|Philips Achieva 3T MR scanner, which was used to obtain the images of the brain and knee shown below.|
MRI at 3T has been available for about 15 years, but it was primarily a research tool. Now, the major vendors are selling systems intended for routine clinical use. Do you need one? And if you purchase one, what can you expect?
What Makes 3T Better than 1.5T?
The chief appeal of 3T is the higher signal-to-noise ratio (SNR), which can be traded for greater speed, higher spatial resolution, or both.1 (For physical reasons, the SNR is not quite double that of 1.5T.1) The stronger gradient also permits thinner slices and increases the conspicuity of gadolinium, allowing reduction of the contrast dose.2,3
The literature is beginning to illustrate the many advances of imaging at 3T imparts. The stronger magnets are particularly beneficial for neuroimaging. For example, in one series of 20 patients with intracranial tumors, functional MRI at 3T affected the surgeon’s view of lesion operability in nine cases, altered the surgical approach in 13, and led to changes in the planned extent of resection in 12.4 Magnetic resonance spectroscopy at 3T may permit noninvasive assessment of the degree of malignancy of gliomas, which had much higher choline:creatine ratios,5 and 3T images are superior to those obtained at 1.5T in predicting the local invasiveness of sellar lesions and planning surgery for them.6 The sensitivity in determining local infiltration (as judged by the intraoperative findings) was 83% at 3T versus 67% at 1.5T, with specificities of 84% and 58%, respectively.
|Transverse T1-weighted, inverse recovery MR image of the brain illustrating excellent gray-white contrast.|
The higher field strength is advantageous for studies with blood oxygen-level dependent (BOLD) contrast and for diffusion imaging.2 In the musculoskeletal system, the greater speed of 3T means better patient tolerance and, thus, fewer motion artifacts and repeat scans, making it possible to schedule many studies at 15-minute intervals.7
Images of the heart can be captured with 1.5-mm resolution and 5.5-mm slice thickness, a complete image being obtained every 120 msec for replay at 24 frames per second.8 In a series of volunteers, 3T scans improved blood-myocardial contrast by more than 200% relative to 1.5T.8 Black-blood studies of the carotid arteries demonstrate better wall SNR and a higher lumen-wall contrast-to-noise ratio (CNR) than studies at 1.5T.9 Myocardial perfusion imaging, even with small contrast doses, also is superior to what is possible at 1.5T.10
Because of the excellent results that can be obtained with angiography at 3T,11 MRI is competing with CT to replace traditional catheter angiography in the abdomen and some other sites. Using parallel imaging and a variable refocusing angle technique, radiologists at the University of Bonn (Germany) have been able to increase the spatial resolution in the female pelvis more than threefold, with fewer motion artifacts, in the same scan time, enabling detection of small lesions, such as tumor-invaded lymph nodes.12 Dynamic contrast-enhanced MR has proved highly accurate for preoperative definition of the site and extent of prostate cancers,13 a capability that might become more important as urologists consider resecting only the tumor—not the entire prostate gland—as a means of reducing morbidity and as image-guided treatments, such as cryosurgery and brachytherapy, are used in more patients.
|A proton density-weighted sagittal view of the knee demonstrating a high-grade meniscal tear.|
Another appeal of 3T systems is their speed. “In the early days of MR, the rate-limiting step was the time it took to do the examination,” recalls Benjamin Seckler, MD, medical director at Columbia Magnetic Imaging (Hudson, NY). “Now, the rate-limiting step is more evenly distributed along the whole process, and examinations that account for about 80% of all MR work in an outpatient facility—the joints, the spine, the brain—can be completed in 30 to 35 minutes. With a 3T scanner, you can do breath-hold imaging for entire examinations where previously that was not possible. In the past, each pulse sequence you used to evaluate the liver would require about 8 minutes. Now, we go through them in 20 seconds. Moreover, not only are anatomic structures visible that previously we could not see, but structures that were seen at 1.5T are portrayed in such great detail that we can narrow the differential diagnosis where previously we had to be nonspecific.”
So Why the Complaints About 3T?
Such testimony could suggest that everyone would be ecstatic to have a 3T scanner, but some purchasers gave expressed considerable disgruntlement. Why?
Some of the early problems with 3T scanners resulted from lack of appropriate coils, according to Nancy Gillan, vice president of MRI at Siemens Medical Solutions (Malvern, Pa). “The 1.5T scanners have been around for a long period of time, and as a result, any clinical application or RF coil you might want is available,” she says. “However, coils for the 3T scanners were slower in reaching the market, in part because most of them are made by third parties who were initially reluctant to invest in coil development as it was not clear that 3T scanners were going to sell in significant numbers.”
The 3T scanners caused problems for some sites by being noisier and heavier than the 1.5T magnets they replaced. A greater difficulty, however, was the differences between the physics of 3T and 1.5T.
