RADTH 305 Final Exam Flashcards - Combined (Modules 4 + 5 )

RADTH 305 - MRI Physics. University of Alberta

What is the B0 fringe field value below which there are no safety concerns?

The fringe field threshold at which there are no safety concerns for the general public is 5 gauss (0.5 mT). The 5 gauss line is the outside boundary outside which pacemakers, implants, and the general public are considered safe. Inside the the 5 gauss line, ferromagnetic attraction, device malfunction, and other safety risks become relevant

• some places have a 9 gauss line as well

define and explain DSV

DSV (diameter of spherical volume) is the diameter of an imaginary sphere around isocenter where the magnetic field’s homogeneity meets specification requirements. manufacters specifiy that the magnetic field is uniform within a certain tolerance (3 ppm) inside the DSV.

• defines the usable region of high-quality homogeneity for imaging

What is the advantage of a shielded gradient coil over an unshielded one?

the shielded gradient coil has an additional shielding winding that produces an opposing magnetic field outside the gradient coil. it reduces eddy currents in nearby conductive structures which results in less image distortion.

This allows for faster gradient switching because the system doesn’t have to wait eddy currents to decay. This is mandatory for EPI sequences that rely on rapid gradient switching

Name two potential hazards of the gradient system for the patient

• Peripheral nerve stimulation

  • rapidly switching gradients induce electric fields in patient’s tissues which can cause involuntary muscle twitching, tingling, or discomfort.

  • At higher levels this is painful, so limits are enforced

• Acoustic noise (hearing damage)

  • fast gradient switching causes loud mechanical vibrations in the gradient coils.

  • Sound levels can exceed 110-130 dB, posing a risk of temporary or permanent hearing loss without proper ear protection

• Gradient Induced Heating (minor compared to RF)

  • rapidly switching gradients induce eddy currents in the patient, which can lead to tissue heating

What is a Faraday Cage? Describe the design

Faraday cage is a continuous conductive enclosure—typically made of copper or aluminum—that blocks electromagnetic fields by redistributing electrical charges across its surface. In MRI suites, the room itself functions as a Faraday cage, with metal shielding on the walls, ceiling, and floor that is fully bonded to avoid any gaps. RF-shielded doors, mesh-shielded windows, and a filtered penetration panel allow controlled access while preventing RF leakage. Waveguides are used for ventilation ducts (long metal tubes that block RF frequencies and filters are used for electrical cables to stop RF noise. This design keeps external radiofrequency noise from entering the MRI environment and prevents the scanner’s RF energy from escaping the room.

Why do we need a strong magnet for MRI

Higher magnetic field means increased SNR, and increased SNR means

• higher resolution images

• faster imaging

• more imaging techniques (diffusion MRI, fMRI, cardiac MRI)

• more contrasts: SWI and spectroscopy

Convert 5 Gauss to SI units

1 gauss = 0.0001 T

5 Gauss = 0.0005 T = 0.5 mT

How Strong is Earth’s magnetic field?

0.3 - 0.7 G

• 30-70 uT

name three types of magnets

Superconducting magnetic

• Use coils cooled to very low temperatures so they have zero electrical resistance. This allows for very high, stable, magnetic fields with low operating power. These are the most common in clinical MRI

Permanent magnets

• Made of ferromagnetic materials that produce a constant magnetic field without power. They are simple and stable but extremely heavy and limited to low field strengths (typically 0.2-0.4T)

  • This is a steady magnet but the field with drift with temperature and may change over time

Electromagnet (resistive magnet)

• Uses electric current through copper coils to generate the field. It can be switched on and off but requires continuous high power and cooling, so it is generally limited to low field strengths

  • Field is only as stable as the current that is supplying it

give one pro and one con of permanent magnets for MRI

Pro: require no power or cryogens, making them simple, stable, and relatively inexpensive to operate.

Con: prone to field drift, magnetic field can change over time due to temperature or environmental effects

What is a Halbach ring?

