • At $3.0\,T$: $127.70\,MHz \times 3.5\,\text{ppm} \approx 440\text{--}447\,Hz$ difference.

• Time-of-Flight ($\text{TOF}$) Magnetic Resonance Angiography ($\text{MRA}$):

  • Mechanism: Uses short $TR$/$TE$ gradient echo pulses to continuously saturate stationary background tissues (driving them into a low-signal steady state). Fresh, unsaturated blood flowing into the slice yields bright signal (Flow-Related Enhancement).

  • Spatial Presaturation Pulses (Presats):

    • Applied parallel to the slice stack to eliminate unwanted directional flow.

    • Carotid MRA: Superior presat band placed above the slices saturates descending venous blood in internal jugular veins to display arterial flow only.

    • Popliteal MRA (Lower Extremity Arteries): Presat band placed inferior to imaging slices saturates ascending venous blood flowing toward the heart.

• Diffusion-Weighted Imaging ($\text{DWI}$):

  • Physiological Basis: Measures micro-displacement of water molecules driven by thermal energy (Brownian motion / sodium-potassium pump activity).

  • Restricted Diffusion (Ischemic Stroke / Cytotoxic Edema): When arterial blood supply fails, cellular sodium pumps collapse, trapping intracellular water. Water molecules cannot diffuse freely (restricted diffusion).

  • Pulse Structure: Incorporates two strong, symmetrical diffusion-sensitizing gradients applied on either side of a $180^\circ$ pulse, combined with an Echo Planar Imaging ($\text{EPI}$) readout.

  • Gradient Action:

    • Normal, freely mobile water precesses through the gradients, acquires phase dispersion, and loses signal (dark).

    • Restricted water (stroke cell) remains stationary between gradient applications, rephases completely, and retains high signal intensity (bright hyperintensity).

  • b-Value: Defines the degree of diffusion weighting based on gradient amplitude, duration, and spacing. Higher b-values (e.g., $1000\,s/mm^2$ vs. $500\,s/mm^2$) increase stroke detection sensitivity.

• Scan Time Formula Compendium:

  • 2D Conventional Spin Echo Scan Time:         $\text{Scan Time (ms)} = \text{TR} \times \text{Phase Matrix} \times \text{NEX}$         $\text{Scan Time (minutes)} = \frac{\text{TR} \times \text{Phase Matrix} \times \text{NEX}}{60,000}$

  • 2D Fast Spin Echo ($\text{FSE}$) Scan Time:         $\text{Scan Time (minutes)} = \frac{\text{TR} \times \text{Phase Matrix} \times \text{NEX}}{\text{ETL} \times 60,000}$

  • 3D Volumetric Sequence Scan Time:         $\text{Scan Time (minutes)} = \frac{\text{TR} \times \text{Phase Matrix} \times \text{NEX} \times \text{Partitions (Slices)}}{60,000}$

MRI Artifacts & Remediation Strategies

• Categories of Artifacts:

  1. Physiological: Patient motion, respiration, cardiac pulsation, peristalsis.

  2. Inherent / Physics-Based: Chemical shift, aliasing, magnetic susceptibility, cross-talk.

  3. Hardware / External: $RF$ leaks, gradient non-linearity, bad channels, electrical spikes.

• Motion and Pulsatile Flow Artifacts:

  • Appearance: Ghosting, smearing, or repetitive signal replication occurring exclusively in the phase encoding direction.

  • Cause: Time delay between phase encoding gradient application and signal readout during which tissue shifts position.

  • Remediation: Patient coaching/comfort, physical immobilization, switching phase/frequency directions, fast scanning, respiratory gating, and Flow Compensation (Gradient Motion Nulling).

  • Gradient Motion Nulling Mechanism: Employs extra balanced bipolar gradients (+1, -2, +1 structure) before readout to rephase flowing spins (adds to net zero phase shift at $TE$). Extends minimum $TE$.

• Radiofrequency ($\text{RF}$) Zipper Artifact:

  • Appearance: Dense band or line of alternating bright/dark pixels extending across the image in the phase encoding direction.

  • Cause: Stray external $RF$ noise entering the scanner room through broken Faraday cage shielding (copper/brass seals), open scan room doors, or electronic devices.

  • Note: The artifact's physical placement along the axis depends on the specific frequency of the invading signal.

• Aliasing / Wrap-Around / Fold-Over Artifact:

  • Cause: Tissue outside the selected $FOV$ receives $RF$ excitation and is under-sampled along the phase encoding axis, violating the Nyquist Theorem (which states that an analog wave must be sampled at least twice per cycle to reconstruct frequency and location accurately).

  • Remediation: Increase $FOV$, utilize phase oversampling / anti-aliasing / no-wrap parameters, or utilize surface coils that do not detect signal beyond the $FOV$

  • 3D Slice Wrap: Occurs in 3D volume imaging when tissue outside the selected slice volume wraps into outer slices along the slice-select axis.

• Chemical Shift Artifact:

  • Chemical Shift of the First Kind:

    • Appearance: Alternating bright band on one edge and dark band on the opposite edge of fat-water interfaces (e.g., renal cortex adjacent to perinephric fat).

    • Direction: Occurs exclusively in the frequency encoding direction.

    • Occurrence: Seen on Spin Echo and Fast Spin Echo sequences.

