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The Evolution of Magnetic Resonance Imaging: 3T MRI in Clinical Applications
Terry Duggan-Jahns, RT(R)(CT)(MR)(M)
*Manager, Outpatient Diagnostic Imaging, St. Joseph Medical Center, Tacoma, Washington.
Address correspondence to: Terry Duggan-Jahns, RT(R)(CT)(MR)(M), Manager, Outpatient Diagnostic Imaging, St. Joseph Medical Center, 1717 South J Street, Tacoma, WA 98405. E-mail: firstname.lastname@example.org.
Disclosure Statement: Ms Duggan-Jahns reports having no significant financial or advisory relationships with corporate organizations related to this activity.
Until recently, 3 Tesla magnetic resonance imaging (3T MRI) was only used in research applications. However, as MRI technology evolves, 3T MRI studies (as opposed to 1.5T) are increasingly common in the clinical setting. The higher field strength of 3T MRI results in an increase in signal-to-noise ratio, spatial resolution, and speed, all of which may provide substantial benefits. However, radiologists familiar with 3T MRI have cited several limitations to the increased field strength, such as a greater amount of noise, imaging contrast issues, and safety concerns. This article will discuss the present challenges, benefits, and limitations of 3T MRI in the clinical setting in specific clinical applications, including imaging studies of the brain, spine, chest, abdomen, pelvis, extremities, cardiac system, vascular system, and breast.
Magnetic resonance imaging (MRI) is an imaging technique that uses a magnetic field and radio waves to image the body. The MRI modality differs from X-ray imaging because MRI does not use iodizing radiation to produce images. The advent of MRI technology has resulted in considerable medical advances, because clinicians have been able to arrive at more precise diagnoses and provide more focused disease management in many therapeutic areas, including orthopedics, oncology, and neurology. The imaging field is constantly advancing, and radiologists may soon have the option to switch from traditional MRI machines to those that offer greater field strength. As the availability of the stronger 3 Tesla (3T) MRI appears on the horizon, radiologists are faced with the process of weighing the pros and cons of adopting this newer technology. This article will review the benefits and limitations of using 3T MRI in the clinical setting, addressing important issues in specific clinical applications.
A Brief History of MRI
The MRI modality is based on a physical phenomenon called nuclear magnetic resonance (NMR), which was discovered in 1931 by Isidor Rabi and his colleagues.1 The NMR phenomenon is observed when a substance is placed in a magnetic field and radio waves are applied. As a result of this process, the atoms of the substance will emit tiny, detectable radio signals.2 The strength of the magnetic field is measured in the unit referred to as the Tesla (T). Today's clinical MRI scanners typically operate at a strength of between .35T to 3T. MRI systems in operation today are classified as either low field (.35T), mid field (.5-.7T), high field (1-1.5T), or ultra high field (≥3T). In contrast to MRI field strengths, the strength of the Earth's gravitational pull is approximately 0.00005T. Consequently, a 1T MRI scanner uses a magnetic field that is 20 000 times the gravitational pull of the Earth, and a 3T MRI scanner operates at a strength of 60 000 times the gravitational pull of the Earth.
Magnetic resonance imaging uses a magnetic field and radiofrequencies (RFs) to create images. The RF used in the MRI is determined by the strength of the magnetic field. The Larmor equation determines the frequency of the RF based on the field strength of the magnetic field. This frequency is directly proportional to the applied magnetic field strength. The Larmor equation is as follows:
ω0 = γ B0
The symbol ω0 represents the angular frequency of the precession of protons in an external magnetic field, the symbol γ is a proportionality constant called the gyromagnetic ratio, and B0 is the strength of the external magnetic field.3
At a magnetic strength of 1T, the proportionality constant or gyromagnetic ratio (γ) is equal to 42.56 MHz. To find the Larmor frequency for different magnetic field strengths, one must use this equation. For example, the Larmor frequency for a 1.5T magnet is 1.5(T)*42.56 (MHz T-1) = 63.8 MHz. For a 3T magnet the Larmor frequency would be 127.6 MHz. Stronger magnetic fields require stronger RF to generate MR images.