A Halbach ring is an arrangement of permanent magnets designed to create a strong magnetic field on one side while canceling the field on the opposite side. This configuration is often used in applications like MRI systems and particle accelerators to achieve efficient magnetic field management.

• strong homogenous field on one side and weak fringe field on the other

what is a cryostat?

A cryostat is the insulated vessel that houses the superconducting magnet and keeps it at extremely low temperatures (below 9K) using liquid helium so coils remain superconducting. It prevents heat transfer, maintains the cryogenic environment and contains the helium bath. Also has various insulating and vacuum layers to shield the coil from the warm environment

What is a coldhead?

A cold head is the cryocooler device that recondenses helium gas back into liquid inside the cryostat. It continuously removes heat from the system, reducing helium boil off and helping maintain the magnet at super conducting temperature

name 2 pros and 2 cons of superconducting magnets

pros: strong, stable, homogenous magnet

cons: expensive, large fringe field, higher safety concern of eddy currents and energy deposition

Describe 2 ways a magnet can be shimmed

Active shimming: using shim coils with adjustable electric currents to fine tune and homogenize the magnetic field

passive shimming: placing small pieces of ferromagnetic material around the main magnet to correct for field inhomogeneities

describe 2 types of magnetic shielding

Active shielding: uses additional superconductive coils to generate an opposing field. Actively cancels out the outer fringe field, allowing for a more compact magnet footprint

Passive Shielding: using ferromagnetic materials (steel) around the magnet room to contain and redirect the fringe field. Reduces how far the magnetic field extends outside the scanner

describe the gradient system in MRI

The gradient system consists of three orthogonal gradient coils (X,Y,Z) inside the bore that produce small, linearly varying magnetic fields. These gradients are rapidly switched on and off to encode spatial information, enabling slice selection, frequency, and phase encoding, and image formation

Describe gradient strength and give a typical value/range

the maximum amplitude of the gradient field

• 10-50 mT/m - will be higher for diffusion MRI

describe rise time and give a typical value / range

the time required for the gradient to go from 0 to maximum strength

• 300 us

describe slew rate and give a typical value / range

how quickly the gradient can change it’s strength (dG/dT)

• 20-200 T/m/s

describe linearity and give a typical value / range

how accurately the gradient volume field changes in a perfectly linear manner across the imaging volume

• 0.1 %

describe maximum duty cycle and give a typical value / range

the percentage of time the gradient can be active at high power without overheating

• 60-80%

What are eddy currents and what problems can it cause

Eddy currents are unwanted electrical currents induced in nearby conductive materials when gradients switch rapidly. They create secondary magnetic fields that distort the intended gradients, leading to image artifacts, geometric distortion, and slowed gradient responses.

what are 3 ways to mitigate eddy currents and their effects

1. pre-emphasis (pre-shape wave form to account for it)

2. active gradient shielding

3. reduce inductance (L) in design of gradients

give an example of an eddy current related artifact

Metal from hearing implants or dental crowns can develop eddy currents when exposed to switching gradients, producing magnetic field distortions. This causes signal voids, geometric distortion, and blurring around the metal, typically seen as dark, misshapen regions or streaking near the implant or crown.

Why does gradient non-linearity occur?

Real gradient coils cannot produce a perfectly linear magnetic field over the entire imaging volume. The physical design and geometry of of the coils only approximates linear fields near isocenter, so further from from centre the field deviates from ideal linearity.

what is a consequence of gradient non-linearity?

Geometric distortion: structures appear stretched, compressed, or shifted, especially toward edges of FOV

Why is a high voltage required in gradient amplifiers?

