Your workplace has just purchased a new 3T magnet and you have been asked to lead the protocol development and sequence optimisation for the new system with regard to your non-contrast musculoskeletal (MSK) protocols.
Your administrative manager has expectations that you will be able to perform twice the number of daily examinations by “…utilising the same sequences from the 1.5T system and somehow halving the scan time because the new scanner is twice as strong”. Taking into account the material that has been covered in Weeks 1-8, write an overview of the issues that will need to be addressed when tailoring your protocols and workflow to suit the new field strength system.
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Since the development of 3 Tesla MRI magnets, its use has grown from a predominant research and academic oriented market to paving its way into everyday clinical practice. The transition to a higher field strength magnet is fuelled predominately by its approximate doubling of the signal-to-noise ratio (SNR) compared to the conventionally used 1.5T MRI magnets (Bernstein et al., 2006). This increase in SNR allows for 3T MRI systems to produce images of higher quality. Additionally, the increase in SNR can also be utilised to effectively reduce scan time and improves the spatial resolution of images.
In particular, MRI using 3T field strengths have become increasing popular in the areas of musculoskeletal imaging (MSK) as the properties of the 3T systems has provided for visualisation of previously indistinct or concealed detail on 1.5T magnets. Due to the improvement in image quality, these obscured anatomical structures such as cartilage, tendons and ligaments can be clearly visualised and pathology more easily detected on 3T MRI systems (Schmitt et al., 2004). Subsequently, radiological reporting of scans is more agreeable, this ensures an increase in diagnostic accuracy and an improvement in overall patient management.
|Fig 1. FSE sequence of the knee acquired at1.5 T (a) and 3.0T (b) demonstrating the increased SNR of the image acquired at 3T compared to 1.5T. Cartilage defect along with delamination (arrow) can be appreciated more clearly on 3T (Wong et al., 2009)|
Despite the promising advantages the 3T MRI system offers, the increased magnetic field strength present challenges. In terms of clinical settings, technicians have to be exercise caution when using the 3T system to avoid compromising image quality when scanning over a shorter time frame. For MSK imaging, this particularly holds true as the lack of motion and the need for high resolution images result in protocols being a trade-off between imaging time, resolution, and SNR (Gold, 2011).
Furthermore, as the field strength is the only external factor being increased when transitioning from a 1.5T to 3T magnet, artifacts previously accounted for at 1.5T also appear at 3T, however more intensely. Artifacts affect image quality as they can mimic or hinder the visibility of pathology present, resulting in incorrect diagnosis (Wilfred & Chan, 2001). Artifacts on MR images arise from a variety of reasons. This includes intrinsic differences in tissue relaxation times, sensitivity to magnetic susceptibility and chemical shift difference between fat and water. Other major issues which arise with increasing field strength are that of specific absorption rate (SAR) and overall safety and screening criteria of patients.
Understanding the artifacts and why they arise is crucial for protocol development and design. With careful adjustment of the imaging protocols when transitioning from a 1.5T to 3T system, these artifacts can be minimised to optimise imaging and overall workflow.
Tissue Relaxation times:
According to the Larmor equation (ω0 = γ · B0,) the resonance frequency of protons (ω0 ) increases proportionally as the main magnetic field strength(B0 )increases. At 3T, the resonance frequency of the excited spins increases from 64mHz at 1.5T to 128mHz. This higher frequency of the spins reduces the efficiency of energy transfer resulting in longer T1 relaxation times. In MSK tissues, measurements of relaxation times have shown a decrease in T2 of about 10% and increase in T1 of about 15-20% (Gold, 2011). The changes in these parameters from a 1.5T to a 3T system impact user selection of appropriate TR (repetition time) and TE (echo time) which ultimately have implications on the acquisition time, contrast and SNR of the image produced.
At 3T, since the T1 relaxation time has increased, the TR should be extended so as to achieve the same type of contrast seen on the 1.5T system. In gradient echo sequences, the flip angle should be lower to account for the increased T1 relaxation times. Increasing the TR alone can lead to longer scan times which lead to an in increase in patient motion artifacts. These factors contribute to an overall decrease in workflow productivity (Soher et al., 2007). Thus if this option was utilised on a 3T, scans cannot be performed at half the time in comparison to a 1.5T scanner.
To counteract the increase in scan time, other parameters can be adjusted such as decreasing the number of signal averages, phase encodes or echo-train length. Whilst this assists in decreasing the overall scan time it also decreases the SNR. Similarly 3D pulse sequences or parallel imaging can be used. These features maximise the SNR whilst reducing scan times.
