Figure 1. Axial T2-weighted spinal cord images at 3T visualize multiple sclerosis (MS) plaques within the spinal cord (arrows).

Three Tesla MR imaging (3T MRI) theoretically provides two to 10 times more signal than other commonly employed MR systems operating between 0.3T and 1.5T. Initial concerns regarding patient safety have been resolved, and 3T MRI has been approved by the FDA for general patient evaluations. The primary concern relative to 3T MRI was the potential adverse biological effects from specific absorption rate (SAR) that have been overcome with advanced technological approaches to the use of higher field strength scanners. The advantage of increased signal at 3T MRI means a greater amount of signal can be used to provide improved resolution for the detection of smaller lesions. The availability of greater susceptibility at 3T MRI improves detection of hemosiderin secondary to brain hemorrhage as well as allowing for shorter acquisition times for functional brain imaging. In general, 3T MRI has been shown to provide improved imaging as compared to lower field strength systems. This has led to a significant increase in the sales of higher field strength systems in the 3T range with several hundred in clinical use today.1

Figure 2. Sagittal FLAIR sequence 3T MR image shows multiple sclerosis (MS) plaque adjacent to the corpus callosum (arrow).

One of the significant differences between 3T MRI and imaging at lower magnetic field strengths is a marked increase in the detection of white matter disease in the brain and spinal cord. This directly impacts the diagnosis of patients with certain disease conditions, such as multiple sclerosis (MS). In fact, 3T MRI has had a major impact on the diagnosis of spinal cord MS where definitive spinal cord lesions were infrequently identified with confidence and now are seen routinely with a high level of diagnostic accuracy (Figure 1, above right). Similarly, brain MS studies are considerably more sensitive at 3T, greatly increasing clinicians’ diagnostic confidence. Brain MS lesions are readily identified on 3T that would have been difficult to find, or not found at all, on lower field strength systems, due to lower signal to noise, artifacts, or insufficient resolution. Also, 3T MRI has reduced lesion subtlety, improving visualization and enabling the confidence to call abnormal lesions (Figure 2 at right). And 3T MRI improves the quality of new techniques, such as diffusion tensor imaging (DTI) with fiber tracking (FT). This method of MR evaluation enables the tracking of white matter fiber tracts within the brain by evaluating the directional flow of water molecules. DTI with FT provides a remarkable image of the white matter fiber tracts of the brain on 3T MRI (Figure 3, below right). The improved technology as well as sensitivity and specificity of higher field strength magnets are not achieved without a concomitant increase in expense. A 3T MRI is not only two to 10 times more powerful than lower field strength MRI systems, but it also costs two to 10 times as much to purchase with higher operational costs.

Reimbursement Implications

The strategies employed by 3T users to offset the added expense of high-field MRI in the past has been to rely primarily on funding from state or privately supported universities and/or research grants. However, private imaging centers are increasingly acquiring 3T MRI systems because of the necessity for improved diagnostic quality as well as the opportunity to increase market share through the availability of improved image quality and increased diagnostic capabilities. High-end imaging centers with 3T MRI systems require special considerations relative to financing and management. The added burden of higher monthly payments and the expense of more specialized personnel mean that equipment downtime must be kept to a minimum, and that lost scan time—due to such issues as personnel absenteeism—is not an option. Because government agencies and third-party payors continue to bargain for the lowest cost health care with little or no regard for quality, it can be very difficult, and impossible in some instances, for a higher-quality 3T MRI to compete with lower-cost 0.3T to 1.5T MRI systems in a marketplace that emphasizes only reduced cost. Currently, there is no reimbursement differential for quality with reimbursements being the same for both lower field and high field strength 3T MRI systems.

Figure 3. Sagittal FLAIR sequence MR image with superimposed DTI white matter fiber tracking of the corpus callosum (color) in a normal subject.

However, both referring physicians and patients are becoming more knowledgeable about the importance of quality. They are increasingly aware that—as with their automobiles, where a car is not just a car—in medical imaging, an MRI is not just an MRI. Just as they do not expect to pay the same for a compact car with limited accessories as they would for a luxury car with full amenities, they are becoming better informed health care consumers; when they pay full price for health insurance, they should not be stuck with the MRI equivalent of a compact car. When the health insurance provider refuses to pay for access to a 3T MRI because it has contracted for lower-field, cheaper MRI services, consumers are increasingly willing to pay the difference.

Perhaps the most telling aspect of operating a private 3T MRI center is the number of physicians and their families who come to the center for their imaging. This may even be the case when the physician is an owner or investor in lower-field MRI centers. In some instances, referring physicians will demand that third-party payors reimburse for 3T MRI, especially if the patient is a physician. A significant and increasing number of referrals to 3T MRI systems are for the evaluation of “negative” lower-field MRI studies, often with “positive” findings on the 3T scans. This is not say that lower-field MRI systems are inadequate or obsolete. The availability of 3T MRI systems is such that it would not be possible for every MRI patient to access a 3T MRI, and there are limitations on the use of 3T MRI due to patient size or metallic implants in some instances. However, when a lower-field MRI study is inconclusive or negative, a 3T MRI could offer a definitive diagnosis.