Gradient coils behave like inductors (resist change in current), the voltage determines how quickly the current can change. A higher voltage lets the system change current faster, which allows for faster gradient ramp times.

give 2 advantages of array coils

• higher SNR

  • multiple sensitive small coil elements placed close to the patient give higher sensitivity than a large single coil

• parallel imaging capability

  • array coils allow for accelerating imaging (reducing amount of k-space lines collected), reducing scan times and motion artifacts

compare receive only (Rx) and transmit/receive (Tx/Rx) coils

Rx Coils: used to collect MR signals only

• pros: smaller, fit closer to patient to maximize SNR, use in arrays

• cons: require seperate Tx coil to transmit RF

Tx/Rx Coils: both transmit RF pulses and receive signals

• pros: can be used independently, no sperate Tx coil needed

• cons: usually larger, lower SNR than small Rx-only elements. slower imaging

Explain gradient non-linearity and it’s consequences

Gradient non-linearity occurs when the magnetic field produced by gradient coils is not perfectly linear across the imaging volume. Ideal linear gradients only exist near the isocenter, farther away the field deviates from the intended linear slope. Consequences include geometric distortion (structures appear stretched or shifted at edge of FOV) and spatial misregistration (voxels mapped incorrectly, affecting measurement accuracy)

Describe an RF transmit amplifier

RFPA boosts the low-power radiofrequency signal from the MRI system to a high-power pulse capable of exciting the nuclear spins in the patient. It drives the transmit coil to generate the B1 field needed for MRI with precise control over amplitude, phase, and duration of RF pulse. Amplifies signal by a factor of 100 (1 V signal from computer → 100 V signal)

Describe an RF receive amplifier

A Low-Noise Pre-Amplifier boosts the very weak signal detected by the receive coils. It amplifies the signal without adding significant noise, ensuring high SNR before signal is digitized for image reconstruction.

• RF Pre-amp will send signal to a demodulator which will extract the encoded information from the amplified RF signal.

what is the purpose of the Tx coil? Give one desirable characteristic

The purpose of the Tx coil is to generate the B1 magnetic field that excites nuclear spins during an MRI sequence. A high field homogeneity may be desirable across the imaging volume to to ensure uniform spin excitation

Why are there a wide variety of Rx coils?

Because different coils are optimized for specific anatomy, size, and imaging requirements. Smaller specialized coils provide higher SNR for localized regions while larger or flexible coils cover multiple body parts / areas. Arrays allow for parallel imaging and faster acquisition

Describe the basic circuit of an RF coil

The basic circuitry of an RF coil is a resonant LC circuit consisting of

• Inductor (L): the coil itself, generating the magnetic field

• Capacitor (C): tunes the circuit to the desired Larmor frequency

• Resistor (R): represents coil loss of energy/current

Circuit resonates at the Larmor Frequency to efficiently transmit or receive MR signals

Give two examples of RF volume coils

Birdcage Coil: cylindrical coil with longitudinal rungs connected by end rings. Produces a homogenous B1 field inside the cylinder, commonly used for head, wrist, or body imaging as a volume coil

Saddle Coil: two curved coils shaped like a saddle on opposite sides of the target. Generates a linearly polarized B1 field, less homogenous than the bird cage. Often used for small animal imaging or localized Tx/Rx

What is a CP Coil?

Quadrature (Circularly Polarized - CP) Coil

• Transmit: produces a rotating Rf field rather than a single axis oscillating field to excite the nuclei more efficiently

• Receive: the rotating magnetization induces signals in two coils placed 90 degrees to each other, creating a two channel system with a combined signal, giving a stronger and cleaner signal

What are the two advantages of a CP coil

The transmit coil requires root 2 less power and the receive coil has an SNR improvement of root 2.

• overall uniformity and efficiency for both transmission and reception

Can you use a 1.5 T coil on a 3 T system?

You cannot use a 1.5 T coil on a 3T system because coils are tuned to the Larmor frequency of a specific field strength. A 1.5 T coil is tuned for 64 MHz while a 3 T field has a resonant frequency of 128 MHz. Using the wrong coil will result in low signal and inefficient excitation and reception.

What is the purpose of an array coil?

Phased array coils = multiple coils and pre-amps create and detect multiple signals. These images can then be added together

• Increase SNR: multiple small coil elements placed in close proximity to the anatomy detect more signal than a large coil

• Enables parallel imaging: allows accelerated imaging to reduce scan times and motion artifacts

• Covers larger areas: combining multiple elements, arrays can image larger regions while maintaining high sensitivity

What is a matrix coil?