T2* effects are also doubled at 3T versus 1.5T. This is due to an increase in field inhomogeneity because of increased tissue susceptibility effects at high field. This causes T2* to shorten significantly, changing image contrast owing to decay of transverse magnetization (T2). To account for the changes in T2 and T2* at 3T, the TE values should be reduced accordingly so as to maintain optimal contrast (Gold, 2011).
The changes in the relaxation times of tissues highlight the fact the sequence timings set for a 1.5T system cannot be used on a 3T system. The need to re-optimise pulse sequence parameters to suit the conditions of a stronger magnet is imperative in acquiring images that are of diagnostic quality at a sufficient acquisition time.
Specific Absorption Rate (SAR):
The Specific Absorption Rate (SAR) is a measure for energy deposition within the tissue of the patient’s body. SAR increases with increasing strength of the magnetic field and at 3T, the radiofrequency (RF) power for excitation at 3.0T is four times higher than at 1.5T. In addition, the shorter wavelength of the RF used at 3T results in inhomogeneous power deposition and the formation of localised “hot spots” particularly around metal implants (Soher et al., 2007). Safety concerns arise as the energy absorbed can cause patients to experience an uncomfortable sensation of warmth or heating from an increase in tissue temperature. As a result, SAR safety limits have been established to ensure the core body temperature of patients does not increase more than 1°c (Soher et al., 2007).
Sequences that may suffer from high RF power deposition include fast or turbo spin-echo sequences with multiple closely spaced 180-degree refocusing pulses or pulse trains. Additionally, larger body parts such as the spine and pelvis are more susceptible to SAR as opposed to smaller regions such as the ankle. This is because RF power deposited is a function of tissue volume excited.
Advances in sequence design can elevate the effects caused by SAR at higher magnetic field strengths. Under a 3T system, this requires decreasing the flip angle which in turn reduces the RF deposition and subsequently SAR. Generally, decreasing the flip angle by approximately 40° is within acceptable limits in regards to patient comfort and also does not significantly impact the image contrast (Tanenbaum, 2006). Additional protocol adjustments are necessary when changing from a 1.5T to a 3T magnet so that SAR is reduced and similar imaging results are produced with the same ease of workflow. This includes increasing the TR to allow enough cooling time before the next excitation pulse and by decreasing the number of 180° RF pulses per repetition. Unfortunately, the adjustments of these parameters cause an increase in scan time and so sequences performed at 3T cannot be expected to be completed in half the time of a 1.5T system.
To further alleviate SAR issues, parallel imaging technique can be utilised with the aid of phased array coils. This technique is based off the system knowing the placement of receiver coils used which reduces the number of phase-encoding steps required during image acquisition. Parallel imaging ultimately allows for a decrease in the overall scan time and also the number of RF pulses transmitted to the patient, decreasing effects attributed to SAR. In a clinical setting this option is not always practical as it causes a decrease in overall SNR (Tanenbaum, 2006). This could prove especially problematic for MSK where contrast and providing images of high resolution is essential.
Chemical Shift :
Chemical shift artifact of fat pixels in the frequency encoding direction is twice as great, at a fixed imaging bandwidth, on a 3.0T system compared to a 1.5T system. This is because the difference in resonance frequency between fat and water is directly proportional to the main magnetic field strength. This type of artifact typically appears as bands of high or low signal intensities and is commonly seen around water-containing structures surrounded by fat, such as vertebral end plates, lipomas and cysts (Wilfred & Chan, 2001).
For imaging, particularly in MSK it is important that chemical shift artifact is minimised as occurrence can lead to incorrect or missed diagnosis of pathology. To counteract this artifact, the receiver bandwidth can be increased to double at 3T compared to the corresponding 1.5T protocol. Increasing the bandwidth causes the number of frequencies sampled per pixel to increase. If there is a shift in frequencies because of chemical shift, fewer pixels are involved and there is a decrease in the visible size of the chemical shift. This technique however causes a reduction in SNR. Another option is to repeat the MR pulse sequence with chemical shift fat saturation, inversion nulling, or water excitation, which will reduce chemical shift artifacts and allow imaging at the lower bandwidth without any loss of SNR.