Water or hydrogen protons are abundant in the human body and constitute approximately 66% of the human body weight. This high water content is responsible for the applicability of MRI in medicine. Because water content varies in different tissues and organs, many diseases or pathologic processes result in changes in water content, which are reflected in the MRI study. Multiple pulse sequences are obtained during an MRI procedure. The resulting signals are measured in both the phase and frequency directions, thus displaying a range of intensities throughout the image matrix. These varying intensities are reflected in the image slices. The MR images can be scanned directly in either 2 dimensions (2D) or 3 dimensions (3D), in the sagittal, coronal, or axial planes, as well as any oblique in-between plane. The benefit of obtaining 3D isotropic volumetric images is that they can be post-processed or reconstructed in any desired plane without loosing resolution. This is because the isotropic voxels are equal in all 3 dimensions.
A few of the MRI parameters adjusted during scanning include the following:
- TE: time echo (the length of time the signal is measured)
- TR: time repeat (time between pulse sequences) or time interval between successive 180° pulses (or successive 90° pulses)
- T1: spin-lattice relaxation (recovery of z-magnetization)
- T2: spin-spin relaxation (loss of xy-magnetization)
- ρ: proton density parameters
- Flip angle of the RF pulse
- TI time: time for inversion, or the time between the 180° pulse and the 90° pulse
- STIR: short TI (or Tau) inversion recovery
- FOV: field of view
- NEX or NA: Number of excitations or signal averages
- Phase and frequency matrix
- Slice thickness
- Slice gap (%): distance between each slice
- Parallel imaging (PI) techniques: generalized autocalibrating partially parallel acquisition (GRAPPA), modified sensitivity encoding (mSENSE), SENSE, array spatial sensitivity technique (ASSET), or simultaneous acquisition of spatial harmonics (SMASH) algorithms
- BW: receiver bandwidth (a measurement of the range of RF)
- ETL: echo train length (number of echoes used in fast spin echo [FSE] sequences)
The manipulation of these parameters helps to differentiate between normal anatomy and pathologies, and also provides vital pathologic information in disease states. Also, these manipulations ultimately affect spatial resolution, contrast-to-noise ratio (CNR), signal-to-noise ratio (SNR) information and scan time, all of which contribute to the overall quality and clarity of the image.
Early Discoveries and Research Applications
Felix Bloch and Edward Purcell were both awarded the Nobel Prize in 1952. Both of these individuals independently discovered the MR phenomenon in 1946. In the subsequent 2 decades, NMR was developed and used for chemical and physical molecular analysis.2
The Road to Clinical Application
Raymond Damadian discovered in 1971 that the nuclear magnetic relaxation times of tissues and tumors differed, which motivated scientists and physicians to consider MR for the detection of disease. Subsequently, in 1973, Paul Lauterbur demonstrated the MRI phenomenon on small test tube samples using a gradient approach to scanning. In 1975, Richard Ernst proposed MRI using phase and frequency encoding, and the Fourier Transform. This technique is the basis of current MRI techniques for 2D and 3D image/slice reconstruction.2
Meanwhile, Raymond Damadian founded the FONAR (Field Focused Nuclear Magnetic Resonance) Corporation to produce commercial MRI scanners in 1978. His prototype whole-body scanner, named Indomitable, produced the first whole-body patient images in 1977. Although the Indomitable prototype did not immediately result in a commercially viable product, the FONAR Corporation began producing commercial scanners in 1980.4
Current MRI Systems in Clinical Practice
The MRI technology first became available in clinical practice in the 1970s, most of which operated at a strength of .6T.2 During the following decade, stronger 1.5T MRI systems were introduced. Imaging systems with a strength of 1.5T are now considered the clinical gold standard for current MRI modalities. In 1998, the US Food and Drug Administration (FDA) gave marketing clearance for scanners operating at strengths of up to 4T, and in 2002, the agency approved some 3T scanners for the brain and the whole body. Four years ago, more than 100 of these machines had been installed, and it was suggested that 3T was likely to become the eventual standard in MRI.5
Magnetic susceptibility measures the response generated by exposure of a substance or material to an applied magnetic field, in this case the field applied by an MRI scanner. This factor is determined by the degree to which electrons in the substance become magnetized and align with or against the applied magnetic field. Substances are categorized relative to these magnetic properties as being diamagnetic, paramagnetic, or ferromagnetic.6
Diamagnetic compounds represent over 99% of biologic tissues within the body. These compounds weakly oppose the applied magnetic field. They have no unpaired electrons. When they are placed in a magnetic field, a weak magnetic field is induced in the opposite direction. They are basically nonmagnetic.
Paramagnetic compounds possess unpaired electrons that align with the applied magnetic field. They become magnetized while the magnetic field is on but become demagnetized once the field has been turned off.
