MAGNETIC RESONANCE IMAGING Flashcards
What is MRI?
A computer-based imaging modality using magnetic fields and radio waves.
What was MRI originally called?
Nuclear Magnetic Resonance.
What is the main advantage of MRI?
Best low contrast resolution.
Why is MRI non-invasive?
It does not use ionizing radiation.
How does MRI differ from conventional radiography in contrast?
MRI contrasts depend on electromagnetic interaction, not x-ray attenuation.
Who discovered the properties of magnetic resonance in the 1940s?
Felix Bloch and Edward Purcell.
What year did Paul Lauterbur publish the first MRI cross-sectional image?
1973
What was the name of the first MRI machine?
Indomitable.
A spurious finding or distortion in the image.
ARTIFACT
What is “free induction decay”?
Signal emitted by tissue after RF excitation.
What is the gyromagnetic ratio of hydrogen?
43 MHz/T.
What does the Larmor frequency describe?
The frequency at which a nucleus precesses in the magnetic field.
indicates power absorbed during RF irradiation.
Specific Absorption Rat (SAR)
What is T1 relaxation time also called?
Spin-lattice relaxation time.
What does T2 relaxation time represent?
Spin-spin or transverse relaxation time.
What is the SI unit of magnetic field strength?
Tesla (T).
Why is hydrogen used in MRI?
It is the most abundant element in the body and creates the strongest MRI signal.
What is the purpose of spin echo sequences?
To produce T1-weighted images and proton-density-weighted images.
What is the function of FLAIR sequences?
Suppressing signals from cerebrospinal fluid (CSF).
What is the primary use of T1-weighted images (T1W)?
Showing anatomical detail.
What are T2-weighted images (T2W) useful for?
Highlighting pathology.
Localizes the MRI signal, enabling slice selection and spatial encoding.
gradient magnetic field
Produces gradient magnetic fields for imaging.
gradient coils
What is the main function of the operating console in MRI?
To control the computer and imaging parameters.
What type of storage is used for raw MRI data?
Magnetic tape or optical disk for permanent storage.
What are the three groups of MRI magnets?
Permanent, resistive, and superconducting magnets.
What is an advantage of permanent magnets in MRI?
Low operating cost.
What is a disadvantage of resistive magnets in MRI?
High power consumption.
What are superconducting magnets cooled with?
Cryogens like liquid helium and liquid nitrogen.
Improves the homogeneity of the magnetic field.
shim coils
What are the three types of gradient coils in MRI?
Z (transaxial slice), X (coronal slice), and Y (sagittal slice) gradients.
What is the main function of RF coils in MRI?
Transmitting RF signals and/or receiving the MR signal.
Imaging small body parts like the spine or shoulders.
surface coils
Head imaging due to its excellent RF homogeneity.
birdcage coil
shields the MRI system from external radio frequency interference.
faraday cage
What is the effect of increasing slice thickness in MRI?
Reduced spatial resolution but less grainy images.
How does decreasing slice thickness affect pathology visualization?
It improves visualization of small pathologic lesions.
The ability to differentiate tissues with small differences in spin density or relaxation times.
contrast resolution
What is the principal biological effect of RF emissions in MRI?
Tissue heating, measured as Specific Absorption Rate (SAR).
What are the three physical fields in MRI that might cause biological effects?
Strong static magnetic field (B0)
time-varying gradient magnetic field (B1)
RF emissions.
A rapid release of cryogens caused by a fault or manual deactivation of the magnet.
quenching
What are gadolinium-based contrast agents used for in MRI?
Enhancing signal in specific tissues for better imaging.
reduces motion artifacts by synchronizing image acquisition with a specific phase of motion.
gating
An image wrap-around effect caused by anatomy outside the FOV appearing in the image.
aliasing artifact
A bright signal in tendons caused by their alignment at 55° to the magnetic field.
magic angle artifact
What causes a metal artifact in MRI?
Field inhomogeneity due to the presence of metal in the scan area.
An artifact caused by thick slices or lack of overlap between slices.
staircase artifact
What is the primary solution for RF overflow artifacts in MRI?
Reducing the receiver gain manually.
What is the appearance of CSF on T2-weighted images?
Very bright signal (white).
What is the principal component of MRI systems responsible for creating the magnetic field?
magnet
How does the Fourier Transform function in MRI?
It converts raw MRI signal data into an image spectrum.
Calculates the precession frequency of nuclei in a magnetic field.
Larmor equation
What is the role of the net magnetization vector (M0) in MRI?
It represents the overall magnetic moment in equilibrium, contributing to MRI signal intensity.
What type of tissues produce high signal intensity on T1-weighted images?
Fatty tissues and structures with short T1 relaxation times.
How does flow phenomena affect MRI signals?
Moving blood produces weak signals, while stagnant blood produces stronger signals.
What is the purpose of phase-encoding gradients in MRI?
To determine spatial location in the Y-axis.
determines spatial location in the X-axis.
frequency-encoding gradients
What is the main advantage of superconducting magnets in MRI?
High field strength and field homogeneity.
What is the disadvantage of permanent magnets in MRI?