A matrix coil is a large array coil made of many small overlapping coils, where you select what coils you want to use for the specific scan. It allows for parallel imaging, improves SNR, and allows imaging of different body parts without physically changing coils

Name five desirable characteristics of an MRI scan

• high SNR

• high spatial resolution

• fast imaging

• artifact free imaging

• appropriate contrast / High CNR

Name at least 4 ways to increase SNR

• Increased B0: higher field → stronger signal = increased SNR

• Smaller / custom Rx coil: reduces noise and closer to patient = detects more signal

• Increase voxel volume: more signal is received, with the tradeoff of lower resolution

• Increase number of signal averages (NSA/NEX): repeating scans and averages reduces noise, but takes longer to finish scan

• Increase FOV while keeping number of pixels the same: increase pixel size, increases SNR

• Decrease bandwidth: reduces noise, ½ bandwidth = root(2) reduction of noise

Define/explain CNR

CNR measures how well two different tissues can be distinguished in an image relative to the background noise. It is the difference in signal between two tissues divided by the noise level. Higher CNR means better ability to see contrast between structures

Name at least 4 ways to increase image resolution

• Smaller voxel size: resolution is inversely proportional to voxel size. So decreasing volume increases resolution

• Smaller FOV while keeping number of pixels same: voxels become smaller, resolution increases

• Larger Matrix: going from 128 → 256 matrix size reduces pixel size by 4x, but signal will also be reduced by factor of 4

• Thinner slices: improves through-plane resolution

• Go further out into k-space edges: edges = resolution, center = contrast

what is the scan time formula for 2DFT images (GE, SE)

What is the scan time formula for 3DFT images

What is the scan time formula for FSE

What is the scan time formula for EPI

How does signal averaging (NSA) affect SNR and scan time

SNR increases by the square root of number of averages

Scan time increases linearly with number of averages

How can slice thickness affect CNR?

The optimal contrast is when the slice thickness is the same size as the lesion. Larger slices will increase the amount of signal collected (increasing SNR), which can improve CNR as both tissues produce stronger signals relative to noise. However, thicker slices also reduce contrast between adjacent structures due to the partial volume effect (two tissues in same voxel washes out contrast)

Explain the trade-off between spatial resolution and SNR

Higher spatial resolution requires smaller voxels, but smaller voxels capture less signal, reducing SNR

• Increase resolution → smaller voxels → lower SNR

• Increase SNR → larger voxels → lower resolution

One must balance detail (resolution) with image quality.

How does Slice Width affect SNR and scan time

how does Matrix size affect SNR and scan time

How does FOV affect SNR and scan time

how does pixel bandwidth affect SNR and scan time?

rank these pulse sequences from slowest to fastest: EPI, GE, FSE, SE

SE < GE < FSE < EPI

how does doubling the number of averages (NEX/NSA) affect SNR

Doubling the number of averages (NEX/NSA) increases the Signal-to-Noise Ratio (SNR) by a factor of the square root of 2, which improves the image quality by reducing noise.

how does doubling the number of averages (NSA/NEX) affect total scan time

Doubling the number of averages doubles the total scan time (2× longer).

give three acronyms of organizations involved in MR-RT

• CADTH

  • Canadian Agency for Drugs and Technologies in Health

• CPQR

  • Canadian Partnership for Quality Radiotherapy

• ISMRM

  • International Society for Magnetic Resonance in Medicine

Name a cancer centre in Canada that currently has a MR-LINAC in use

• Arthur J Child Comprehensive Cancer Centre

• Princess Margaret Cancer Centre

• Odette Cancer Centre

What is the MOMENTUM Study?

An international registry that collects clinical, imaging, technical, and patient reported data from patients treated with MR-guided radiotherapy to evaluate the safety, feasibility, and clinical benefit of MR-guided adapted radiation therapy and supporting evidence-based adoption of the technology.