Ferromagnetic substances are strongly attracted by the magnetic field. They become permanently magnetized even after the magnetic field has been turned off. Ferromagnetic substances include materials made of iron, cobalt, and nickel. Examples of ferromagnetic products encountered in the clinical environment include aneurysm clips and shrapnel.6
Susceptibility effects are more pronounced at higher field strengths. This can result in signal loss, in addition to the need to consider possible safety hazards. The benefits and challenges of susceptibility effects in MRI will be further discussed later in this article.
Benefits of 3T MRI Scanners
The ability to scan with a strength of 3T has multiple potential benefits. First of all, 3T MRI results in an increase in SNR compared with 1.5T MRI. The use of 3T imaging results in an SNR that is roughly twice that of a 1.5T scanner. This gain in SNR can be used to either improve image quality or decrease the scan time in contrast to 1.5T imaging. Another benefit of 3T MRI is an improvement in image quality and resolution. With 3T MRI, one can choose to increase in-plane resolution or decrease slice thickness. Increased SNR and higher spatial resolution results in improved image clarity and diagnostic strength. Scanning with 3T MRI also provides an increase in spatial and temporal resolution. This results in the ability to perform smaller FOVs and thinner slices as a result of almost double the SNR compared to 1.5T MRI.
Decreased scan times can also be chosen in 3T MRI to help reduce data artifacts related to patient motion in individuals who are unable to or who have difficulty holding still during the MRI process. This results in a preservation of image quality and resolution even at reduced imaging times.
An increase in chemical shift with 3T MRI results in better MR spectroscopy (MRS) imaging when compared with MRS at 1.5T, due to a doubling of chemical shift with 3T MRI. This results in improved spectral resolution or the ability to visualize changes in peaks in metabolites. Fat-water suppression techniques are also improved at 3T. This is especially beneficial for musculoskeletal studies in which fat saturation imaging techniques are important.
Advanced functional MRI sequences are also possible with 3T MRI, including diffusion-weighted imaging (DWI), diffusion tensor imaging (DTI) and blood oxygen level dependent (BOLD) imaging, all of which are much improved at 3T.
Finally, a higher CNR is possible with 3T MRI. Longer T1 values at 3T provide for better background tissue suppression. The result is a tremendous improvement in time of flight (TOF) imaging, especially in Circle of Willis imaging of the brain (Figure 1). The visualization of smaller vessels is also improved.
Limitations of 3T MRI Scanners
An increased specific absorption rate (SAR), the measurement of the energy deposited by an RF field in a given mass of tissue, is also possible with 3T MRI. The SAR in MRI studies is limited by the international electrotechnical commission (IEC) guidelines and should not exceed 8W/kg of tissue for any 5-minute period or 4 W/kg for a whole body scan averaged over 15 minutes. Clinicians should note that dissipation of RF energy in the body could result in tissue heating. Consequently, the IEC states that body core temperature should not increase beyond 1°C during MRI studies.7
Compared with 1.5T MRI, increasing or doubling the field strength to 3T results in a quadrupling of SAR.7 This phenomenon is demonstrated particularly with RF-intensive pulse sequences, such as FSE, echo planar imaging, and fluid attenuation inversion recovery (FLAIR). As a result of increased SAR in 3T MRI, RF energy deposition limits as prescribed by the US FDA are quickly reached. Manufacturers of 3T MRI systems are incorporating modified pulse sequences to avoid this problem, which can also be addressed by restricting the volume of tissue that is studied in detail.5 As a result, imaging at 3T could result in longer scan times, a reduction in the number of slices, and delays between imaging sequences, all of which could limit 3T scanner performance.7 Doubling the flip angle in MRI also increases SAR 4-fold. PI techniques, such as GRAPPA, mSENSE, SENSE, ASSET, or SMASH, reduce SAR in 3T MRI and could allow for reductions in RF exposure and scan times.
Images obtained with 3T MRI are also more subject to flow artifacts. Although these can be reduced by such measures as cardiac gating or flow compensation techniques, there may remain cases of routine spine, cardiac, body, or shoulder imaging in which 1.5T is superior.
Institutional MRI policies and procedures should be updated on a regular basis to reflect new MRI safety concerns and current American College of Radiology MRI safety guidelines.8 Controlling access to any field strength MRI suite is critical and is of the utmost importance for patient and personnel safety.
