MRI Physics
What are two inherent ways in which a gradient echo sequence differs from a spin echo sequence? Why is this advantageous?
(1) Flip angle of < 90 (2) do not have a 180 rephasing pulse. The smaller flip angle leads to a faster recovery, shorter TR/TE times and a faster scan. Cons: susceptible to field inhomogeneities/susceptibility artifacts.
Write the equation for the Larmor frequency
ωØ = γBØ ωØ = Larmor (precessional or resonance) frequency γ = gyromagnetic ratio (42.58 MHz / T for H) BØ = external magnetic field strength (Tesla)
What are the three categories of magnetic susceptibility and which has the largest effect on MRI?
- Ferromagnetic: Strongest form (eg. Iron) - Diamagnetic (negative susceptibility and oppose the magnetic field; calcium, water, most organic materials), - Paramagnetic: slight positive susceptibility and enhance magnetic field but have no measurable self-magnetism; molecular oxygen, gadolinium- based contrast agents and some blood degradation products
Recommended dose of Gd-containing contrast media is about _____ mMol/kg body weight.
0.1 However, doses 2 to 3 times as high are often use.
In which of the following sequences will white matter be hyperintense to grey matter? a. T1-weighted b. T2-weighted c. Proton density d. Gradient echo
A - T1-weighted, and d if T1-weighted?
Choose which of the following factors will cause T2-prolongation (hyperintensity)? a. Fast molecular tumbling rates b. Freely-bound molecules c. Large molecules
A and B only.
True/ False: TE is always longer than TR
False TE is always shorter than TR
Which of the following can cause burns to the patient during an MRI scan? a. Nicotine patch on the skin b. Body parts directly touching magnet bore c. Pacemaker leads d. Eyeliner tattoo e. ECG wires f. Jewelry g. Closed loop of appendages touching (e.g. knees)
All of the above are correct.
Which one of the following is correct? a. TE is longer in proton density weighting compared to T2 weighting b. TE is shorter in proton density weighting compared to T2 weighting c. TR is longer in proton density weighting compared to T2 weighting d. TR is longer in proton density weighting compared to T2 weighting
B Proton density contrast weighting relies on differences in the number of magnetized protons per unit volume of tissue. Contrast is generated from variations in proton density (CSF > fat > gray matter > white matter). Long TR minimizes T1 relaxation differences of the tissue. Short TE preserves the proton density differences without allowing significant T2 decay.
What is the difference between spin echo and gradient echo?
Both spin and gradient echoes are susceptible to static magnetic field inhomogeneities such as degraded blood products and metal objects) however in spin echo, a refocusing pulse cancels out this T2* effect.
Proton relaxation by Gd-DTPA is most likely due to which property of gadolinium? a. Nuclear field b. Chelate (DTPA) c. Unpaired electrons d. K-edge energy e. Electron density
C
In regards to gadolinium intravenous contrast material and MRI a. Lengthens T1 relaxation time b. Shortens T2 relaxation time c. Shortens T1 relaxation time d. Lengthens T2 relaxation time
C Also B at higher doses (or longer times)
How does Gd-DTPA appear on T2 weighted images?
Dark as it shortens T2
In a traditional spin echo sequence, what factors determine the length of the study? What about in a fast spin echo sequence?
Duration = TR x Phase matrix x NEX FSE - acquisition time is proportional to 1/ETL (echo-train length)
What are the two factors that affect the flip angle?
Duration and amplitude of the RF pulse applied
The RF pulse applied should be [less than/greater than/equal to] the Larmor frequency of hydrogen for the protons to absorb the energy and jump to a higher energy state.
Equal to
Describe at least two ways in which a quench can be dangerous.
Explosive conversion of liquid helium to gas, which can cause (1) displacement of oxygen in the room and subsequent suffocation and/or (2) severe increase of pressure in the room, preventing the door from opening.