Define margin reduction in the context of MR-RT

Margin reduction refers to decreasing the planning target volume (PTV) margins around the tumor because real-time MRI visualization, daily adaptive planning, and improved soft tissue contrast reduce set-up uncertainty, organ motion, and anatomical variation, allowing for more precise targeting while sparing normal tissue

Define dose escalation in the context of MR-RT

Safely increasing the radiation dose to the tumor by using real-time MRI guidance and daily adaptive planning to account for anatomical changes and motion, improving target coverage while maintaining normal tissue dose constraints

Define target delineation in the context of MR-RT

The process of accurately identifying and outlining the tumor and relevant target volumes (GTV, CTV, PTV) on imaging to ensure the prescribed radiation dose delivered to the intended area while minimizing dose to surrounding normal tissues

Define tumor tracking in the context of MR-RT

The continents or near-real-time monitoring of tumor position using MRI during radiation delivery, allowing the beam to be gated or adapted to tumor motion (ex. respiration) to maintain accurate dose delivery and reduce irritation of surrounding normal tissues.

List at least four differences between diagnostic MR and MR-Sim Equipment

• patient positioning

  • MR-Sim allows treatment position using flat tabletops and immobilization devices whereas diagnostic MRI uses curved tables for comfort

• RF Coils

  • Coil bridges are often used on patients to prevent deformity of patient outline

• Geometric Accuracy

  • MRI systems prioritize geometric fidelity and distortion correction for RT planning, while diagnostic MRI prioritizes image contrast and diagnostic quality

• Laser Systems

  • MR-Sim includes external laser alignment systems for reproducible patient setup and reference marks; diagnostic MRI does not

• Comfort

  • diagnostic MRI has cushions on the treatment couch, MR-Sim does not as we are using immobilization devices

• Bore Size

  • the bore in an MR-Sim is larger than diagnostic MRI so that immobilization devices can fit in machine

Describe the electron return effect in terms of it’s physics and RT significance

In MR-guided radiotherapy, the strong magnetic field causes secondary electrons produced by photon interactions to curve due to the Lorentz force. At tissue-air interfaces, electrons that would normally exit the patient are bent back toward the tissue, leading to increased electron fluence at the interface. The ERE results in dose enhancement at tissue-air interfaces (skin, lung, bowel gas) and potential dose reduction just beyond the interface, which can increase the risk of skin or interface toxicity and must be accounted for during treatment planning and dose calculation

what three factors affect the ERE

• Direction of B₀ relative to beam

  • magnitude and location of dose enhancement depends on orientation

• Strength of B₀ field

  • higher field strength causes greater electron deflection, increasing ERE severity

• prescence and size of air cavities

  • ERE occurs in tissue-air interfaces, larger or more numerous gaps (lungs, skin) will increase effect

What two MR-LINACs are minimally affected by ERE and why?

• Aurora-RT

  • electrons experience less lateral deflection as their velocity is aligned with the magnetic field

• ViewRay MRIdian

  • lower field strength causes less deflection of secondary electrons

which MR-LINAC is most affected by the ERE

Elekta Unity

• has a higher magnetic field strength and a B0 field oriented perpendicular to the photon beam, which causes greater lateral deflection of secondary electrons at tissue-air interfaces

give 5 main features of the ViewRay MRIdian system

• 6 MV Beam

• 0.35 T split superconducting magnet

• magnet perpendicular to the beam

• bore is 70 cm wide

• couch moves in three directions

• system is commercially available and there are many clinical publications on it

give 5 main features of the Aurora-RT System

• 6 MV beam

• 0.5 T biplanar T_c superconducting magnet with steel yoke

• magnetic field is parallel to the photon beam

• bore is 110 cm wide

• couch moves in three direction

• not commerally available with no clinical outcomes published

give 5 main features of the Elekta Unity System

• 7 MV beam

• 1.5 T closed superconducting magnet

• magnet perpendicular to the beam

• bore is 70 cm wide

• commercially available and has many clinical outcomes

In the Elekta system, how is the system engineered so that the beam reaches the patient unobstructed by the magnet

The LINAC and gantry rotate around the MRI magnet in the Unity machine, and the photon beam passes through the magnet via a coil-free central gap in the magnet design. Specifically, the central 15 cm of the magnet contains no coils, creating a clear path for the radiation beam. This gap allows a maximum field size of approximately 24 cm in the head-to-toe (superior–inferior) direction at isocentre.