An important safety issue in imaging at 3T is an increase in SAR that could result in increased tissue heating due to increased magnetic field strength. Moreover, there is an absence of data on scanning of patients with various types of implants at 3T due to the fact that some implants have not been tested at 3T, and new implants and devices are introduced on a regular basis.
Gradient noise increases with magnetic field strength (B0), and gradient noise at 3T can be twice that of 1.5T.9 Measures such as improved gradient design, active noise cancellation, and acoustically vacuum-based bore liners have all kept gradient noise effects to a minimum. Nonetheless, the routine use of earplugs in patients is required.
Magnetic susceptibility is the degree of magnetization that a material or tissue exhibits in response to a magnetic field, and can have either a negative or positive effect on the overall quality of the image. Materials vulnerable to magnetic susceptibility can be either foreign objects in the patient's body, such as implants or metal hardware, or external materials, both of which can present safety issues and result in an increase in magnetic susceptibility artifacts in the imaging area of interest. Magnetic susceptibility artifacts are more prominent at 3T then they are at 1.5T. This phenomenon can be beneficial in functional MRI (fMRI), BOLD, and diffusion or echo planar imaging. Disadvantages are seen in signal void in air/tissue interfaces during brain diffusion sequences, such as in the frontal sinus, skull base, orbits, and frontal lobes of the brain. Increasing bandwidth, decreasing voxel volume, and increasing ETL can be used to adjust for magnetic susceptibility in both 1.5T and 3T imaging.
Chemical shift doubles in moving from 1.5T to 3T. This results in an improvement in spectral resolution, but chemical shift artifacts also increase with an increase in field strength. This phenomenon can be advantageous for MRS imaging at 3T but can be a disadvantage for imaging cartilage and bone interfaces of musculosketal areas. To overcome this phenomenon or decrease chemical shift artifacts, the bandwidth can be doubled.
Dielectric artifacts impact MRI studies by causing an increase in inhomogeneity. Dielectric effects are caused by local eddy currents due to the increased conductivity of body tissue. This effect is particularly prominent in 3T body imaging. The RF wavelengths are shorter at 3T than at 1.5T. Inhomogeneities will be seen in larger anatomic areas, such as the abdomen during liver imaging. This effect is usually seen on images of the left lobe of the liver. There is an area of inhomogeneity, shading, or a drop off of signal in the area of imaging resulting in a loss of image clarity in that area. This is especially true when imaging thinner patients. To decrease this effect, most MRI equipment manufacturers supply dielectric pads and encourage the use of these devices, which are placed between the patient and the anterior body array coil during MR abdominal imaging procedures to reduce the inhomogeneity artifacts seen throughout the tissue.10
The prolongation of T1 at 3T provides improved image contrast. Magnetic resonance angiography sequences are improved due to the suppression or saturation of background tissue and the improved CNR between flow and background tissue. However, this same phenomenon makes it difficult to see contrast differences between gray and white matter in spine and brain imaging for T1 sequences. T2 values or effects are unrelated to field strength. T2-star (T2*) values vary with field strength resulting in T2* being shortened at 3T, therefore 3T studies are more sensitive to the deposition of blood products (hemosiderin) and tissue mineralization, making these sequences an improvement over 1.5T imaging for brain hemorrhage seen in patients with head trauma or patients who have experienced a stroke. It is important to note that T2* is shortened at 3T.
Techniques Used to Address Challenges in 3T Imaging
Parallel imaging techniques, such as integrated parallel acquisition techniques, GRAPPA, mSENSE, SENSE, ASSET, or SMASH, help to reduce imaging time by reducing the number of phase encoding steps that are performed during a given scan time. PI acceleration factors can be selected between 2 to 4 times faster than conventional imaging, resulting in decreased scan times but also decreases in SNR. With inherent increases in SNR at 3T compared to 1.5T, this loss can be balanced accordingly. PI works better at higher field strengths due to smaller wavelengths at 3T.
Flip Angle Reduction
Flip angle reduction is helpful for reducing SAR on FSE or turbo spin echo sequences. However, using this technique may affect image contrast for gradient echo sequences (GRE) on 3T images. Siemens uses a technique called sampling perfection with application optimization contrast using different flip angle evolution for 3D FSE imaging. Transition between pseudo-steady state is another technique to refocus echoes close to the center of K-space. These techniques help to further improve image contrast and spatial resolution.11
Adjustment of Duty Cycle
Duty cycle is defined as the number of RF pulses that occur during a given TR period. Using longer repetition times than the minimum necessary, clinicians can build in some gradient cooling time. Although slightly longer scan times are needed with this technique, SAR is reduced.
