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Moving from 1.5T to 3T?

The value of higher field strength for clinical imaging has been indicated in some clinical applications. Research studies are likely to confirm clinical utility of 3.0T vs. 1.5T—studies showing not just signal-to-noise ratio, contrast-to-noise ratio, or even diagnostic sensitivity and specificity—but effects on patient management, and ultimately, effects on patient outcome.

What are the MR physics issues which are relevant when comparing field strengths?

SNR (signal-to-noise ratio) goes up
… but perhaps not as much as you might think.

Ignoring relaxation effects, the MR signal induced in a receiver coil is proportional to the square of the magnetic field (B0). However, the noise has a linear B0 dependence at field strengths greater than 1.0T, and as a result, SNR is linear with B0 in this range. One might expect, therefore, to double the SNR when the field strength is doubled.

But this theoretical SNR increase is not normally realised in vivo. In practice, SNR gains of this order of magnitude are only realised in certain tissues (e.g. cerebrospinal fluid). At 3.0T, susceptibility effects are more significant than at 1.5T; microscopic susceptibility changes cause larger local magnetic field gradients and greater dephasing of spin isochromats results, decreasing the apparent T2 (i.e. T2*) and causing faster signal decay. For example, the value of the T2* in grey brain matter is likely to be predominantly determined by iron concentration (and thus scales with B0). In grey and white brain matter, due to increases in T1 (see later) and due to the low levels of iron, the SNR gain at 3.0T compared to 1.5T is only 30-60%, not 100%. Furthermore, a higher receiver bandwidth is often used to reduce the larger chemical shift seen at 3.0T. Higher receiver bandwidth reduces SNR.

However, the increase in SNR is still probably the most significant consequence of going to 3.0T affecting clinical utility. SNR increases may be traded for higher spatial and/or temporal resolution, if desired, which makes for demonstrably better image quality. One signal average (1 NSA or on some systems, NEX) at 3.0T should yield a SNR comparable or better than 2 averages at 1.5T (changing from one to two averages produces a √2 increase (41%) in signal).

Other techniques, such as the Blood Oxygen Level Dependent (BOLD) susceptibility effect used in functional MRI (fMRI), also benefit from increased SNR at 3.0T. The BOLD effect is very small, and higher SNR allows higher sensitivity to the BOLD effect.

Parallel imaging techniques also benefit. All parallel imaging methods require a sacrifice of SNR, because they allow image creation from fewer acquired data samples. Increased SNR at 3.0T mitigates this SNR loss from parallel imaging.

T1 relaxation times get longer
…requiring sequence parameter changes.

This may seem counterintuitive—you might at first think that a stronger magnetic field would “pull” the net magnetisation vector of any spin isochromat back to alignment with the external magnetic field more quickly—but this classical picture does not help us here. In fact the T1s usually get longer—slower regrowth of the net magnetisation vector in the z-direction. This has to do with the number of resonant protons which are available to transfer energy to the “lattice”, which depends on field strength. You can read more about this in the main section of ReviseMRI.com.

Longer T1 times of tissues means that pulse sequence parameter settings from lower field strengths may not simply be copied over to a 3.0T magnet. The slower recovery of longitudinal magnetisation usually means that a longer TR is required to maintain expected contrast between tissues. This change in TR has consequences on other parameters and metrics such as scan time and coverage. Similarly, preparation-pulse delay times require modification.

Chemical shift increases
…which is both good and bad.

In the frequency-encode direction, the MRI scanner uses the (precessional) frequency of the MR signal to indicate spatial position in the frequency encoding direction. The different electron (i.e. chemical) environments of molecules in which resonant protons reside can shield (or deshield) the external magnetic field. If protons experience changing magnetic fields, their frequency of precession will change (cf. the Larmor equation). This is chemical shift. Since protons in water in organs and muscle resonate at a slightly different frequency than that of protons in lipids (i.e. fat), the MRI scanner will interpret the frequency difference as a spatial (positional) difference, when fat and water signals are in fact from the same voxel. The frequency shift is approximately 3.5 parts-per-million (ppm) which (according to the Larmor equation) is

  • 1.5(T)*42.56(MHz T-1)*3.5*10-6 = 223 Hz at 1.5T, or
  • 3(T)*42.56(MHz T-1)*3.5*10-6 = 445 Hz at 3.0T.

The chemical shifts between water and lipids are actually in a range of 3.3 to 3.5 ppm since chemical shifts can also be affected by temperature and pH. Fat and water are in phase immediately after an excitation pulse, then they go out of phase, and then 1/223 seconds later (4.5 ms) they’re in phase again (for 1.5 T). For 3.0 T, it’s 1/445 seconds after the excitation pulse (2.2 ms). Thus the in-phase and out-of-phase echo times vary according to field strength.

