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1.5T versus 3T MRI

 

Did you know there are different types of MRI’s? Many people are unaware of this and do not understand the differences between a “1.5T MRI” and a “3T MRI”.  You might have heard those terms on the news, in a commercial, or mentioned by your doctor. You may have even read about them online but are still unsure about the differences. The following will explain what the differences are in a practical way, so if you ever need an MRI you will understand which magnet is best for you and why. 

 

The “T” in 1.5T and 3T stands for Tesla. Tesla is defined as the unit of measurement used to describe the strength of the magnet used in an MRI. The magnet is the M in MRI (Magnetic Resonance Imaging).  This magnet is the basis of how images in MRI are acquired.  The strength of the magnet directly affects the quality of those images, however there are several other factors that determine which magnetic strength is best suited for the person being imaged and for the or specific body part being scanned or imaged.

 

So how does the strength of the magnet affect images?  While the body is in an MRI, the cells in one’s body give off what we call a "signal." The strength of the MRI magnet is directly related to the amount of signal that is received from one’s body (technical people call it NMV – Net Magnetization Vector).  The signal from the body is what is used to create the images. So the higher the magnetic field strength, the more signal can be captured from your body by the MR scanner; however, more is not always better. Everything comes with pros and cons, but before I discuss what these are, it’s important to have a basic understanding of the factors (outside of magnet strength) that can affect an image.

 

The primary factor affecting image quality relative to magnet strength is that not every human has the same exact body. Most bodies are composed of approximately 60% water, some fat, muscle, and organs. Unfortunately, one’s body composition can change over time.

 

Additionally, if a physician surgically places anything into the body (e.g., a joint replacement) or if one had an injury caused by a foreign object (e.g., bullet or shrapnel), those items change the body’s composition,  and therefore changing the type of signal the body will give when placed into a magnetic field. There are two things that can be factors with body composition:, safety and image artifact. All implants must be tested for safety before being allowed near the MR scanner because of the magnetic field strength. After implants are tested they are given a status. The status of an implant is as follows:Implant status could be categorized as safe, unsafe, or conditional. Safe means indicates the implant is always safe to go into any strength of magnetic field. Unsafe whereas unsafe means indicates the implant can never go into a magnetic field. Conditional means indicates the implanted device has been tested and is safe only in the named strength of magnetic field and only with particular conditions listed with the manufacturer.

 

This means, that some implants that are safe to go into a 1.5T MR scanner may not be safe to go into a 3T scanner. When dealing with foreign bodies or shrapnel it is important to know the type of material in the body to determine if it will be a safety issue. Implants and foreign bodies can also cause an error in the image which is commonly referred to as “image artifact.” The type of material the object is made of should be discussed with the MR technologist. As the strength of the magnet increases these artifacts become more pronounced and cause more issues when imaging the area around them.

 

Another consideration is that not all body parts are made up of the same types of tissue.  The prostate does not have the same composition as blood vessels, bones, or other organs.  Therefore, some organs are better imaged with different strengths of scanners.

 

Let’s consider the pros and cons of a 3T scanner because once one understands the strengths and weaknesses, one will understand why a 1.5T MR scanner may be a better choice than a 3T MR .scanner in some situations.  In a perfect world, the stronger magnet (3T) would generally give the best images; however, with the previously listed considerations in mind, a 1.5T scanner is often a better choice.

 

The most obvious advantage of a 3T scanner is the stronger signal the magnet produces. As mentioned before, the stronger the magnetic field is, the more signal it will receive from body tissue.  This stronger signal allows greater spatial resolution and increased contrast detection (i.e., it produces a higher resolution image). In theory, resolution images will be better wWith increased contrast and resolution images, in theory, will be better. Howevercontrast. However, this increase in signal from one’s body can cause artifacts on the images when one is scanned in a 3T scanner that may not be present when one is scanned in a 1.5T scanner due to the lower magnet strength.  The artifacts that occur due to the stronger magnetic field present in a 3T scanner are: susceptibility artifacts, chemical shift artifacts, or dielectric resonance affects. These are explained below.

 

Susceptibility is the ratio of magnetization to the magnetizing force. Susceptibility artifacts are due to the differences in the magnetic susceptibility of tissues and materials that are inside one’s body. It is especially an issue around metal objects and implants in areas that have air-tissue and air-bone interfaces. Consequently, implants in the body can cause images to have mis-registration, distortions, or blacked out areas. Because these types of artifacts are worse in high field scanners, patients with foreign bodies or implants typically are not scanned in a 3T scanner. Susceptibility artifact can still occur in images from a 1.5T scanner, but they are less pronounced and the images acquired are still very diagnostic.

