Implications of SENSE MR in routine clinical practice
Introduction
The impact of magnetic resonance imaging in radiological practice has been much stronger than could have been anticipated at the time of its invention. The early recognition amongst clinicians of the potential of MR as a radiation-free tomographic modality caused a very rapid adoption of the technique [1], [2]. The acceptance of MR stimulated significant progress in its diagnostic capabilities, based upon vast improvements in SNR. To a large extent, developments in MR can be attributed to simultaneous developments in various technological areas: increased main magnetic field strength, improved RF coil design, advances in digital electronics, increased gradient performance and last but not least, improved and new contrasts and acquisition methods. The most significant inventive step in obtaining high SNR images from large FOVs, however, was the introduction of phased array coils [3].
Superior soft tissue contrast, attainable at modest scan times, has been the economic differentiator for the success of MR imaging. The increase in SNR provided the opportunity to reduce scan times, simply by reducing the number of signals averaged and/or increasing readout bandwidths. Other substantial increases in imaging speed providing new diagnostic opportunities have been achieved by implementing rapid imaging techniques, such as EPI [4], FLASH [5] or FFE [6], [7] and RARE [8] or TSE, and their variants and hybrids. All scan time reductions incurred by these new methods did, however, affect image contrasts, as their principles rely on employing alternative magnetization dynamics for image formation. Furthermore, attainable resolution for (single-shot) EPI and TSE is compromised by signal decay during the acquisition. This inherent resolution limit becomes more prominent at higher main magnetic fields, as T2 and T2* decrease.
While the wide variety of contrasts was, and is, a significant factor in the success of MR, economic factors, including increased learning curves, can be expected to cause new contrast appearances to be less easily accepted in the clinical community, given the body of clinical evidence based upon existing and known contrasts. Increases in imaging speed that alter scan characteristics (such as TR) and thus contrasts, e.g. by increasing gradient performance, can be expected to penetrate less and less effectively into routine clinical practice. Furthermore, physiological barriers have been reached for dB/dt, acoustic noise, SAR and main magnetic field strength, beyond which regulations on patient safety will limit acquisition speed. Attaining significant scan time reductions without changing contrast-determining scan characteristics or violating physiological barriers, is a highly attractive means to further improve cost effectiveness of MR imaging. Such a method for improving scan performance has been demonstrated recently by the implementation of SENSE [9] on routine clinical MR scanners [10]. Clinical advantages of SENSE for several MR imaging techniques have been discussed in terms of increased speed and/or reduced artifact sensitivity in [11].
This paper briefly outlines the physical and system concepts underlying SENSE, provides examples of the impact of SENSE in clinical practice and indicates the potential future direction for the technique. For several MR applications, limitations on ‘non-SENSE’ systems are identified, the impact of SENSE to alleviate these constraints is discussed and a (few) typical example(s) are shown.