New Pulse Sequences
New pulse sequences are being designed and developed, which reshape RF and gradient waveforms that result in a reduction of the peak RF power to 40% when compared with conventional techniques. Such techniques include the variable rate selection excitation hyperecho technique, which can reduce the peak RF energy by up to 40% to 60% when compared with conventional techniques.9
New Surface Coil Configurations
When 3T first became available for clinical use, there was a limited availability of coils for this new technology. However, as the demand for 3T MRI has progressed, coil development for 3T has evolved. Surface coils with a change in configuration from 8 to 16 channels are now becoming more readily available for 3T imaging and also help to increase SNR, improve spatial resolution, or decrease scan time.
As discussed previously, dielectric artifacts result in increased inhomogeneity with areas of nonuniformity or shading in MRIs. These artifacts are caused by the shortening of RF wavelengths inside the body and exist at all field strengths. However, these effects are increased at higher field strengths and the effects can be increased with multichannel coils. The dielectric effect in muscle appearance is worse than that of fat tissue. To help reduce these artifacts, a high conductivity pad made of a dilute manganese chloride solution is placed between the coil and the patient. Some pulse sequences are more sensitive to dielectric artifacts than others, such as FSE. Also, patients with ascities are more likely to exhibit dielectric effects.
Relaxation times are prolonged at 3T compared with 1.5T in traditional T1 imaging sequences, which leads to a reduction in T1 contrast differentiation, especially between gray and white matter in brain and spine imaging. T1 FLAIR imaging at 3T is a solution for this concern. It should be stated that T1-weighted GRE, fast low-angle shot (FLASH), and spoiled-gradient echo (SPGR) sequences are not affected by field strength.
Advantages and Considerations of 3T in Specific Clinical Applications
Diffusion-weighted imaging is improved at 3T due to the increased SNR. DTI or white matter tracting is also further improved in 3T MRI. fMRI techniques, such as BOLD studies for planning purposes before neurosurgery, display improved capabilities at 3T due to a higher SNR, increased magnetic susceptibility effects, and increased CNR. MRS at 3T results in an improvement in spectral resolution for the evaluation of metabolites that could be obscured at 1.5T. Multinuclear spectroscopy to analyze many disorders is better visualized with 3T MRI.9 Perfusion imaging is also significantly improved at 3T due to the increase of magnetic susceptibility effects. Perfusion imaging at 3T takes advantage of this effect to help determine brain tissue viability following a stroke or transient ischemic attack. With the effect of prolongation of T1 relaxation, imaging at 3T with T1-weighted GRE sequences, such as SPGR or magnetization prepared rapid gradient echo, can be used. Standard doses of intravenous (IV) gadolinium-based contrast media (GBCM) may result in greater sensitivity, and using a half dose of GBCM is also being investigated.12 Images representative of neuroimaging studies of the brain with 3T MRI are available in Figures 1, 2, and 3.