Although the beam passes through the magnet structure, the total material in the beam path is minimized and is equivalent to about 8.2 cm of aluminum, ensuring acceptable beam attenuation and dose delivery.

In the Viewray system, how is the system engineered so that the beam reaches the patient unobstructed by the magnet?

The MRI magnet is split into two halves, creating a central open gap at isocentre. The radiation beam passes directly through this gap rather than through the magnet structure, allowing it to reach the patient unobstructed and with minimal attenuation, while still maintaining the magnetic field required for MRI guidance.

In the Aurora-RT system, how is the system engineered so that the beam reaches the patient unobstructed by the magnet

The MRI magnet is designed such that the B₀ magnetic field is always parallel to the photon beam. To achieve this, the magnet rotates together with the treatment gantry, ensuring that the beam path remains aligned with the magnetic field at all times. As a result, the radiation beam passes directly through the magnet without obstruction or attenuation, regardless of gantry angle, allowing continuous, unobstructed delivery of the prescribed dose.

Give the field strength and beam energies of 3 different MR LINAC Systems

ViewRay MRIdian: 6 MV beam and 0.35 T magnet

Elekta Unity: 7 MV beam and 1.5 T magnet

Aurora-RT: 6 MV beam and 0.5 T magnet

Australia MR-LINAC: 6 MV and 1.0 T magnet

Give the field orientation of three different MR LINAC systems

ViewRay MRIdian: split superconducting perpendicular

Elekta Unity: superconducting closed bore perpendicular

Aurora-RT: rotating superconducting parallel magnet

Australia: superconducting open bore parallel and perpendicular orientation.

• The patient rotates in this machine!

Which was the first commercially available MR-RT Machine?

ViewRay MRIdian received FDA clearance in 2012. It combined a 0.35 T MRI scanner with a linear accelerator for real-time imaging and adaptive radiation therapy.

Name three disadvantages of CBCT vs. MRI for MRgRT applications

• Poor Soft Tissue Contrast: limited differentiation between tissues, making it harder to visualize tumors/OARs

• Limited Real Time Imaging/Motion Tracking: CBCT acquires images slowly and cannot continuously monitor tumor motion during treatment. Compare this to MRI that allows for real-time visualization and gating of moving targets

• Higher Imaging Dose: CBCT uses ionizing radiation, MRI is non-ionizing

why is MRI particularly useful for proton therapy?

It provides superior soft-tissue contrast for accurate tumor and organ-at-risk delineation, which is critical in proton therapy due to its sharp dose fall-off (Bragg peak). Small errors in target definition or positioning can lead to significant underdosing of the tumor or overdosing of nearby normal tissue, so MRI’s high-resolution imaging helps maximize treatment precision and safety.

Describe a standard RT workflow without MRI

A typical radiation therapy workflow involves patient consultation, treatment planning using CT imaging, verification of patient positioning, administration of radiation doses, and follow-up assessments.

Describe the MR-CTgIGRT workflow

Descibe the MRgRT workflow

What are some pros of an MR-only workflow

• improved target localization

• real-time tracking/gating possible

• normal tissue sparing

• adaptive planning

• fMRT= bioguided RT

• decreased fractionation

what are some cons of an MR-only workflow

• complications from tissue overdose

• decreased dose rate (600 MU/min c.f 1200 MU/min)

• slower gantry rotation

• decreased field size

What is adaptive RT and when/who might it be useful for

ART is an approach in which the radiation treatment plan is modified during the course of the treatment to account for changes in patient anatomy, tumor size/shape, or organ motion using updated imaging. ART is particularly useful when there are significant anatomical or biological changes, such as tumor shrinkage/progression during tx, daily organ motion/deformation, weight loss or body contour changes, targets near critical organs where maintaining dose constraints is prioritized, and for pediatric patients who have rigorously regulated dose prescriptions to minimize long term effects

What does the term PseudoCT mean

A pseudoCT is a synthetic CT image generated from MRI data that assigns CT-like electron density (HU) information to tissues. This is how dose calculations are performed in MRI-only workflows, where conventional planning CTs are not acquired.