So, what are the consequences of going to 3.0T?
Chemical shift artefact: Increased chemical shift causes increased chemical shift artefact (which occurs in the frequency encoding direction only, except in EPI-based readouts). An increase in receiver bandwidth (rBW) will reduce the artefact (since chemical shift ∝ 1/rBW ), but with a sacrifice of SNR (because SNR ∝ 1/√rBW). On some scanners rBW can be increased directly, on others in can be increased by decreasing the water-fat shift (WFS) value.

Phase cancellation (black boundary) artefact, which occurs in both the frequency and the phase encoding directions, will occur at different echo times compared to 1.5 T:

  • 1.5 T
    • in-phase TEs: 0, 4.5, 9.0, 13.5… (ms)
    • out-of-phase TEs: 0, 2.2, 6.7, 11.2… (ms)
  • 3.0 T
    • in-phase TEs: 0, 2.3, 4.5, 6.7… (ms)
    • out-of-phase TEs: 0, 1.1, 3.4, 5.6… (ms)

Spectral fat suppression is more effective at 3.0T because the water resonant peak and the (main) fat resonant peak are more separated. Applying an RF saturation pulse with a transmit bandwidth covering the fat resonance peak only is more easily achieved. A similar argument may be made for water-only excitation. However, note that at large fields-of-view (FOV), conventional (STIR-type) fat suppression is more efficient, because a larger FOV contains a larger B0 inhomogeneity range, and so a spectrally-selective RF pulse doesn’t work so well.
In addition, slice-selective gradient reversal techniques (SSGR) become feasible for fat suppression in spin-echo based pulsed sequences.

Spectroscopy is more effective at 3.0T, due to the greater separation of spectra of different resonant species (choline, creatine, lactate etc), and because of higher SNR. Smaller voxel sizes are achievable, decreasing partial volume effects.

Magnetohydrodynamic effects increase
…but you can forget about them.

When a conductor moves within a magnetic field (B), an electric potential (V) is generated across the conductor. This effect occurs within moving tissue and within flowing blood, most notably in the aorta. The effect of the voltage produced across a vessel containing flowing blood is the magnetohydrodynamic effect.

At 3.0T the consequence of the magnetohydrodynamic effect is similar, but greater in magnitude, to the consequence at 1.5T; the electrocardiogram (ECG) trace becomes non-diagnostic because of an artificially elevated T-wave. However, the vectorcardiogram (VCG) adequately solves the problem of improper triggering from the elevated T-wave instead of the QRS peak, at both 1.5T and at 3.0T.

Even if the magnetic field strength were as high as 4.0T, the voltage generated by the magnetohydrodynamic effect would still be limited to below 40mV (approximate threshold for cardiac depolarisation). (Use, for example, vessel diameter d=1.6cm, average velocity v=42cm/s, in the equation V = dvBsinθ.) Theoretically, the magnetohydrodynamic effect could retard flowing blood, and produce a rise in blood pressure, but the flow reduction would be at most a few percent at field strengths as high as 5.0T.

Dielectric effects (so-called) increase
…causing signal loss, unless you have a RF-transmit system with multiple fully-independent sources

Signal uniformity problems have been observed on conventional 3.0T MR systems, particularly in applications such as breast imaging, imaging of large patients of certain shapes, patients with ascites, and can be observed to a lesser extent in many other imaging applications. The “shading” artefact which is seen comes primarily from a standing wave effect in which travelling waves from multiple coils/elements interfere. These multiple elements are the rungs of the integrated (birdcage) body coil, which is normally used for RF transmission. As a result, a non-uniform B1 field exists in the body. This means the flip angle varies across the anatomy, and signal variations are the result. Dielectric resonance also plays a small part, in which a wave interferes with its reflection from a boundary, but high physiological electrical conductivity levels ensure the role of dielelectric resonance is minor (though it may be observed in phantoms).

A solution to this prominent shading artefact is to independently control the transmit elements; a combination of different B1 fields allows adjustment of the overall B1 field in the patient. This is called RF shimming, and requires multiple independent transmit sources. Four degrees of freedom for each source (waveform, frequency, amplitude, phase) allow a lot of flexibility in obtaining optimum B1 uniformity. RF shimming should be performed on a per-patient basis, and generally reduces SAR, which can be used to enable faster scanning when protocols are SAR-limited.

SAR (specific [energy] absorption rate) goes up
…which can restrict some sequences.

The energy required to tip spin isochromats is negligible compared to the energy that simply dissipates as heat. The International Electrotechnical Commission (IEC) has issued guidelines for safe MRI, to reduce the risk of thermoregulatory distress or local tissue damage. Limits are stated in Watts of RF power per kilogram of tissue. These limits impose a specific absorption rate of RF energy, to limit heating effects. Separate limits are stated for the whole body, and averaged over the head, and in any one gram of tissue. In the clinical range of magnetic field strengths (0.2T to 3.0T), each doubling of B0 produces a four-fold increase in SAR.