“If you view 3T simply as a faster 1.5T system, you are going to run into trouble,” warns Michael Brandt, vice president of MR marketing at Philips Medical Systems (Andover, Mass). “You need to be ready to adapt.”
Bryan Mock, PhD, global 3T MR product manager at GE Healthcare (Waukesha, Wis), agrees: “The 3T scanner is clinically ready, and we do not have any reservations about its use in a routine clinical practice, but you do have to make adjustments if you are coming from the 1.5T world.”
|Time-resolved imaging at 3T with large field of view capabilities and a 200mT/m slew rate result in temporal resolution achieving new speeds possible as a result of GRAPPA factor of 4 on the Siemens Magnetom Trio with Tim (see last image below). Image courtesy of the University of California, Los Angeles, radiology department.|
Numerous differences between 1.5T and 3T must be accommodated. For example, the T1 relaxation time increases at 3T, and the degree of change differs among tissue, affecting contrast.14 Although this change in relaxation can be exploited for time-of-flight vascular studies, because the signal within the vessels increases, it reduces the contrast differences among tissues if traditional sequences are used.2 Magnetic susceptibility increases with field strength2 and can create artifacts, especially next to gas-filled structures. This makes it more difficult to image the bowel wall, although detection of gas, such as intrahepatic pneumobilia, is easier.1 Sensitivity to chemical shift also scales with the strength of the field, which is an advantage for spectroscopy and fat suppression2 but can cause artifacts if steps are not taken to compensate.
Columbia Medical Imaging’s Seckler describes the effect: “Many of our early images looked cartoonish; the anatomic structures seemed to be outlined by black indelible marker.” Dielectric effects can create areas of shading or signal drop-off, sometimes resulting in variations in SNR from one patient to another,15 a problem that can be corrected in some circumstances by using pads with high electrical permittivity.2 Finally, there are some new artifacts, such as the standing wave effect, in which strong variations in the signal create “holes” in the image. Because of this effect, pregnant patients and those with ascites are not good candidates for 3T scans.1
|As demonstrated by the free breathing T2 axial acquisition, or the VIBE breath-hold acquired at slice thickness of 2.5mm, clear depiction of all anatomies is possible. Image courtesy of Siemens Medical Solutions.|
New safety concerns arise with 3T. Metal implants that are not contraindications to 1.5T scans may be contraindications to 3T scans. A more common problem is the radiofrequency deposition limit. Doubling the field strength quadruples the specific absorption rate (SAR), and without suitable technical or protocol modifications, a scan quickly may run up against the FDA-imposed limits designed to prevent increases in the patient’s body temperature. The vendors have developed various ways to deal with this problem, such as parallel imaging with integrated parallel imaging (iPAT), simultaneous acquisition of spatial harmonics (SMASH), and sensitivity encoding (SENSE). These techniques carry an SNR penalty, but the greater strength of the magnet compensates for it.7 Other techniques for coping with SAR are using transmit?receive coils and reducing the flip angle.2
Help Is Available
Radiologists and technologists are not facing these challenges without help. All of the vendors have training programs to ease the transition to 3T, and on-site experts guide buyers through the early stages. Several meetings, some of which are vendor-sponsored, have been organized that deal with 3T scanning only, and 3T-focused publications are appearing. For example, the first issue of volume 14 of the Magnetic Resonance Imaging Clinics of North America, edited by Mark C. DeLano, MD, of Michigan State University, is devoted entirely to imaging at 3T, with papers on abdominal, musculoskeletal, brain, head and neck, spine, and carotid artery imaging.
“Optimization for clinical use has been reached much faster with 3T scanners than it was at 1.5T,” Siemens Medical’s Gillan notes. “Customers are seeing improvements in a much shorter time, allowing them to benefit from the advantages of the higher field strength.”
|Siemens Magnetom Trio with Tim.|
Despite this support, Seckler says, “You are not going to master 3T by the time the vendor’s specialists leave. What we found extremely helpful was to have our technologists talk to technologists at other institutions that have the same magnet. That helped us tweak our protocols to obtain the best results.”
Close alliances with the vendors have other benefits. “Customers using 3T place more stringent requirements on system performance because research might be a considerable part of their utilization plan,” according to Russell Tanner, national support manager of MRI at Siemens Medical. “So we pay very careful attention to our specifications and tuning the scanner to the highest level to ensure that the customer can obtain the maximum results. For example, echo planar studies require a level of system stability beyond the needs of a customer doing only routine imaging.”
So Do You Need a 3T Scanner?
“Do not simply conclude that because it is bigger, it must be better,” Tanner advises. “The proper question is, ‘Better for what?’ It’s like choosing a hammer. If you are hanging a picture, you do not need a sledgehammer. Before you choose, you need to know the particular qualities of a 3T system and what the trade-offs are. Then you can make an intelligent choice.”
