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Current carrying coils designed to produce a desired magnetic field gradient (so that the magnetic field will be stronger in some locations than others).
Proper design of the size and configuration of the coils is necessary to produce a controlled and uniform gradient. Three paired orthogonal current-carrying coils located within the magnet that are designed to produce desired gradient magnetic fields, which collectively and sequentially are superimposed on the main magnetic field (B0) so that selective spatial excitation of the imaging volume can occur.
Gradients are also used to apply reversal pulses in some fast imaging techniques. Gradient coils in general vary the main magnetic field, so that each signal can be related to an exact location. The gradient coil configuration for the z-axis consists of e.g., Helmholtz pair coils, and of paired saddle coils for the x- and y-axis.

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Further Reading:
Magnetic Field Simulation of Golay Coil(.pdf)
2008   by    
Electrical eddy currents in the human body: MRI scans and medical implants
Thursday, 26 August 1999   by    
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Shielded Gradient CoilsInfoSheet: - Coils - 
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Current-carrying gradient coils with reduced gradient fringe field inside of the magnet cryostat structures like cryoshields and He-vessel. The shielding can be accomplished by secondary actively driven coils or by passive screens, which are inductively coupled to the gradient coils. In both cases eddy currents outside of the gradient system will be reduced.
See also Eddy Currents.

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Faraday's Law
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DeviceForum -
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Types of Magnets, 
Magnetic resonance imaging (MRI) is based on the magnetic resonance phenomenon, and is used for medical diagnostic imaging since ca. 1977 (see also MRI History).
The first developed MRI devices were constructed as long narrow tunnels. In the meantime the magnets became shorter and wider. In addition to this short bore magnet design, open MRI machines were created. MRI machines with open design have commonly either horizontal or vertical opposite installed magnets and obtain more space and air around the patient during the MRI test.
The basic hardware components of all MRI systems are the magnet, producing a stable and very intense magnetic field, the gradient coils, creating a variable field and radio frequency (RF) coils which are used to transmit energy and to encode spatial positioning. A computer controls the MRI scanning operation and processes the information.
The range of used field strengths for medical imaging is from 0.15 to 3 T. The open MRI magnets have usually field strength in the range 0.2 Tesla to 0.35 Tesla. The higher field MRI devices are commonly solenoid with short bore superconducting magnets, which provide homogeneous fields of high stability.
There are this different types of magnets:
Resistive Magnet
Permanent Magnet
Superconducting Magnet
The majority of superconductive magnets are based on niobium-titanium (NbTi) alloys, which are very reliable and require extremely uniform fields and extreme stability over time, but require a liquid helium cryogenic system to keep the conductors at approximately 4.2 Kelvin (-268.8 Celsius). To maintain this temperature the magnet is enclosed and cooled by a cryogen containing liquid helium (sometimes also nitrogen).
The gradient coils are required to produce a linear variation in field along one direction, and to have high efficiency, low inductance and low resistance, in order to minimize the current requirements and heat deposition. A Maxwell coil usually produces linear variation in field along the z-axis; in the other two axes it is best done using a saddle coil, such as the Golay coil.
The radio frequency coils used to excite the nuclei fall into two main categories; surface coils and volume coils. The essential element for spatial encoding, the gradient coil sub-system of the MRI scanner is responsible for the encoding of specialized contrast such as flow information, diffusion information, and modulation of magnetization for spatial tagging.
An analog to digital converter turns the nuclear magnetic resonance signal to a digital signal. The digital signal is then sent to an image processor for Fourier transformation and the image of the MRI scan is displayed on a monitor.