Due to the prolongation of T1, T1 FLAIR sequences can be used for T1 spinal imaging. For patients with implanted hardware, increasing receiver bandwidth and using longer ETLs will help to reduce magnetic susceptibility effects and still produce excellent image quality (Figures 4 and 5).12
Full Body Imaging
3T imaging allows for thinner slices and higher resolution with higher matrices to provide for better lesion characterization, especially in images of the liver. Abdominal imaging at 3T is still compromised by respiratory motion, vascular pulsation, increase of SAR using FSE techniques, and image quality or artifacts due to reduced B1-homogeneity, or the dielectric effect. As previously discussed, improvements in coil design, PI techniques, coil configuration, dielectric pads, improved breath-hold monitoring techniques (prospective acquisition correction [PACE]/Navigator corrections), hyperecho techniques, and variable flip angles have made clinical imaging at 3T a reality (Figures 6 and 7). Because of the changes of resonance at 3T, time echoes for in phase and opposed phase have to be adjusted. These values are almost inverted when compared to 1.5T. In-phase values are found at 2.3 msec and 4.6 msec, whereas opposed-phase values are 1.1 msec, 3.5 msec, and 5.74 msec. Dual phase imaging also can be problematic because the phase echoes are too close together. This could make adrenal gland imaging challenging. The effects of fat saturation are improved at 3T because of the stronger chemical shift between fat and water, but at the same time, homogeneity decreases as a result of the increase in field strength and may result in less optimal fat saturation results. Radiologists should note that chemical shift artifacts are more pronounced at fat/water interfaces.10
Magnetic resonance imaging is an established modality for the diagnosis, characterization, and identification of benign tumors versus malignant disorders. 3T imaging of the pelvic area particularly for prostate cancer staging is extremely beneficial. The use of 3T along with an endorectal coil and IV GBCM provides for improved image spatial resolution as compared with 1.5T imaging. This is helpful in imaging malignancy extension and determining preoperative planning. The development of MRS of the prostate at 3T could provide additional clinical information for treatment in terms of prostate cancer staging.12
Musculoskeletal Imaging: Joints
Because of the increase in SNR with 3T MRI, a smaller FOV, thinner slices, and increased spatial resolution can be obtained. As noted previously, limited coil availability has been a concern with 3T MRI in the past. SAR issues related to fat saturation sequences and longer ETL with FSE sequences related to duty cycle also have been areas of concern. However, with new advancements in dedicated coil technology and pulse sequences, these issues have been largely resolved. The higher SNR and increased spatial resolution at 3T may be beneficial to help enhance the detection of tears of the articular cartilage of the labrum of the shoulder and hip, triangular fibrocartilage complex (Figure 8) tears of the wrist, and to aid in the diagnosis and staging of various internal derangements of the knee and elbow.12
Breast imaging with MRI is a growing area that has demonstrated increased use over the past several years. This increased use is due, in part, to recent guidelines from the American Cancer Society recommending the use of MRI as a screening tool, in addition to mammograms, in women who meet at least 1 of the following criteria13:
- Presence of a BRCA1 or BRCA2 mutation
- First-degree relative (parent, sibling, or child) with a BRCA1 or BRCA2 mutation
- Lifetime risk of breast cancer of 20% to 25% or greater (based on family history and other factors)
- Radiation to the chest between the ages of 10 and 30
- Presence of Li-Fraumeni syndrome, Cowden syndrome, or Bannayan-Riley Ruvalcaba syndrome, or a history of one of these syndromes in a first-degree relative
Magnetic resonance breast imaging at 3T results in improved spatial and temporal resolution capabilities. MRI breast imaging, in conjunction with dynamic contrast, provides for improved detection and characterization of breast cancer, and some clinicians believe that these benefits will be better seen with 3T imaging.
Cardiac MRI is an evolving and challenging area. To date, most of the MRI cardiac studies or procedures have been performed on 1.5T scanners. Cardiac MRI requires a team of highly trained and dedicated team of radiologists, cardiologists, and MRI technologists. Cardiac MRI is performed to help evaluate the structures and function of the heart, valves, and major vessels, and is useful in diagnosing and managing coronary heart disease, in addition to a variety of other cardiovascular problems. Cardiac MRI also provides useful information to help the cardiologist develop a treatment plan and monitor patient progress. A good electrocardiogram reading before the study is essential to obtaining adequate image quality.
Cardiac MRI is valuable because it can help to examine the size and thickness of the chambers of the heart (Figure 9), determine the extent of damage caused by a heart attack or progressive heart disease, detect the build up of plaque and blockages in the blood vessels, and assess patient recovery following treatment. In 1.5T imaging, these studies can be very time consuming. Cardiac patients are often very sick and may suffer from irregular heart rates, and could therefore experience difficulty holding their breath during the procedure. In analyzing cardiac MRIs, the MRI technologist must be experienced in post-processing software to determine ejection fraction values and flow quantification of cardiac valves and vessels.
Important advantages of cardiac imaging with 3T MRI include the ability to obtain higher spatial and temporal resolution, an increase in SNR, and a decreased imaging time compared to 1.5T imaging.
Radiologists should note that MRI heart cine sequences using steady-state free precession (SSFP) sequences for cardiac imaging to display cardiac wall motion and left ventricular ejection fraction at 3T can be problematic because of increased artifacts and energy deposition limits. Optimization of the SSFP essentially consists of obtaining images without dark banding or flow artifacts. Most 3T systems now allow for manually changing the transmit frequency to reduce this artifact (Figures 10A and 10B).12 A scout of several images at the same location using different transmit frequencies are obtained, and the technologist picks the best image with regard to flow and frequency artifacts. Another technique is to use a T2 FLASH cine technique, which is less susceptible to these frequency artifacts.