 

What does this means for you? If one has an artificial joint or implant especially in the area to be imaged, one should be scanned in a 1.5T MRI or lower for those reasons listed above.

 

Chemical shift is a little confusing if without a basic understanding of the physics of MRI. When a body is placed into an MRI scanner, the Hydrogen protons in the body resonate (vibrate) to a specific frequency (hence the Resonance in MRI). This frequency will be slightly different in every part of the body (i.e., muscle, fat, water, blood, bones). Chemical shift is the spatial displacement of water and fat due to those differences in frequency. This artifact is not typically noticeable in a 1.5T scanner but doubles when seen in the 3T scanner.

 

What does this means to you? Due to increased chemical shift, which is increased in the 3T scanner, good, diagnostic high quality and diagnostic images of specific body parts  may be more easily acquired for specific body parts when scanned in other than a 3T MRI scanner. For example, if an institution has both a 1.5T scanner and a 3T scanner, examinations of breasts, coronary, and G.I. tracts are preferred to be scanned done in the 1.5T scanner. Other exams such as orthopedic, neurologic, and vascular imaging are best in the 3T (given there are no other contraindications, as discussed earlier regarding such things as implants and foreign objects).

 

Dielectric affects occur due to the radio frequency field (RF-field) component of the MRI. While getting an MRI, one will be positioned on the MRI table with what is called a coil. This coil will be placed over the body part being imaged and will work like an antenna to receive the signal from the body. Once the body is moved into the scanner, an RF pulse will be applied. Although it will not be felt, this RF pulse is what excites the protons in the body. A dielectric effect is an interaction that can occur in certain tissues due to the electrical component of the RF-field. It is more significant in 3T imaging and is most common in brain and abdominal imaging. While newerNewer  MRI software has developed ways to compensate for this artifact, but it is still something to consider when scanning with a high field MR such as the 3T scanner (especially on an older model scanner).

 

What does this means to you? The dielectric effects can cause a dark shading artifact when imaging the brain or abdomen- especially on older scanners which makes the image less desirable from a diagnostic perspective.  In this instance, a 1.5T scanner may be a better option.  

 

Specific Absorption Rate (SAR).  SAR is the estimated rate of energy that is being absorbed by a volume of tissue when RF energy is deposited into the body during the MRI exam. This occurs in all MR scanners but will increase as the magnet strength increases. This means while SAR really isn’t an issue on a 1.5T scanner, it is an issue on a 3T scanner due to the increased magnetic field. The FDA regulates how much SAR the body can receive in a given time and those safety limits are built into the MR software so that warnings will appear when one is close prescribed limit. If the warning is ignored, most scanners will not allow the scan to proceed when those limits have been met. Fortunately, there are no known long term effects of SAR. It is not an accumulated type of energy which means that when one leaves the scanner, the “SAR level” in one’s tissue returns to zero and would start over should one need another MRI at a future time.   

What does this means to you? SAR means the body can heat up when MRI is performed.  The MR technologist will provide an emergency squeeze ball so if one feels too hot or it becomes too hot feels anyor any burning sensations occur during the scan, one can squeeze the ball to set off an alarm so that the technologist will stop the scan. This heat can occur on any MR scanner but can be more likely to happen in a 3T scanner. Just remember there are no known long term affects when it comes to SAR and it will be closely monitored throughout the scan.

 

MRI of the prostate is very unique. The location of the prostate is deep in the pelvis which requires a great deal of signal to get diagnostic images. If there are no prosthetic implants, this is easily achieved with a 3T scanner. It is also achievable in a 1.5T scanner if a high quality specialty coil designed specifically for prostates is used (such as that employed by FirstScan). However, there is one important image that is taken during the prostate scan called a Diffusion, which is very sensitive to the artifact discussed above called chemical shift. If the rectum (which is located right behind the prostate) has a large amount of gas, it will cause severe artifact in the images. In some cases, it can be to the point that the images are no good and cannot be interpreted by a radiologist. Therefore, it is very important that all the pre-scan patient preparations listed by the imaging center be followed as closely as possible.

 

In conclusion, there is no straight answer for which MRI scanner is the overall “best” scanner; rather, the best scanner really depends on what is best for the exam needed and based on one’s body composition. If you or your physician are unsure about which is best for your situation, please have your doctor consult with a radiologist or an MRI technologist.

 

References:

 

 

The National Center for Biotechnology Information:

http://www.ncbi.nlm.nih.gov/books/NBK174456/

 

MRI in Practice, 3rd Edition / Edition 3

by Catherine Westbrook, Carolyn Kaut Roth, John Talbot

 

Questions and Answers in MRI:

http://mriquestions.com/index.html

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