SAR limitations can necessitate longer TR times, or poorer coverage, or longer RF pulse durations, or lower flip angles, or some combination of the above. However, a number of SAR management features are applied in recent MRI magnets, in order to maintain pulse sequence parameters and image quality. These include: optimised body coil design; a priori knowledge of energy deposition throughout the body; anatomy-specific dynamic SAR limits; independent RF sensing hardware for feedback control; automatic protocol optimisation for each patient; parallel imaging to reduce the number of RF excitations; modulated refocusing pulses (flip angle sweeps) in turbo spin-echo echo-trains; and most recently, multiple-transmit RF-system architecture which generally reduces energy deposition hotspots (which are often the limiting factor).

Attraction and torque (and Lenz effect) forces increase
…the usual safety procedures are followed.

Attraction is the pulling force that draws ferromagnetic objects into the bore of the MR magnet. It can make ferromagnetic objects into projectiles, which can produce injury or death to a patient in the scanner bore. Even a paper clip has a terminal velocity within the bore of 40mph at 1.5T, and 60mph at 3.0T.

There are no special consequences for 3.0T compared with 1.5T; continued strict and careful management of the MR unit should minimise associated risk. Positive, documented evidence of safety and/or compatibility of all equipment and devices for the field strength used must be obtained as usual, and the implementation of MR safety should be documented.

Torque is the twisting force which tries to align a ferromagnetic object along magnetic field lines. It is at a maximum at the centre of the imaging volume. It is significant for materials of high magnetic susceptibility, e.g. ferromagnetic materials. Torque is largely shape dependent, and may be more significant than the attractive force. For example, a 1cm needle shaped object will experience a twisting force up to 90 times the attractive force.

Implant contraindications may be more restrictive at 3.0T, and testing is required at that field strength. As usual, positive documented evidence must be obtained that an implant is safe for the field strength used.

The Lenz effect describes a force opposing the motion of an electrical conductor moving in a magnetic field. It may be significant for certain patients with artifical heart valves. You can read more about the Lenz effect in a previous blog-post.
Faster magnetic field changes cause a stronger Lenz effect. Thus, careful observation of at-risk patients whilst moving them into a stronger magnetic field is prudent.

Considering occupational exposure
…an increase in mild, transient sensory effects may occur, no evidence of long term effects is reported.

Long term effects.There is no evidence for cumulative or long-term effects of exposure to magnetic fields up to 4T. Time-averaged static-field exposure limits are not likely to be exceeded; in fact exposure is more like 100 times below recommended exposure limits set in the UK, which are based on ICNIRP guidelines.

Short term effects. Movement within the magnetic field at 3.0T (as opposed to 1.5T) may yield an increase in mild, transient sensory effects such as vertigo, nausea, magnetophosphenes, and taste sensations. Magnetic-field related vertigo results from both magnetic susceptibility differences between vestibular organs and surrounding fluid, and induced currents acting on the vestibular hair cells. Interestingly, it has been shown that the perception of dizziness is not necessarily related to a high value of the rate of change of magnetic field. Magnetophosphenes are not a practical problem for MRI since they are rarely reported for normal MRI exposures even up to 7T. Perception of metallic taste (the electrogustatory effect) depends on direction and rate of head motion, and the threshold for perception of metallic taste varies from one person to the next (and does not depend on the presence of metallic tooth-fillings).

The threshold for minor changes in heart rate, blood pressure changes, and induction of ectopic heart beats, is thought (by the World Health Organisation) to be in excess of 8T. Any effect observed at 3.0T is within the range of normal physiology.

When a conductor moves within a magnetic field (B), an electric potential (V) is generated across the conductor. This effect occurs within moving tissue and within flowing blood, most notably in the aorta. The effect of the voltage produced across a vessel containing flowing blood is the magnetohydrodynamic effect.

At 3.0T the consequence of the magnetohydrodynamic effect is similar, but greater in magnitude, to the consequence at 1.5T; the electrocardiogram (ECG) trace becomes non-diagnostic because of an artificially elevated T-wave. However, the vectorcardiogram (VCG) adequately solves the problem of improper triggering from the elevated T-wave instead of the QRS peak, at both 1.5T and at 3.0T.

Even if the magnetic field strength were as high as 4.0T, the voltage generated by the magnetohydrodynamic effect would still be limited to below 40mV (approximate threshold for cardiac depolarisation). (Use, for example, vessel diameter d=1.6cm, average velocity v=42cm/s, in the equation V = dvBsin

Apologies to the California Milk Processor Board