The gradient slew rate is the speed rate of ascent or descent of a gradient from zero to its maximum amplitude, either positive or negative. Which is the amplitude divided by the rise time in msec. Measured in mT/m/msec or T/m/sec. The shorter the rise time, the faster the gradients and therefore echo spacing. Gradients with a shorter echo spacing will have a better resolution and more slices per TR.

From Hitachi Medical Systems America Inc.;
Hitachi expanded its portfolio with the Echelon™ 1.5T. The MRI scanner combines a compact magnet and a scalable 8-channel RF system with high-performance gradients and slew rate to select short echo times, small field of views, high matrices and thin slices. Standard features of the Echelon MRI system include higher-order active shim, RAPID (parallel imaging for use on brain MRI, body, cardiovascular imaging, and orthopedic coils), multiple coil ports, and an advanced reconstruction engine.

From Siemens Medical Systems;

FDA cleared and CE Mark 2011.
The Biograph mMR has a fully-integrated design for simultaneous PET/MRI imaging. The dedicated hardware includes solid-state, avalanche photodiode PET detector and adapted, PET-compatible MR coils.
The possibility of truly simultaneous operation allows the acquisition of several magnetic resonance imaging (MRI) sequences during the positron emission tomography (PET) scan, without increasing the examination time.
See also Hybrid Imaging.

(MRI-CA, MRCA) The noninvasive imaging of the coronary arteries using magnetic resonance imaging of the heart.
For cardiac MRI-CA, high performance machines are necessary with minimum 40mT/m and 300μsec slew rate.
2D and 3D acquisition are used for fast gradient echo sequences with techniques for minimizing cardiac and respiratory motion and suppressing the high signal of pericardial fat. The optimal sequences seem to be trueFISP, Balanced FFE or FIESTA with SMASH and SENSE techniques. Respiratory motion is minimized for 3D acquisitions by using respiratory gating, especially using navigator echoes (Navigator Technique) to track diaphragmatic and cardiac movement. Optimization of MR technique can provide mapping of long segments of the coronary arteries.
Blood pool agents are being applied to improve the reliability of coronary MR angiography. The major current clinical indication is the identification of coronary artery anomalies because the diagnostic accuracy's for identifying haemodynamically significant stenoses are variable depending of the image quality.

See also Magnetic Resonance Angiography, and Cardiac MRI.

The word gradient (from grade) means the inclination of a surface along a given direction. In MRI, gradient stands for gradient field and/or gradient coil. Inside the main magnet are three gradient coils located, which produce the desired gradient (magnetic) fields. These fields are used to alter (collectively and sequentially) the influence of the static magnetic field B0 on the imaged object by inc- or decreasing the field strength and changing the direction. Through this influence selective spatial excitation and spatial encoding (each voxel resonate at a different frequency) is possible. Gradients are also utilized in another way for fast imaging sequences.

See also Slew Rate and Duty Cycle.