FLIP THIS CARD 1. Add a slice gap 2. Slice interleaving
FLIP THIS CARD Cross-talk refers to interference between adjacent slices in MR because the actual slice profiles are not perfectly rectangular, which decreases the contrast. What can be done to avoid this?
FLIP THIS CARD B Long TR minimizes T1 relaxation differences of the tissues. Long TE allows T2 decay differences to be manifested.
FLIP THIS CARD Figure below represents which one of the following? a. T1 weighted contrast b. T2 weighted contrast c. Proton density contrast
FLIP THIS CARD a Short TE preserves T1 tissue differences by allowing significant T2 decay to occur.
FLIP THIS CARD Figure below represents which one of the following? a. T1 weighted contrast b. T2 weighted contrast c. Proton density contrast
FLIP THIS CARD 3
FLIP THIS CARD What type of MR sequence is shown in this image? 1. DWI 2. Gradient echo 3. Inversion recovery 4. T2 5. T1 6. PD
FLIP THIS CARD 6
FLIP THIS CARD Which one of the following is incorrect regarding the figure below? 1. Magnetic field strength (B0) has no effect on T2 relaxation (unlike T1 relaxation). 2. T2 relaxation occurs due to inherent spin-spin interactions between the protons and their environment. 3. Tissue A is more likely to have faster molecular tumbling rate compared to B 4. Tissue A is more likely to have more freely-bound molecules compared to B 5. Tissue A is more likely to have smaller molecules compared to B 6. More spin to spin to interaction occurs in Tissue A compared to Tissue B
T/F: The Larmor frequency is the same no matter the magnetic field strength when imaging for the same element (e.g. hydrogen).
False - Equation (wø = yBø) is directly related to magnetic field strength, and so the Larmor frequency increases with increasing magnetic strength.
T/F: In a gradient echo sequence, dephasing and rephasing of the FID signals occur in opposite directions relative to the main magnetic field.
False - they occur in the same direction as Bo, thus the external magnetic field variations are not cancelled allowing sensitivity to field nonuniformities and magnetic susceptibility agents.
What is the cause of free induction decay (loss of phase coherence)?
Intrinsic micromagnetic inhomogeneities in the sample structure whereby individual protons in the bulk water and hydration layer coupled to macromolecules precess at different frequencies arising from the slight changes in local magnetic field strength. = T2* decay
Why is quench a safety concern?
It releases helium which can lead to suffocation.
Using the Larmor equation, gyromagnetic ratio can be expressed as?
Larmor frequency (angular precessional frequency) / external magnetic field (MHz/T)
How does an increase in field strength B0 affect T1 and T2 relaxation times?
Magnetic field strength (B0) has no effect on T2 relaxation and will increase T1 relaxation
Why is hydrogen considered an MR active atom?
Mass number (P+N) is odd, and thus there is a net angular momentum that will be responsive (i.e. align the axis of rotation) to an applied magnetic field.
To obtain a thin slice thickness, either the RF bandwidth can be (narrow/wide) or the gradient strength can be (low/high).
Narrow bandwidth and high gradient strength produce a thin slice thickness.
What are the biological effects of a magnet at 1.5T? What about 4.0T? 10.0T?
No biological effects at 1.5T other than heating. At 4.0T may experience dizziness or disorientation. At 10T and above (particularly 20T), can see enzyme kinetic changes, increased membrane permeability and altered biopotentials.
On post T2 relaxation contrast study (e.g. ferumoxide agent), how do normal liver, spleen, and bone marrow appear on T2*? What about lesions for example metastases?
Normal liver spleen, and bone marrow which selectively absorb the ferumoxides by the reticulo-endothelial cells has a low signal on T2* (= dark, short T2). Lesions that do not contain RET cells do not take up the agent therefore have a normal signal on T2*.
MR active nuclei will have (odd/even) mass numbers.
Odd
What is the mechanism by which gadolinium contrast agents create contrast in MRI studies?
