Specifications for Gradients
The MR sales representative is telling me about his scanner's strong gradients. How do I interpret the specification sheet?
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Key gradient specifications: Slew rate is defined as Peak gradient strength ÷ Rise time.
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Both spatial resolution and imaging speed depend upon good gradient performance, and scanners do differ in their gradient specifications. Electrical currents are pulsed on and off during imaging, and gradients typically have a trapezoidal waveform as shown in the diagram to the left from which various measurements are derived.
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The first value to look for on the specification sheet is maximum (or peak) gradient strength. This is quoted in units of millitesla per meter (mT/m). Most 1.5T to 3.0T superconducting whole body scanners have maximum gradient strengths in the range of 30-45 mT/m, while lower field (<0.5T) permanent scanners are in the 15-25 mT/m range. For the best performance concerning peak gradient strength, bigger is better.
Rise time is measured in milliseconds, and is typically in the range of 0.1-0.3 msec for most scanners. Rise time is meaningless by itself, because a scanner with very weak gradients might ramp up to full strength very quickly, but have overall poor gradient performance for imaging. What is important is not the absolute value of rise time, but rise time scaled according to the maximum achievable gradient strength. This is encapsulated in the slew rate, defined as
Rise time is measured in milliseconds, and is typically in the range of 0.1-0.3 msec for most scanners. Rise time is meaningless by itself, because a scanner with very weak gradients might ramp up to full strength very quickly, but have overall poor gradient performance for imaging. What is important is not the absolute value of rise time, but rise time scaled according to the maximum achievable gradient strength. This is encapsulated in the slew rate, defined as
Slew Rate = Peak Gradient Strength ÷ Rise Time
Slew rates are measured in units of Tesla per meter per second (T/m/s). Thus a gradient that ramps from 0 to peak amplitude of 30 mT/m in 0.5 msec would have a slew rate of 60 T/m/s.
The slew rate influences the minimum attainable TR and TE for conventional MR imaging and influences the echo spacing in fast spin echo and echo planar applications. In the current marketplace, high field superconducting scanners boast slew rates in the 150-200 T/m/s range; superconducting open scanners in the 100-120 T/m/s range; and lower field permanent scanners on the order of 50 T/m/s.
The need for strong gradients and high slew rates depends on your intended scanner use. If cardiac or brain imaging is anticipated, then powerful gradients are mandatory. If the intended scanner use is for orthopedics, however, such demanding gradients may not be required.
Caveat emptor: The typical gradient strengths and slew rates quoted above are per axis (i.e. values for the x-, y- and z-gradients individually). Some vendors, often those with somewhat inferior gradient performance, will instead quote effective gradient strengths and slew rates. Because three gradients are averaged together, the effective gradient strength and slew rates are 1.73 (=√3) times larger than the per axis values.
The slew rate influences the minimum attainable TR and TE for conventional MR imaging and influences the echo spacing in fast spin echo and echo planar applications. In the current marketplace, high field superconducting scanners boast slew rates in the 150-200 T/m/s range; superconducting open scanners in the 100-120 T/m/s range; and lower field permanent scanners on the order of 50 T/m/s.
The need for strong gradients and high slew rates depends on your intended scanner use. If cardiac or brain imaging is anticipated, then powerful gradients are mandatory. If the intended scanner use is for orthopedics, however, such demanding gradients may not be required.
Caveat emptor: The typical gradient strengths and slew rates quoted above are per axis (i.e. values for the x-, y- and z-gradients individually). Some vendors, often those with somewhat inferior gradient performance, will instead quote effective gradient strengths and slew rates. Because three gradients are averaged together, the effective gradient strength and slew rates are 1.73 (=√3) times larger than the per axis values.
An additional gradient parameter commonly measured is duty cycle. The duty cycle does not have a fixed value, but varies by pulse sequence. It represents the percent of time the gradient is able to work at maximum amplitude during that sequence. Maximum duty cycle performance of close to 100% should be expected for most modern scanners.
References
Hidalgo-Tabon SS. Theory of gradient coil design methods for magnetic resonance imaging. Concepts Mag Res Part A 2001; 36A:223-242.
Hidalgo-Tabon SS. Theory of gradient coil design methods for magnetic resonance imaging. Concepts Mag Res Part A 2001; 36A:223-242.
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