Reading the Magnetic Spatial Gradient Map

How to Read a Magnetic Spatial Gradient Map

By Matt Rederer MBA RT(R)(MR)(CT) MRSO MRSE (MRSC)

Magnetism is obviously a very important part of MRI. Objects becoming attracted to the MRI unit are a big threat in MRI. This threat it’s caused by the very strong static magnetic field used in MRI. The MRI unit contains many components including gradient coils, RF transmitters, and other electronics. We find these components within the gantry of the MRI unit. Patients are advanced into the bore of the MRI unit and tissues that are being imaged are located in the center of the unit. This is called isocenter. All imaging in MRI occurs in this area. As we move away from isocenter, our magnetic field begins to deteriorate, and this holds no diagnostic benefits. This is referred to as the fringe field. Ferrous objects are materials that demonstrate strong attractive forces when exposed to an external magnetic field. These objects also maintain a weak magnetic field for a duration of time after leaving this external magnetic field. The properties of ferromagnetism depend on the type of magnetic material as well as its remanence and coercivity. A misconception in MRI is that ferrous objects are attracted to a magnetic source because it’s a magnetized object. The truth is that ferrous objects are attracted to a magnetic source because they are exposed to multiple magnetic fields at once.

As a ferrous object is exposed to multiple magnetic fields, the object is drawn to the stronger magnetic field. When the ferrous object reaches this new location, it is then exposed to multiple magnetic fields and is drawn to the stronger magnetic field. This process continues until the ferrous object reaches an area where the magnetic field is not changing or collides with another object. This changing magnetic field becomes an important component when evaluating a threat associated with ferrous objects in the MRI environment. We refer to this changing magnetic field as we approach a magnetic source the magnetic spatial gradient. The magnetic spatial gradient can be defined by the formula dB/dx. The “d” stands for a change in, the “B” stands for magnetic flux, and the “x” stands for distance. We can then say that the magnetic spatial gradient describes a change in magnetic field over a change in distance.

The unit of measurement used to define this is Tesla per meter (T/m) or Gauss per centimeter (G/cm). 1  T/m = 100G/cm

Implanted devices which receive MR conditional labeling must define a magnetic spatial gradient limit. To understand this, we need to understand the shape and behavior of our MRI unit. Every MRI manufacturer will provide a system manual to their users. Within this manual, many important facts and details about the MRI system can be found here. One of these is the magnetic spatial gradient map. Every MRI manufacturer will represent magnetic spatial gradient in slightly different ways. Understanding this map is crucial when attempting to meet MR conditional labeling for an implanted device.

The first obstacle in understanding this plot is to identify the view that the spatial gradient is presented in this map. For example, they may present this as a profile view or sagittal view of this system. They may represent this as a top view of this system, or a front view of the system. Once we orientate ourselves as to what we are looking at, we can then look at the X and Y axis of this map.

GE OR76 spatial gradient map

The X and Y of the graph is typically labeled as a distance. Many times, this direction is labeled in meters. This means that we are referring to the distance in the direction of the table movement. Zero on this map represents isocenter. As we move a distance away from isocenter, we can see that magnetic fields change over a distance. These are defined as T/m or G/cm

Siemens Verio spatial gradient map

To use this spatial gradient map appropriately, we need to understand the location of the implanted device in relation to isocenter. When preparing our patient, we will set a landmark as to where we want to center our imaging. This is the location which will be found at isocenter. If we fix our table, we will be sure that our patient will not move away from this location. We can then measure the distance from our landmark to the implanted device. This will help us understand how far from isocenter the implanted device will be. We can then look at our magnetic spatial gradient map and identifying the amount of attractive force will be presented on the implanted device. If we can meet the MR conditions, the patient may be scanned safely when referring to only the magnetic spatial gradient. It is important to note that the implanted device must never be exposed to a spatial gradient greater than defined MR conditions. This means that when advancing the patient into the gantry, we must not exceed this limit.

There is another term we should be familiar with when evaluating the spatial gradient. The “maximum” spatial gradient refers to the spatial gradient (dB/dx) time our field strength flux (B0). In other words, if an implant will remain at a spatial gradient of 3T/m while imaging a patient at 3T, the maximum spatial gradient would be 9T/m at this location (3T * 3T/m).

Although understanding the magnetic spatial gradient map can be tricky to understand, we can simplify the process by breaking it down in two steps. Gaining an understanding of our manufacturer plot is the first obstacle. Once we understand what we are looking at, we can identify a distance from isocenter the implanted device will be and therefore understand the level of risk associated with our magnetic spatial gradient.

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