Presence of contrast agents enhances relaxation of water protons in their vicinity -> faster T1 relaxation times (more hyperintense) which helps identify areas of hypervascularity (e.g. malignant tumor).
What does the 5G (Gauss) line indicate?
Risk to implanted devices (e.g. pacemaker, insulin pump). Importantly, this has nothing to do with translational (pulling) forces.
What is the relationship between SAR and magnet strength, flip angle and TR?
SAR = Bo^2 x Alpha (flip angle)^2 x Duty Cycle (inverse relationship with TR) Double Bo = quadruple SAR Double flip angle = quadruple SAR (thus SE has higher SAR than GRE) Double duty cycle (make TR ½) = double SAR
What are advantages and disadvantages for TOF and phase contrast MR aniography each?
TOF Advantage - Very fast acquisition Disadvantage - Pulsative arterial flow causes artifact: ghosting - Incomplete suppression of stationary tissue Phase contrast Advantage - Better background suppression - Flow in each direction can be imaged - Quantization of flow possible in each direction
The description below explains which MR technique? It exploits contrast between the high signal intensity of inflowing fully magnetized blood and the low signal intensity of saturated stationary tissue. During the acquisition, the imaging volume repeatedly experiences radiofrequency pulses that result in saturation of the nonmoving spins.
Time of flight
What are two reasons why free radial gadolinium (Gd3+) are chelated as a MR contrast agent?
To reduce the toxicity and improve biologic tolerance. Free radical Gd3+ is very toxic, and causes renal tubular damage, chelation reduces toxicity. Free radical Gd3+ has low water solubility, and chelation increases water solubility.
T/F: Amplitude of current in a coil is directly related to magnetic strength.
True
T/F: Magnetic field strength has no effect on T2 relaxation, but it does affect T1 relaxation.
True
True/false? Narrowing FOV in MR decreases the signal to noise ratio (SNR). Thinner slice thickness decreases the signal to noise ratio (SNR).
True True
True/False: The difference in precessional frequencies between fat and water are greater with a stronger magnet
True - Per the Larmor equation (Precessional frequency = gyromagnetic ratio X field strength). This is why fat saturation techniques are more successful with higher strength magnets.
How is a spin echo made, and name three different spin echo weightings?
Use of a 90 degree then 180 degree refocusing pulse It includes T1, T2, and proton density.
Can a pregnant woman get an MRI?
Yes. Caution is advised, however. No studies have proven deleterious effects, however there is concern over auditory damage of the fetus, as well as exposure to free gadolinium ions (they are known to pass the placental barrier).
Which of the following is likely to have the shortest T1 time? a.Fat b.Liver c.Kidney d.White Matter e.CSF
a. FAT
Which gradient is used to identify location on the x-axis. a. Frequency encoding gradient b. Phase encoding gradient c. Slice-selection gradient
a. FEG
For each of the following, indicate whether they are diamagnetic, paramagnetic or ferromagnetic: a. Water b. Iron c. Calcium d. Gadolinium e. Methemoglobin f. Cobalt g. Methylcellulose
a. Water Diamagnetic b. Iron Ferromagnetic c. Calcium Diamagnetic d. Gadolinium Paramagnetic e. Methemoglobin Paramagnetic f. Cobalt Ferromagnetic g. Methylcellulose Diamagnetic
If the TE has a duration of t seconds, the TI will have a duration of: a. t/4 b. t/2 c. t d.2t
b. TI = TE/2
What should you do if your patient codes in the magnet? Choose all that apply. a. Call for a code team to resuscitate the patient in the MRI room (zone IV) b. Quench the magnet c. Start CPR yourself in the MRI room d. Bring the patient to zone II before attempting resuscitation
c or d depending on the circumstance
Superconducting MR magnets are most likely kept cold by using liquid: a. Air b. Carbon Dioxide c. Helium d. Nitrogen e. Oxygen
c. Helium