The high field advantage: making the most of 3T MRI

There are a multitude of factors that need to be considered when deciding between the

purchase and use of a 3.0 Tesla (T) or a 1.5T MRI system. Many institutions have well-

established protocols for 1.5T MRI systems and may be wary of the challenges of a higher field

strength 3T MRI system. If an institution decides to purchase a 3.0T MRI system, it is important

for technologists to play a role in maximizing the advantages gained at this field strength while

feeling confident in managing any disadvantages at a higher field strength. Many of the

challenges of a 3.0T system can be overcome by considering the MR safety of devices, protocol

optimization, utilizing the gain in the signal to noise ratio (SNR), employing methods of

reducing specific absorption rate (SAR), reducing dielectric effects, proper coil selection, and

taking advantage of reduced scan times.

Magnet safety is important at all field strengths and should be considered first and

foremost. Technologists should not assume that if a device is safe at 1.5T that it is safe at 3.0T.

Therefore, all patients should be screened appropriately, and the device should be MR safe at a

3.0T level for the patient to be admitted into the scan room.

Moving from a 1.5T system to a 3.0T system requires that MRI protocols must be

updated. It is not feasible to copy protocols that are optimized for a 1.5T system and expect for

equivalent imaging quality to be achieved at 3.0T. In order to obtain the best imaging possible,

one factor to take advantage of is the increase in the SNR at 3.0T. The increase in SNR allows

for the imaging matrix to be increased to aid in obtaining better spatial resolution; therefore,

radiologists will be better able to resolve small structures without detriment to the SNR. The

result is better Neuro and Musculoskeletal imaging. The increase in SNR also lengthens T1

relaxation time which means that for a particular repetition time (TR) the image is more T1

weighted at 3.0T.1 Increased T1 relaxation time allows for studies that use gadolinium as a

contrast agent to be performed at a reduced dose in comparison to a full dose that must be given

at 1.5T. The reduced gadolinium dosage is optimal for patients with nephrogenic systemic

fibrosis.

Another factor to consider for a 3.0T system is the increase in diamagnetic susceptibility

artifacts. This is a challenge when imaging patients with dental braces or certain ferromagnetic

implants such as surgical pins and rods that cause a signal void in the area of the metal implant.

Conversely, there is a clear advantage to increased diamagnetic susceptibility when evaluating

acute hemorrhage in stroke patients through the use of susceptibility weighted imaging (SWI).

SWI is extremely sensitive to venous blood flow. SWI has the ability to detect an increase in

deoxyhemoglobin levels in small vessels due to slow or restricted blood flow. Normally, these

vessels would not be visible. SWI shows radiologists changes in oxygen saturation and other

differences in susceptibility. Changes in oxygenation and susceptibility can show the origin of

the stroke and effected vascular areas. SWI at 3.0T allows technologists to image small

structures that would be missed in conventional imaging.

The 3.0T systems are more sensitive to chemical shift artifacts than a 1.5T system.

Chemical shift artifact is due to the different resonant frequencies of fat and water that are

excited within the same slice. This is misregistered by the Fourier transform and appears as a

bright or dark band in the frequency direction.2 Increasing the bandwidth will accommodate for

the increase in chemical shift artifacts and reduce this banding problem. In certain

circumstances, however, chemical shift at 3.0T is beneficial. MR Spectroscopy benefits from an

increase in chemical shift because it produces greater signal to noise and better spatial resolution

of individual metabolites. The metabolite peaks are separated better at 3.0T than at 1.5T which

results in more accurate metabolite identification. Clinically, this is beneficial in determining the

degree of malignancy in brain tumors, brain ischemia and infarction, types of brain trauma,

infectious diseases, and early onset of Alzheimer’s disease.3

SAR management is another important factor to consider when working with a 3.0T MRI

system. Although SAR levels are regulated on MRI systems, the SAR increases with field

strength, radio frequency (RF) power, transmit coil, and patient size. These increases in SAR

level can be overcome with the use of a longer TR, reduced number of slices, pulse sequence

order, time between sequences, and the use of gradient echoes. In an effort to reduce SAR, the

technologist can increase the TR while holding the number of slices constant. This practice will

allow a longer time period to collect data while reducing the SAR. Increasing the TR will

increase the scan time. A secondary method of reducing SAR is to reduce the number of slices

while holding the TR constant. This means that less RF pulses are necessary because there are

fewer slices. It is also important to consider the pulse sequence order. Technologists may want

to alternate between fast spin echoes and gradient echoes to reduce the effects of SAR. Fast spin

echoes use multiple 180 degree refocusing pulses that increases SAR. In gradient echo imaging,

a flip angle of 90 degrees or less is followed by a refocusing of the gradients. Therefore,

gradient echoes reduce the effects of SAR because there are not multiple 180 degree RF

refocusing pulses. Technologists who take time to speak with patients between sequences may

also help with SAR reduction. The result of exposing a human to RF irradiation is heat, and the

act of pausing between sequences to check on the patient allows time for the patient to release

the heat that is created. Patient size also affects SAR, which is proportional to the power of five

for the patient’s circumference; therefore, larger patients are not able to release the thermal load

as efficiently as smaller patients.4 Finally, the technologist can use 3D imaging with isotropic

voxels to reduce SAR. This will allow the images to be mathematically stacked and

reconstructed at any angle, and multi-planar reconstruction will allow a number of projections to

be obtained with a single data set.

The dielectric effect, or B1 field inhomogeneity, is caused by non-uniform RF

distribution. The non-uniform RF distribution causes darkened, shaded areas within the image

and is more prominent at 3.0T than at 1.5T and is more common with multi-channel coils. The

dielectric effect is particularly problematic in body imaging since certain tissues in the body

shorten the RF wavelength causing the artifact. These artifacts are seen more in obese patients,

patients with ascites, and pregnant patients.5 One way to correct for dielectric darkening and

shading is through the use of dielectric pads. This high conductivity pad is placed between the

coil and the patient over the area of interest. Some MRI system manufacturers have developed

multitransmit RF technology which nulls dielectric shading to correct for image artifact.

Proper coil selection and loading for your patient population is essential in producing

diagnostic images. The surface coil selection at 3.0T is limited for pediatric imaging at this time.

Brain imaging in infants requires technologists to be creative in coil selection because coils at

3.0T have been developed for the adult patient population. The use of surface coils or a knee

coil to maximize coil filling is necessary in order to receive the SNR that is suitable for

diagnosing hydrocephalus, tethered cord, and in discerning the developing white matter tracks of

the infant brain. Placing an infant in a large head coil significantly reduces the SNR because the

coil elements are too far from the infant’s head. The use of an integrated head and spine coil that

is designed for infants will not only give technologist the ability to scan both the head and spine

in a timely manner without changing coils, but it will provide the resolution and SNR necessary

to produce diagnostic images.6 Coils that are suitable for pediatric use are being developed and

tested by MRI coil manufactures at this time.

The final advantage that a department could gain in moving from 1.5T to 3.0T is a

reduction in scan time. Parallel imaging and phased array coils shorten scan times at 3.0T.

Radiology departments need to investigate all of the advantages and challenges when deciding

which MRI system is the best fit for their needs. Advantages include, gains in SNR, MR

spectroscopy, SWI, and faster scan times. Many of the challenges of a 3.0T system can be

overcome through protocol optimization and technologists feeling confident in their ability to

screen patients appropriately, use gains in SNR, reduce artifacts, make parameter changes within

a sequence, choose the appropriate coil, and reduce SAR. Future applications of the system

including MRA and fMRI are also important considerations.