Power to Move

A distinctive advantage of 3T MRI over lower field strength systems is improved spatial and temporal resolution during real-time cine MR imaging (RTC MRI). This method of imaging provides RTC images of cerebrospinal fluid (CSF) flow as well as vascular flow in arterial, capillary, and venous phases. Recent reports have confirmed that contrast-enhanced MR angiography (CE-MRA) is significantly improved at 3T as compared to lower field strengths.2 Concerns regarding adverse side effects at 3T MRI from devices, such as are found in patients with a ventricular shunt with a programmable valve, have proven to be significantly less than anticipated with no alternations from multiple exposures to a 3T scanner or from exposure to various MR imaging conditions. Even these implanted devices are now considered safe for patients undergoing MR imaging at 3T.3

Figure 4. Sagittal dynamic 3T MR images of CSF demonstrate dynamic changes during real-time imaging of CSF within the cerebral aqueduct. Note the change in intensity in the progressive images. View the video (1.1MB)

RTC MRI at 3T provides physiological data that cannot be appreciated on routine nonmotion displayed MR images. Sagittal images often are considered the easiest to view and interpret; however, angled axial imaging is typically preferred for quantitative analysis of volumes and/or velocities (Figure 4).4–9 Technical factors employed for routine CSF flow include a routine imaging time of 5 to 6 minutes, with fields of view in the sagittal plane of 250 mm (0.98 x 1.39 mm in-plane resolution), slice thickness of 10 mm, and 15 phases of the cardiac cycle. A field of view of 150 mm, slice thickness of 4 mm, and 12 cardiac phases are acquired in the axial plane (Intera Achieva 3T MRI from Philips Medical Systems, Andover, Mass).

Routine MR angiography (MRA) is a commonly employed method in the evaluation of suspected vascular disease.10,11 [View a video (3.4MB) of lower extremity imaging]. However, one of the limitations of routine MRA is that the rate of blood flow within the abnormality must be within the range of the flow being measured during the MRI procedure. Normal flow rates for arterial and venous flow in the brain range from 36 to 46 cm/sec (arterial) and 6 to 21 cm/sec (venous).12,13 MRI systems performing routine nondynamic imaging are programmed to track blood flow at these rates. Blood flow at velocities different from these ranges will not be seen; therefore, vascular pathology may be missed. A major advantage of RTC MRA is that all phases of blood flow are evaluated during a single venous injection of contrast with the availability of real-time flow analysis. Figure 5 demonstrates images from a 63-year-old male patient with a suspected prior brain hemorrhage. Routine MRI examinations and MRA studies at lower field strengths demonstrated no abnormalities. The nondynamic 3T MRA and MR venography (MRV) failed to show evidence of vascular pathology (Figures 5a and 5b). However, the RTC MRA (Figure 5c) shows a slow flow aneurysm, which was not identified on routine nondynamic studies.

Figure 5. Sagittal routine MRA (a) and MRV (b) identify no abnormalities. Sagittal dynamic 3T MRA images in slow flow aneurysm demonstrate dynamic changes during real-time imaging of vascular flow within the brain. Note the change in signal intensity (arrows) in the progressive dynamic images. View the video (0.9MB)

The technique of RTC MRA involves the intravenous injection of gadolinium DTPA and simultaneous imaging over the area of interest. The vasculature evaluated is limited to the area centered within the magnetic field; however, stepping table technology is enabling larger areas to be examined. The RTC MRA technique used at our center acquires a 3D volume at a 250 field of view (0.49 x 0.65 mm in-plane resolution, reconstructed to 0.49 x 0.49 mm), 10 slices that are 20 mm thick with 10 mm of overlap between slices, temporal resolution of 5.6 seconds per dynamic acquisition, with a total of 15 temporal images per slice.

RTC MRI is now a routine part of clinical practice, particularly at 3T, where the advantage of improved speed and resolution combines to make this a very powerful diagnostic tool. This method can assist in regularly evaluating patients with suspected normal pressure hydrocephalus and other CSF flow abnormalities as well as both typical and atypical forms of vascular disease. In many instances, the need for more invasive procedures can be eliminated, resulting in improved patient care with lower risk and significant reductions in cost.

William W. Orrison, Jr, MD, MBA, is chief of neuroradiology at Nevada Imaging Centers and AMIGENICS, Las Vegas. Jon B. Gibson, RT(R)(MR)(CT), is chief 3T MR technologist for Nevada Imaging Centers. David Chang and Ranjit Kapil are students at Touro University School of Osteopathic Medicine, Las Vegas.


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