A radiologist wants to maximize T1 contrast between white matter and gray matter on a spin echo brain sequence. Which parameter combination achieves T1 weighting?
Short TR and short TE; Short TR and long TE; Long TR and long TE; Long TR and short TE
Answer: Short TR and short TE. T1 weighting requires a short TR, so tissues with different T1 recovery rates are caught at different degrees of longitudinal recovery, and a short TE to minimize T2 effects. The tempting distractor of long TR with short TE instead produces proton density weighting, because the long TR lets all tissues recover fully and removes T1 contrast.
On a T2-weighted fast spin echo sequence of the brain, which of the following appears brightest?
Cerebrospinal fluid; Cortical bone; Air in the paranasal sinuses; Hemosiderin within an old hemorrhage
Answer: Cerebrospinal fluid. Simple fluid such as CSF has a very long T2 relaxation time, so its transverse magnetization persists at long TE and it appears bright on T2-weighted images. Hemosiderin is a tempting distractor because it is conspicuous on T2, but it causes signal loss through susceptibility-induced dephasing and appears dark, as do cortical bone and air, which have few mobile protons.
Which TR and TE combination produces a proton density weighted spin echo image?
Short TR and long TE; Short TR and short TE; Long TR and long TE; Long TR and short TE
Answer: Long TR and short TE. Proton density weighting minimizes both T1 and T2 contrast: the long TR allows full longitudinal recovery for all tissues, and the short TE is read out before significant T2 decay, so signal differences reflect proton concentration. Short TR with short TE is the tempting distractor, but the short TR reintroduces T1 contrast and yields a T1-weighted image instead.
Compared with a gradient echo sequence, a spin echo sequence is less sensitive to static magnetic field inhomogeneities. What feature of the spin echo sequence accounts for this?
A shorter repetition time that limits time for dephasing; A 180-degree refocusing pulse that reverses dephasing from fixed field inhomogeneities; A smaller flip angle that deposits less energy in tissue; Stronger gradient amplitudes that overwhelm local field variations
Answer: A 180-degree refocusing pulse that reverses dephasing from fixed field inhomogeneities. The 180-degree refocusing pulse flips the dephasing spins so that phase accumulated from fixed, time-invariant field inhomogeneities is rephased at the echo, leaving signal decay governed by true T2. Gradient echo sequences refocus only the gradient-induced dephasing, so their signal decays with the faster T2*, which includes inhomogeneity effects. A smaller flip angle is a tempting distractor because gradient echo sequences use one, but flip angle governs SAR and contrast, not inhomogeneity compensation.
A patient with a total hip arthroplasty undergoes pelvic MRI, and the initial gradient echo images show severe signal voids and distortion around the prosthesis. Which protocol change best reduces this susceptibility artifact?
Decreasing the receiver bandwidth to improve signal-to-noise ratio; Moving the patient to a higher field strength magnet; Switching to a longer TE gradient echo sequence; Switching to fast spin echo with an increased receiver bandwidth
Answer: Switching to fast spin echo with an increased receiver bandwidth. Fast spin echo sequences refocus static field distortions with their 180-degree pulses, and a wider receiver bandwidth shrinks the spatial displacement produced by metal-induced frequency shifts. Decreasing bandwidth is the tempting distractor because it does improve SNR, but it worsens the metal distortion; longer TE and higher field strength both increase susceptibility effects.
On an abdominal MR image at 1.5 T, a dark band is seen at one fat-water interface of the kidney and a bright band at the opposite interface. Along which direction does this chemical shift artifact of the first kind occur, and how can it be reduced?
Along the phase-encoding direction, reduced by respiratory gating; Along the phase-encoding direction, reduced by lowering receiver bandwidth; Along the frequency-encoding direction, reduced by lowering receiver bandwidth; Along the frequency-encoding direction, reduced by increasing receiver bandwidth
Answer: Along the frequency-encoding direction, reduced by increasing receiver bandwidth. Fat protons resonate approximately 3.5 ppm (about 220 Hz at 1.5 T) below water, so fat is mismapped along the frequency-encoding direction, producing the classic bright and dark bands at fat-water interfaces. Increasing the receiver bandwidth increases the frequency range per pixel, shrinking the pixel shift. The phase-encoding options are tempting because motion ghosts occur in that direction, but chemical shift of the first kind is a frequency-encoding phenomenon.
On an axial T1-weighted brain image, periodic ghost images of the pulsating carotid arteries are replicated across the brain. Along which direction do these motion ghosts propagate?
The frequency-encoding direction; The slice-select direction; Diagonally across the field of view regardless of encoding axes; The phase-encoding direction
Answer: The phase-encoding direction. Phase encoding is built up over many TR intervals, so motion between excitations produces inconsistent phase and discrete ghosts along the phase-encoding axis. Frequency encoding is the tempting distractor, but each frequency readout is completed in milliseconds, far faster than physiologic motion, so periodic motion ghosts are essentially confined to the phase direction. Swapping phase and frequency axes is a common strategy to move ghosts off the anatomy of interest.
Gadolinium-based contrast agents increase lesion conspicuity on which sequence, and by what mechanism?
T2-weighted images, by increasing proton density; T1-weighted images, by shortening T1 relaxation of nearby water protons; T1-weighted images, by increasing proton density; T2-weighted images, by prolonging T2 relaxation
Answer: T1-weighted images, by shortening T1 relaxation of nearby water protons. Gadolinium is paramagnetic and markedly shortens the T1 of adjacent water protons, so enhancing tissue recovers longitudinal magnetization quickly and appears bright on T1-weighted images. The T2-prolongation distractor is backwards: gadolinium actually shortens T2 as well, which at high concentrations causes signal loss rather than gain. Contrast agents do not change proton density appreciably.