With the increased SNR at 3T, heart perfusion images have improved in providing a better visual delineation of perfusion abnormalities and cardiac ischemic evaluation.
Time of flight imaging is improved at 3T because of the longer T1 of background tissue. This results in background tissue suppression and a higher visibility of contrast in vascular structures. Lower flip angles help to reduce SAR and pulsation artifacts, thus reducing acquisition time. The use of PI with both non-contrast and contrast-enhanced techniques allows for shorter scan times with increased resolution. This results in greater image quality, especially in small vascular structures (Figure 11), when compared with 1.5T imaging.9 Therefore, 3T MRI may be extremely beneficial in imaging peripheral vascular areas, such as the bilateral calf and feet areas following IV GBCM contrast injection.
Magnetic resonance imaging technology will continue to pose challenges with the evolution of 3T MRI and other advanced techniques. Understanding and mastering the challenges of 3T imaging today is crucial and will ultimately result in superior spatial resolution, speed, and consistent image quality when compared with present day 1.5T systems. With further developments in coil design, SAR reduction techniques, pulse sequences, and protocol development for 3T, advanced MR procedures, such as functional neuroimaging, in addition to high-resolution musculoskeletal, body, and vascular imaging will be possible. In today's diagnostics environment, some consider cost to be the only true barrier to the widespread adoption of 3T systems for clinical imaging and predict that 3T MRI may eventually replace 1.5T MRI as the clinical gold standard. Only time will tell if today's hurdles to 3T MRI can be overcome. Regardless of the future of 3T MRI specifically, MRI modalities will continue to evolve and provide new challenges to the radiology community.
1. Rabi II, Zacharias JR, Millman S, Kusch P. A new method of measuring nuclear magnetic moment. Phys Rev. 1938;53:318.
2. About.com: Inventors. Magnetic Resonance Imaging. Available at: http://inventors.about.com/od/mstartinventions/a/MRI.htm. Accessed May 14, 2008.
3. ReviseMRI.com. What is the Larmor equation? Available at: http://www.revisemri.com/questions/basicphysics/larmor_eqn. Accessed May 28, 2008.
4. Massachusetts Institute of Technology. Inventor of the Week Archive: Raymond Damadian. Available at: http://web.mit.edu/invent/iow/damadian.html. Accessed May 29, 2008.
5. Bronson J. High-field MRI: is it time for 3T? Imaging Econ. Available at: http://www.imagingeconomics.com/issues/articles/2004-02_04.asp. Accessed May 14, 2008.
6. Hashemi R, Bradley W, Lisanti C, eds. MRI: The Basics. Baltimore, MD: Lippincott Williams &Wilkins; 2004.
7. DeLano MC, ed. 3T MR imaging. Magn Reson Imaging Clin N Am. 2006;14:1-126.
8. American College of Radiology. ACR Guidance Document for Safe MR Practices: 2007. Available at: http://www.acr.org/SecondaryMainMenuCategories/quality_safety/MRSafety/safe_mr07.aspx. Accessed May 29, 2008.
9. Tanenbaum L. 3T in clinical practice. Appl Radiol. 2005;34(suppl). Available at: http://www.appliedradiology.com/articles/Article.asp?ID=1134&IssueID=136&ThreadID=. Accessed May 29, 2008.
10. Schoenberg SO, Zech CJ, Panteleon A, et al. New perspectives and challenges in abdominal 3T MR imaging. Appl Radiol. 2007;36(suppl). Available at: http://www.appliedradiology.com/articles/Article.asp?ID=1330&IssueID=170&ThreadID=. Accessed May 29, 2008.
11. Runge VM, Nitz WR, Schmeets SH, Schoenberg SO. Clinical 3T Magnetic Resonance. New York, NY: Thieme Medical Publishers; 2007.
12. Roberts TPL, ed. 3.0T versus 1.5T imaging. Neuroimaging Clin N Am. 2006;16:217-370.
13. American Cancer Society. ACS advises MRIs for some at high risk of breast cancer. Available at: http://www.cancer.org/docroot/NWS/content/NWS_1_1X_Society_Advises_MRIs_for_Some_Women_at_High_Risk_of_Breast_Cancer.asp. Accessed May 29, 2008.
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The Evolution of Magnetic Resonance Imaging: 3T MRI in Clinical Applications
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