BW of ±15 kHz BW of ±30 kHz
|Fig 2. Chemical shift artifact at 3T. (a) A small BW results in a visible chemical shift artifact (b) Increasing the receiver BW reduces the artifact (Bernstein et al., 2006)|
Alternatively, a clinical advantage of an increase in chemical shift between fat and water at 3T is the ability to acquire better fat suppression sequences compared to a 1.5T MR system. Because fat and water resonates at twice the frequency on 3T compared to 1.5T, more complete suppression of the fat peak, without saturation of the water peak can easily be achieved. The time per slice spent in fat saturation at 3.0T during a multi-slice acquisition is less than at 1.5T. This means that if fat saturation is applied under the influence of 3T, more slices can be acquired at a given TR, slice thickness and bandwidth compared to 1.5T. The use of fat water separation methods is important in musculoskeletal imaging due to its ability to provide excellent separation in areas of poor field homogeneity. It also allows for accurate diagnosis of bone marrow pathology and soft tissue inflammation (Gold, 2011). However, the fat suppression at 3T can also be too complete and lead to the loss of anatomic margins between fat and hypo-intense tissues such as tendons and ligaments (Mosher, 2006).
Therefore, MRI technicians are required to have an understanding for the need to balance scanning parameters and have an awareness of the trade-offs associated when varying these parameters to meet clinical requirements.
Susceptibility variation near the interface of materials of different magnetic susceptibility, such as bone-soft tissue or air-tissue and metallic implantscan cause local non-uniformity of the main magnetic field. This can result in a variety of artifacts on images such as image distortion, signal loss and focal areas of high or low signal intensity ( Bernstein et al., 2006). Susceptibility artifacts increase with the main magnetic field strength, and are slightly larger at 3T compared with standard 1.5T MR imaging. Therefore suppression of these artifacts needs to be accounted for in protocol design as it is possible that enlarged susceptibility artifacts may obscure important findings at 3T MR imaging that may have previously been visualised on 1.5T MR imaging.
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A common 3.0T artifact is incomplete inversion of the magnetisation in regions of rapid susceptibility variation. Many protocols at both 3T and 1.5T make use of inversion recovery (IR) pulses (eg, Fluid Attenuation Inversion Recovery [FLAIR]) to achieve desired contrasts or the suppression of fat signal.E-mail address:firstname.lastname@example.orgMayo Clinic College of Medicine, Rochester, Minnesota, USA
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Gradient echo (GE) and echo-planar sequences are mostly affected by susceptibility artifacts as they do not have 180° refocusing pulses and have long echo times, which mean there is more time for the protons to de-phase and acquire different precession frequencies (Wilfred & Chan, 2001). In MSK these sequences are used for cartilage and meniscal segmentation and the artifact produced can cause bone to appear larger and adjacent soft tissue smaller.
There are a variety of techniques which can be used to ensure susceptibility artifact occurrence is minimised. The readout direction can be changed to alter the location of the artifact so that it does not obscure the region of interest. Careful shimming of the main magnetic field can be performed to even out any variations in the magnetic field.Corresponding Author Additionally decreasing voxel size can be an effective means of decreasing susceptibility artifact. This is achieved by increasing the spatial resolution and decreasing slice thickness. This reduces the amount of intra-voxel dephasing with metal hardware. Consequently the SNR is reduced, however due to the increased SNR proficiencies associated with a 3T system; images are still comparable to those obtained under a 1.5T system (Ramnath, 2006).
When transitioning from a 1.5T to a 3T magnet, new equipment, more rigorous site planning, and more stringent safety measure is imperative to support the higher field system in a safe manner. Safety issues associated with the static, gradient and RF field all need to be revised and accounted for to suit the conditions associated with a 3T magnet.
Increasing SAR issues associated with 3T magnet calls for the need for an updated screening method via implementation of a newly revised patient questionnaire is imperative. This is because previously safe implants on a 1.5T may be contraindicated on a 3T system. Additionally, safety information from manufacturers is needed for safe operating of implants to prevent effects of torque on implants within the patient’s body. ( Jerrolds & Keene, 2009)
With the safety difference between the two systems, work flow may be hindered which in turn makes a facility less productive due to implementation of stricter safety guidelines.
Magnetic Resonance Imaging at 3T provides a powerful non-ionising medical imaging method for visualising connective tissues within the body. However, trade-off between producing images of high resolution and increased SNR whilst reducing the acquisition time of scans remains a limiting factor. Transitioning from a 1.5T magnet to 3T magnets encompasses pitfalls which hinders the possibility of performing scans at half the time to that of a 1.5T magnet. These pitfalls need to be accounted for when designing appropriate imaging parameter and pulse sequence protocols so as to minimise the increase in artifacts, SAR depositions and maintain an MRI safe environment, ensuring an efficiency of workflow.
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