A patient scheduled for contrast-enhanced MRI is screened for nephrogenic systemic fibrosis (NSF) risk. Which situation carries the highest NSF risk?
A group I linear agent in a patient on dialysis or with eGFR below approximately 30 mL/min/1.73 m2; A group I linear agent in a patient with acute allergy to iodinated contrast; A group II macrocyclic agent in a patient with normal renal function; A group II macrocyclic agent in a patient with eGFR of 45 mL/min/1.73 m2
Answer: A group I linear agent in a patient on dialysis or with eGFR below approximately 30 mL/min/1.73 m2. NSF is associated almost exclusively with older group I linear gadolinium agents given to patients with severe renal impairment, classically eGFR below approximately 30 mL/min/1.73 m2 or dialysis dependence, where slow clearance allows gadolinium dissociation. The group II distractor with moderately reduced eGFR is tempting, but group II macrocyclic and newer agents have an extremely low NSF risk even in renal failure. Iodinated contrast allergy is unrelated to NSF.
According to the ACR MR safety zone scheme, which zone is the magnet room itself, where the static field poses a projectile hazard at all times?
Zone IV; Zone II; Zone I; Zone III
Answer: Zone IV. Zone IV is the scanner room, where the magnet is always on and only screened individuals under direct supervision may enter. Zone III is the tempting distractor because it is also restricted, but it is the control-room region immediately outside the magnet room where free public access is already prohibited. Zone I is unrestricted public space and Zone II is the supervised transition area for screening.
A ferromagnetic oxygen cylinder is accidentally brought toward an MRI scanner. The translational projectile force on the cylinder is greatest in which location?
In Zone II, far from the magnet; Uniformly throughout the magnet room; Near the bore opening, where the spatial gradient of the fringe field is steepest; At the magnet isocenter, where the field strength is maximal
Answer: Near the bore opening, where the spatial gradient of the fringe field is steepest. Translational force on a ferromagnetic object depends on the product of the field and its spatial gradient, which peaks near the bore entrance where the fringe field falls off most rapidly. The isocenter is the tempting distractor because the field is strongest there, but at isocenter the field is nearly homogeneous, so the spatial gradient and therefore the translational force approach zero (torque, however, can still be substantial).
A protocol is moved unchanged from a 1.5 T scanner to a 3 T scanner. Approximately how does the specific absorption rate (SAR) change?
It doubles, because SAR is directly proportional to field strength; It is unchanged, because SAR depends only on the gradient system; It halves, because higher fields are more efficient at excitation; It increases approximately fourfold, because SAR is proportional to the square of field strength
Answer: It increases approximately fourfold, because SAR is proportional to the square of field strength. SAR scales approximately with the square of the main field strength (and also with the square of flip angle and with duty cycle), so doubling B0 from 1.5 T to 3 T roughly quadruples RF power deposition for an identical sequence. The linear-doubling distractor is tempting but underestimates the effect; this quadratic penalty is why 3 T protocols often reduce flip angles or lengthen TR to stay within SAR limits.
A pacemaker is labeled MR Conditional. What does this labeling mean for scanning the patient?
The device is safe at 1.5 T but the label carries no other requirements; The device may be scanned only when all specified conditions, such as field strength, spatial gradient, and SAR limits, are met; The device is safe in any clinical MRI environment without restrictions; The device may never be scanned and MRI is absolutely contraindicated
Answer: The device may be scanned only when all specified conditions, such as field strength, spatial gradient, and SAR limits, are met. MR Conditional means the device has demonstrated no known hazards only within an explicitly specified MR environment, so every labeled condition, including static field strength, maximum spatial field gradient, SAR mode, and any required device programming, must be satisfied. The 1.5 T distractor is tempting because many conditions specify 1.5 T, but field strength alone is insufficient; MR Safe (no restrictions) and MR Unsafe are the other distinct categories.
During a superconducting magnet quench, liquid helium rapidly boils off. What is the primary hazard to people inside the magnet room if venting fails?
Asphyxiation from helium gas displacing oxygen in the room; Ionizing radiation released from the cryogen; Permanent magnetization of implanted devices; Electrocution from the magnet power supply
Answer: Asphyxiation from helium gas displacing oxygen in the room. A quench converts liquid helium to an enormous volume of cold gas; if the quench pipe fails to vent it outside, helium can displace room air and cause hypoxia, and positive pressure can make the door difficult to open. Electrocution is a tempting distractor because the magnet carries large currents, but the stored energy dissipates as heat in the cryogen; helium is not radioactive and releases no ionizing radiation.
In a spoiled gradient echo sequence with a short TR, how should the flip angle be adjusted to increase T1 weighting?
Flip angle has no effect on gradient echo contrast; Decrease the flip angle toward 5 degrees; Keep the flip angle at exactly 90 degrees regardless of TR; Increase the flip angle (for example, toward 60-80 degrees)
Answer: Increase the flip angle (for example, toward 60-80 degrees). With short TR, a large flip angle leaves tissues more fully saturated between excitations, so signal depends strongly on how quickly longitudinal magnetization recovers, which is T1 contrast. The small flip angle distractor is the classic trap: small angles disturb longitudinal magnetization very little, minimizing T1 effects and producing proton density or T2*-weighted contrast instead. In gradient echo imaging, flip angle largely takes over the contrast role that TR plays in spin echo.