Limited and fixed field strength.
cools the magnet coils and maintain superconductivity.
cryogens
What is the purpose of surface coils in MRI?
For high-resolution imaging of small body parts like the spine or joints.
What is a neurovascular coil used for?
Imaging the brain and neck regions.
How does increasing MRI signal acquisition affect spatial resolution?
It improves spatial resolution.
What is the significance of spatial resolution in MRI?
It determines the clarity of anatomical detail in images.
What effect does a stronger magnetic field have on the precession rate?
It increases the precession rate.
creates gradient magnetic fields for slice selection and spatial encoding.
gradient coils
What is the main role of RF pulses in MRI?
To excite hydrogen nuclei and create an MRI signal.
What is spin density in MRI?
The concentration of hydrogen nuclei in tissue, affecting signal strength.
What is the relationship between T1 relaxation time and tissue type?
T1 relaxation time varies depending on the tissue’s molecular environment.
What type of images are most useful for identifying pathologies in MRI?
T2-weighted images.
How does fat appear on T1-weighted images?
Bright (high signal intensity).
What is the primary biological hazard associated with RF emissions in MRI?
Tissue heating.
What is the significance of a Faraday cage in MRI facility design?
It prevents external radiofrequency interference from affecting imaging.
suppresses specific tissue signals, like fat or fluid.
inversion recovery sequences
What causes phase wrapping artifacts in MRI?
Anatomy outside the field of view being misrepresented in the image.
What is the purpose of short tau inversion recovery (STIR) sequences?
To suppress fat signals in MRI images.
What causes partial volume artifacts in MRI?
Insufficient resolution leading to mixing of signals from adjacent tissues.
How does cerebrospinal fluid (CSF) appear on T1-weighted images?
Dark (low signal intensity).
How is a high signal intensity on T2-weighted images interpreted?
It often indicates the presence of pathology or fluid.
What is the significance of echo time (TE) in MRI?
It determines the time for signal acquisition after the RF pulse.
What does repetition time (TR) affect in MRI?
The contrast between tissues and the overall signal strength.
What is a typical use of fat suppression techniques in MRI?
To enhance contrast between fat and adjacent tissues, such as in breast or musculoskeletal imaging.
What does the term “spin-spin relaxation” refer to?
The process of signal decay due to loss of phase coherence among spins.
reconstructs images from raw data, though it is rarely used today.
back-projection reconstruction
It allows rigorous mathematical reconstruction of images.
2D Fourier transformation
What does the term “signal-to-noise ratio” (SNR) refer to in MRI?
The ratio of useful signal to background noise in an image.
What is the principal advantage of increasing the magnetic field strength?
Higher SNR and better resolution.
How can aliasing artifacts be reduced in MRI?
By increasing the field of view or using oversampling techniques.
What is the function of shimming in MRI?
What is the function of shimming in MRI?
How does slice thickness affect spatial resolution in MRI?
Thinner slices improve spatial resolution.
What is the typical strength of clinical superconducting magnets?
1.5T to 3T.
What is the appearance of fat on T2-weighted images?
Bright but not as intense as fluid.
What is the impact of longer echo times (TE) on T2 contrast?
It enhances T2 contrast, making fluid and pathology more visible.
What is a common application of gradient echo sequences?
3D imaging and vascular studies.
What is the function of fast spin echo (FSE) sequences?
To acquire T2-weighted images quickly.
What is the principal hazard of strong static magnetic fields in MRI?
Interaction with metallic implants or devices in the patient.
How does gadolinium enhance MRI imaging?
By shortening T1 relaxation times, increasing signal intensity in affected areas.
What type of artifact is common with turbulent blood flow in MRI?
Flow-related artifacts or signal voids.
What is the purpose of ramping up the magnet in MRI?
To gradually increase the magnetic field strength to its operating level.
How does increasing receiver bandwidth affect image quality?
It reduces chemical shift artifacts but may lower SNR.
What are common uses of fluid-attenuated inversion recovery (FLAIR) sequences?
Imaging brain lesions or conditions like multiple sclerosis.
What is the significance of repetition time (TR) in MRI pulse sequences?
It affects image contrast and acquisition time.
What is the purpose of paired saddle coils in MRI?
For knee imaging and providing uniform gradients.
converts time-domain signals into frequency-domain images.
fourier transform
What is the advantage of short repetition times (TR) in T1-weighted imaging?
Faster imaging with higher T1 contrast.
What is the function of saturation pulses in MRI?
To suppress specific tissue signals.
What is the main limitation of resistive magnets in MRI?
High power consumption and limited field strength.
What are the typical field strengths for permanent magnets in MRI?
Up to 0.3T.
What does “homogeneity” of the magnetic field affect in MRI?
The accuracy and quality of image acquisition.
What is the typical appearance of gray matter on T1-weighted images?
Gray (lower signal intensity than white matter).
What is the role of neurovascular coils?
Imaging brain and neck vasculature.
What is the main advantage of birdcage coils in MRI?
Superior RF homogeneity, ideal for head imaging.
What is the role of echo-planar imaging (EPI) in MRI?
Rapid acquisition of images, commonly used in functional MRI (fMRI).