For Ultrasound Imaging (USI) see Ultrasound Machine at

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Radiology-tip.comGamma Camera,  Linear Accelerator
Radiology-tip.comUltrasound Machine,  Real-Time Scanner

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Kyoto University and Canon reduce cost of MRI scanner to one tenth
Monday, 11 January 2016   by    
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Thursday, 30 April 2015   by    
Chemists develop MRI technique for peeking inside battery-like devices
Friday, 1 August 2014   by    
New devices doubles down to detect and map brain signals
Monday, 23 July 2012   by    
Searchterm 'Gradient Coil' was also found in the following services: 
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Acoustic Noise
Vibrations of the gradient coil support structure create sound waves. These are caused by the interactions of the magnetic field created by pulses of the current through the gradient coil with the main magnetic field in a manner similar to a loudspeaker coil. The sounds made by the scanner vary in volume and tone with the type of procedure being performed.
Sound pressure is reported on a logarithmic scale called sound-pressure level, expressed in decibel (dB) referenced to the weakest audible 1 000 Hz sound pressure of 2*10-5 pascal (20 micropascal). Sound level meters contain filters that simulate the ear's frequency response. The most commonly used filter provides what is called 'A' weighting, with the letter 'A' appended to the dB units, i.e. dBA.
MRI system noise levels increase with field strength. Disposable earplugs and/or headphones for the patient are recommended in high-field systems. Noise-canceling systems and special earphones are available, and active acoustic control systems were developed, e.g. softtone, pianissimo. A sequence with low noise gradient pulses is also called 'whisper sequence'.
See also Phon and Decibel.

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Further Reading:
MRI Noise in Utero Not Harmful for Baby's Ears
Tuesday, 28 September 2010   by    
  News & More:
Noise from Magnetic Resonance Imaging Can Have Short-Term Impact on Hearing
Thursday, 22 February 2018   by    
Echo Planar Imaging at 4 Tesla With Minimum Acoustic Noise(.pdf)
GE Healthcare unveils Silent Scan MRI for patient comfort
Friday, 25 October 2013   by    
Sunday, 29 November 2009   by    
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CoilForum -
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A coil consists of one or more loops of conductive wire, looped around the core of the coil. Coils are part of the hardware of MRI machines and are used to create a magnetic field or to detect a changing magnetic field by voltage induced in the wire. A coil is usually a physically small antenna.
The perfect coil produces a uniform magnetic field without significant radiation.
Different types of MRI coils are used in MR systems:
Gradient coils are used to produce controlled variations in the main magnetic field (B0) to provide spatial localization of the signals and to apply reversal pulses in some imaging techniques.
MR imaging radio frequency coils to receive and/or transmit the RF signal.
Shim coils provide auxiliary magnetic fields in order to compensate for inhomogeneities in the main magnetic field of the MRI machine.
See also Gradient Coil, Radio Frequency Coil, Hardware and Coil Loading.

See also the related poll result: '3rd party coils are better than the original manufacturer coils'

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Further Reading:
PET Attenuation Correction for Rigid MR Tx/Rx Coils From 176Lu Background Activity
Tuesday, 6 February 2018   by    
Magnetic Field Simulation of Golay Coil(.pdf)
2008   by    
  News & More:
High-field MRI Coils that work, superbly, even at 750 MHz
New superconducting coil improves MRI performance
Wednesday, 20 July 2016   by    
First European fully superconductive coil reaching a magnetic field of 25 Tesla produced
Wednesday, 8 June 2016   by    
November 2014 New Products: Wearable coil facilitates positioning during prostate MRI
Monday, 10 November 2014   by    
ScanMed Introduces a Groundbreaking New Orbit and Mandible Array to the MRI Market
Wednesday, 26 March 2014   by    
MRI coil optimized for imaging of infants gets FDA clearance
Tuesday, 17 April 2012   by    
Low-profile MRI coil company inching toward commercialization
Wednesday, 21 December 2011   by    
High-Resolution Uniform MR Imaging of Finger Joints Using a Dedicated RF Coil at 3 Tesla
Sunday, 31 January 2010   by    
DOTmed Industry Sector Report: MRI Coil Sales & Service Companies
Thursday, 13 December 2007   by    
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