Define and Explain ATS

ATS (Adapt-to-Shape) is an online adaptive radiotherapy workflow used in MRgRT. It involves re-contouring the target and organs at risk on the daily MRI to reflect the patient’s current anatomy, followed by full reoptimization of the treatment plant before delivery. This accounts for anatomical changes such as organ deformation, tumor shrinkage, or variable filling. This ensures accurate target coverage while maintaining normal tissue dose constraints.

Define and Explain ATP

ATP (Adapt-to-Position) is an online adaptive radiotherapy workflow used in MRgRT. It involves shifting or re-optimizing the existing treatment plant based on the daily position of the target seen on MRI, without changing the target or OAR contours. The plan is adapted to account for rigid translational shifts and limited rotations while the shape of the anatomy is assumed to be unchanged.

When is ATP more useful? When is ATS more usful?

ATP is faster and less resource-intensive than Adapt-to-Shape (ATS) and is more useful when anatomy is stable but target position varies from day to day.

ATS is useful when shape and position of structures change significantly from day-to-day, but it is more time and resource-intensive than simpler adaptive approaches.

Draw a flowchart for a generic online MR workflow

Draw a flowchart for an online prostate-ATP workflow

draw a flowchart for an online pelvic-ATS workflow

pretreatment CT taken to give electron density and a daily MRI is taken. They are combined in deformable registration so that the CT is adapted to the current anatomy seen on the MRI. Contours are then retraced based on today’s MRI and densities are applied to areas of concern (bladder). The online part is where the doses are replanned according to the combined CT/MRI data , then a conformational MRI and dose calcs are done for QA. Treatment is delivered at the same time as an MRI to ensure if the patient moves the dose is shifted. A post treatment is then done to confirm dose was delivered to the right spot

what is an imaging biomarker, and why is this of interest in MR-RT?

An imaging biomarker is a quantitative feature extracted from medical images (signal intensity, diffusion, perfusion parameters) that reflects biological processes, disease characteristics, or treatment response. In MRgRT, imaging biomarkers are valuable MRI acquired during treatment can monitor tumor responses, cellularity, or perfusion in real time. This enables response-adapted treatment like modifying the dose or margins to improve tumor control or minimize toxicities

MR is not sensitive to electron density (which is needed for treatment planning). How do MR-only workflows get around this?

MR-only workflows address this by generating a pseudoCT from the MRI, where tissues are assigned CT-equivalent electron density or Hounsfield unit values using methods such as atlas-based approaches, tissue classification, or machine learning. This synthetic CT enables accurate dose calculation without a planning CT needed.

What is the topic of AAPM TG-284

this task group reported on MRI simulation in radiotherapy. it provided recommendations for clinical implementation, optimization, and quality assurance of MR-Sim in RT. It includes guidance on equipment selection, siting, commissioning, workflow integration, motion and distortion management, safety, and imaging protocols tailored for RT planning

Explain three unmet needs in MR-SIM from the AAPM TG-284 paper

• incomplete geometric distortion correction

  • While vendors provide 3D gradient nonlinearity (GNL) correction, it is not available for all imaging sequences (e.g., 2D, gated, DWI), may not be applied retrospectively, and residual distortions remain at large fields of view with no vendor-provided solution.

• Limited B₀ field mapping and correction

  • Online B₀ field mapping is often restricted to research options, may rely on wrapped phase images, and patient-specific online distortion correction is not clinically available.

• Residual Intensity Non-Uniformity

  • Vendor correction algorithms reduce RF coil intensity variations but residual inhomogeneities remain, affecting image registration and segmentation; clinical correction tools are limited.