Section snippets
Sensitivity encoding
The basic concept of resolving spatial information in magnetic resonance imaging has for more than 2 decades been based upon the insight that a linear spatial variation in main magnetic field strength encodes the location of the signal source in terms of frequency differences [12]. The frequency span of the detected MR signal must be sufficient to avoid aliasing or backfolding of signals from distinct spatial locations at the same frequency (Nyquist theorem). This requirement poses the scan
Examples of SENSE in clinical practice
The speedup obtained with SENSE could be translated directly into shorter scanning, is particularly beneficial for breath hold applications and shorter total exam time in general. Shorter scan times decrease motion artifacts. In dynamic or real-time applications, such as cardiac functional studies, shorter scan times allow better following of dynamic processes with higher frame rates. Another application of SENSE is to translate the speedup to enhance resolution or to increase the number of
CE-MRA of the renal arteries
Contrast enhanced MRA (CE-MRA) of the aorta and renal arteries has matured greatly since first described in 1993 as a free breathing study requiring in excess of 5 min and using a slow (≈0.1 cc/s) i.v. infusion of gadolinium chelate [23]. Current high-resolution protocols for renal CE-MRA, made possible primarily through the use of faster gradient systems, are performed in a breath-hold (≈30 s or less) using a rapid gadolinium bolus (1.5–5.0 cc/s). Using such techniques, many investigators are
CE-MRA in paediatric body imaging
Since the introduction of contrast-enhanced magnetic resonance angiogram (CE-MRA) nearly a decade ago [23], this technique is now the most commonly used technique for obtaining MR angiograms of the chest or abdomen in the adult population. However, there has been limited use of this technique in paediatric body MR imaging until recently [40], [41], [42]. This is despite the obvious clinical need for a non-invasive vascular imaging technique in children that does not require ionizing radiation
CE-MRA of head and neck
Simultaneous visualization of the neck and brain vessels from the aortic arch to the circle of Willis has been a challenge for years. The use of dedicated ‘neurovascular’ or ‘head and neck’ coils made it possible to cover the complete region. Contrast-enhanced (CE) MRA is the emerging method of choice to image the (neck) vessels but venous return is fast in the neck. The available time to perform a CE-MRA study of the head and neck vessels is limited to 1 min or a little more, even when a
Comprehensive abdominal and liver MRI
Compared to the other imaging modalities (ultrasound and helical CT), MR imaging offers better inherent image contrast and truly 3D imaging which constitute an advantage in investigating liver malignancies. In routine comprehensive abdominal MR, it is important to obtain a number of image sets, which provide various image contrasts. MR imaging protocols combine unenhanced T2- and T1-weighted images with both in- and out-of-phase echo times, gadolinium-enhanced, and in selected patients
MR mammography
MRI of the breast is currently considered the most sensitive of all breast imaging techniques, including, in particular, high frequency breast ultrasound and state-of-the-art conventional film-screen or full-field digital mammography. Owing to its superior inherent sensitivity, MRI of the breast is used to non-invasively clarify equivocal conventional imaging findings, or to identify primary or recurrent breast cancer. In women who underwent breast conservation treatment, MRI alone is able to
Imaging of ischaemic heart disease
Imaging is an essential part of the diagnosis of ischaemic heart disease (IHD), either in patients with acute or chronic chest pain, or in patients with congestive heart failure. Imaging of IHD can be directed to visualize and locate the underlying cause of failure of myocardial blood supply (i.e. coronary angiography), to assess the impact on coronary and myocardial perfusion (perfusion imaging) and myocardial metabolism (spectroscopy), to locate and evaluate the extent of myocardial ischaemia
High-resolution routine imaging
Imaging of the skull base and supra- and infra-hyoid neck requires the use of high resolution images in order to distinguish the very small structures in these areas. The use of 512 matrix is mandatory but often results in long imaging times, especially when T1-weighted spin-echo images are used and the use of fat-saturation even prolongs the measurement. Measurement times are then in the range of 8 min and a routine examination including an axial TSE-T2, unenhanced and Gd-enhanced SE-T1 and
Future directions in parallel imaging
From the above-mentioned examples, it can be concluded that SENSE has opened new frontiers in the application of MRI. Exploration of the possibilities of parallel imaging has certainly not ended. In this section, we will introduce a few foreseeable technological and clinical opportunities.
Conclusions
The implications of parallel imaging, and especially SENSE, has been demonstrated for several routine clinical applications. The seemless embedding of SENSE in clinical workflow has enabled us to explore its potential for routine clinical practice in the past 2 years. SENSE MRI has already demonstrated many advantages, several of which have been demonstrated in this paper. New applications of MR imaging can be anticipated for the future, especially in the combination of 3T and SENSE.
Acknowledgements
Assignment of clinical contributions—JHM: Renal Artery CE-MRA; TC and RM: Paediatric CE-MRA; JWC: Neuro imaging and CE-MRA; KY: Tractography; YW and CM: Abdomen and Liver imaging; CKK: Mammography; SD and JB: Cardiac imaging.
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