    • Remediation: Widen the receiver bandwidth ($\text{rBW}$), decrease field strength, or decrease FOV$.\n * **Chemical Shift of the Second Kind (Phase Cancellation / Out-of-Phase Artifact):**\n * **Appearance:** Continuous dark black outline surrounding organs at fat-water boundaries ("India Ink artifact").\n * **Occurrence:** Seen **exclusively on Gradient Echo (\text{GRE}$) sequences** at specific out-of-phase$TE$values where fat and water vectors point$180^\circ opposite each other.\n * **Cause:** Lack of a 180^\circ refocusing pulse allows fat and water vectors to cancel each other's signal within shared voxels.\n\n* **Cross-Talk vs. Cross-Excitation:**\n * **Cross-Talk:** Overlap of adjacent slice RF$excitation profiles due to non-square$RF$pulse beam edges (Full Width at Half Maximum -$\text{FWHM}). Causes partial saturation and signal loss in neighboring slices.\n * *Remediation:* Incorporate a slice gap (10\%\text{--}20\%) or use interleaved slice order acquisitions.\n * **Cross-Excitation:** Signal loss occurring in multi-angle slice stacks (e.g., angled lumbar spine disc spaces at L4-L5 and L5-S1) where intersecting slice slabs excite shared tissue twice within a single TR period.\n\n* **Gradient Warp (Linearity Distortion):**\n * **Appearance:** Geometric distortion or curving of anatomical structures at the outer edges of large FOV images.\n * **Cause:** Loss of magnetic gradient linearity away from the magnet isocenter (B_0 center).\n\n* **Magnetic Susceptibility & Metal Artifacts:**\n * **Mechanism:** Distortions in local magnetic fields caused by materials with differing magnetic susceptibilities:\n * *Diamagnetic:* Weakly repels field (e.g., water, body tissue).\n * *Paramagnetic:* Weakly attracts field (e.g., gadolinium contrast).\n * *Ferromagnetic:* Strongly attracts field and distorts local lines (e.g., iron, steel, metallic implants).\n * **Appearance:** Severe black signal voids bounded by bright distorted borders (**signal pile-up**) in the frequency encoding direction.\n * **Remediation (Metal Artifact Reduction Sequence - \text{MARS}):**\n * Avoid Gradient Echo and Diffusion-Weighted sequences.\n * Use Fast Spin Echo (\text{FSE}$) with high Echo Train Lengths ($\text{ETL}).\n * Widen receiver bandwidth (\text{rBW}).\n * Use thin slice thickness and small TE values.\n\n* **Equipment Hardware Artifacts:**\n * **Corduroy / Herringbone Artifact:** Array of diagonal crisscross lines extending over the entire image caused by electromagnetic spikes or corrupted raw data points in k-space (often due to loose coil connections or power supply failures).\n * **Annefact / Zebra Stripe Artifact:** Artifact arising when active coil elements located outside the scanning FOV pick up signals that fold over into the image.\n\n# Questions & Discussion\n\n* **Question regarding T1 vs. T2 Contrast Difficulty:**\n * *Question:* Why is it inherently harder to achieve consistent T1$contrast compared to$T2 contrast?\n * *Answer:* T1$contrast operates within a narrow window of parameter options.$TR$must remain short enough to prevent full longitudinal recovery while$TE$must remain strictly short to eliminate$T2$contamination. Deviating slightly from these windows rapidly destroys$T1$contrast. Conversely,$T2$contrast provides a broad operational window where long$TR$settings (e.g.,$4000\text{--}8000\,ms$) and generous$TE$settings (e.g.,$80\text{--}120\,ms$) easily establish strong$T2 weighting.\n\n* **Question regarding the Ernst Angle in Gradient Echo:**\n * *Question:* Does the Ernst angle principle apply to gradient echo sequences, such as MRA?\n * *Answer:* Yes. For any given TR$, increasing the flip angle yields higher signal intensity up to the Ernst angle threshold. Pushing flip angles past the Ernst angle saturates stationary background tissue, which is intentionally leveraged in 2D Time-of-Flight ($\text{TOF}$)$MRA to suppress background tissue and increase vessel contrast relative to flowing blood.\n\n* **Question regarding Proton Density in Musculoskeletal Imaging:**\n * *Question:* Why is Proton Density (\text{PD}) imaging extensively used for cartilage and meniscal evaluations?\n * *Answer:* Normal meniscal tissue is fibrous and dense, appearing black (hypointense) on both T1$and$T2$sequences. On$T1 images, fluid inside a meniscal tear is also dark, creating a "black-on-black" appearance with zero contrast. Proton Density sequences depict the dark meniscus clearly while rendering fluid inside torn meniscal fibrocartilage bright, highlighting tear pathology.\n\n* **Question regarding Phase Matrix Selection in Calculations:**\n * *Question:* When provided with a matrix such as 192 \times 256, how is the phase step value identified for scan time calculations?\n * *Answer:* In clinical practice and registry testing, the smaller matrix value (192$) represents the phase encoding steps, whereas the larger value ($256$$) represents frequency encoding steps. If uncertain, running both calculations will quickly demonstrate that using the higher number yields unrealistically long scan times.

• Question regarding Spatial Saturation Bands below the Heart:

  • Question: Where should spatial presaturation bands be placed when performing venous imaging in the lower extremities (e.g., popliteal region)?

  • Answer: Presaturation bands must be placed inferior to the imaging slice block to saturate venous blood flowing upward toward the heart before it enters the imaging slices.