Seckler adds, “The market is very competitive—laypeople and referring physicians often believe that if you need a portion of your body imaged, eventually, you will need MRI. So, we bought our 3T scanner for marketing reasons. In the absence of economic considerations, almost everyone will want a 3T scanner, especially as the price declines. Such scanners are faster, they provide higher resolution, and they offer the radiologist more information from which to make a diagnosis. But since the Deficit Reduction Act of 2005 reduced reimbursement for diagnostic radiologists and radiation oncologists, it does not make sense from an economic perspective to buy these very expensive instruments.”
GE Healthcare’s Mock suggests, “If you are looking to differentiate your practice within a community, 3T might be the right choice.”
Philips Medical’s Brandt notes, “In some areas, it is a no-brainer to use 3T—no pun intended. On the other hand, some areas [with 3T] are more challenging, although those challenges are being overcome. Whether it is the right choice for you depends on your business model.”
Do you need a 3T magnet? Carefully consider your facility’s case mix, as well as the advantages and challenges.
Judith Gunn Bronson, MS, is a contributing writer for Medical Imaging.
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- Tanenbaum LN. Clinical 3T MR imaging: mastering the challenges. Magn Reson Imaging Clin N Am. 2006;14:1?15.
- Sasaki M, Shibata E, Kanbara Y, Ehara S. Enhancement effects and relaxivities of gadolinium DTPA at 1.5 versus 3 Tesla: a phantom study. Magn Reson Med Sci. 2005;4:145?149.
- Van Westen D, Skagerberg G, Olsrud J, Fransson P, Larsson EM. Functional magnetic resonance imaging at 3T as a clinical tool in patients with intracranial tumors. Acta Radiol. 2005;46:599?609.
- Jeun SS, Kim MC, Kim BS, et al. Assessment of malignancy in gliomas by 3T 1H MR spectroscopy. Clin Imaging. 2005;29:10?15.
- Pinker K, Ba-Ssalamah A, Wolfsberger S, Mlynarik V, Knosp E, Trattnig S. The value of high-field MRI (3T) in the assessment of sellar lesions. Eur J Radiol. 2005;54:327?334. Available at: http://www.journals.elsevierhealth.com/periodicals/eurr/article/PIIS0720048X04002797/abstract. Accessed July 21, 2006.
- Ramnath RR. 3T MR imaging of the musculoskeletal system I: considerations, coils, and challenges. Magn Reson Imaging Clin N Am. 2006;14:27?40.
- Nayak KS, Cunningham CH, Santos JM, Pauly JM. Real-time cardiac MRI at 3 Tesla. Magn Reson Med. 2004;51:655?660.
- Yarnykh VL, Terashima M, Hayes CE, et al. Multicontrast black-blood MRI of carotid arteries: comparison between 1.5 and 3 Tesla magnetic field strengths. J Magn Reson Imaging. 2006;23:691?698. Available at: www3.interscience.wiley.com/cgi-bin/abstract/112535158/ABSTRACT. Accessed July 21, 2006.
- Araoz PA, Glockner JF, McGee KP, et al. 3 Tesla MR imaging provides improved contrast in first-pass myocardial perfusion imaging over a range of gadolinium doses. J Cardiovac Magn Reson. 2005;7:559?564. Available at: journalsonline.tandf.co.uk/link.asp?id=q61q1474gq248722. Accessed July 21, 2006.
- Nael K, Saleh R, Lee M, et al. High-spatial-resolution contrast-enhanced MR angiography of abdominal arteries with parallel acquisition at 3.0T: initial experience in 32 patients. AJR Am J Roentgenol. 2006;187:W77?W85. Available at: www.ajronline.org/cgi/content/abstract/187/1/W77. Accessed July 21, 2006.
- Morakkabati-Spitz N, Gieseke J, Kuhl C, et al. MRI of the pelvis at 3 T: very high spatial resolution with sensitivity encoding and flip-angle sweep technique in clinically acceptable scan time. Eur Radiol. 2006;16:634?641. Available at: www.springerlink.com/link.asp?id=fx178h2ll4014624. Accessed July 21, 2006.
- Kim CK, Park BK, Kim B. Localization of prostate cancer using 3T MRI: comparison of T2-weighted and dynamic contrast-enhanced imaging. J Comput Assist Tomogr. 2006;30:7?11. Available at: www.jcat.org/pt/re/jcat/abstract.00004728-200601000-00002.htm. Accessed July 21, 2006.
- Stanisz GJ, Odrobina EE, Pun J, et al. T1, T2 relaxation and magnetization transfer in tissue at 3T. Magn Reson Med. 2005;54:507?512.
- Rodegerdts EA, Boss A, Riemarzik K, et al. 3D imaging of the whole spine at 3T compared to 1.5T: initial experiences. Acta Radiol. 2006;47:488?493.