Authored date:2006-06-02

Introduction

The use of breath-hold techniques and intravenous administration of contrast agents allows MRI to demonstrate a full range of upper abdominal diseases [1–4]. One advantage of the wide variety of MR sequences currently available is that comprehensive examination of disease processes is feasible [1]. The main disadvantage of this variety is that general agreement on MR imaging strategies is difficult to achieve and thus there is a tendency to add new sequences to a protocol rather than replace older sequences [5]. This tendency serves to decrease patient throughput, increase study cost, and increase the likelihood of patient motion. Directly opposing this trend, the idea of whole-body MR screening exams is rapidly gaining popularity [6–8]. This could be particularly useful in the examination of metastatic cancer and other diseases that can occur simultaneously in a variety of organs and anatomical locations [8, 9]. Additionally, and although such views are vividly being discussed in the medical community, it can be considered ethical to do wholebody MR screening of healthy individuals because MR does not use ionizing energy, uses relatively safe contrast agents, and is more effective than CT at detecting most disease processes except in the lungs [10–13]. Whole-body MR screening requires a series of very rapid exams and an easy transition from one anatomical location to another. As a result there is a need to replace longer breathing-averaged sequences with shorter breath-hold or breathing-independent sequences, to eliminate redundant sequences and to otherwise reduce the total time of MR examinations. The term “breathing independent” reflects that sequences are less than 2 seconds in duration, are minimally sensitive to artifacts from patient breathing and motion, and therefore do not require that patients breathe in a regular fashion or suspend respiration. 
In addition to rapid sequence software, the hardware must also support the rapid imaging of each anatomical location as well as the rapid transition from one location to another. Siemens has addressed these needs with the following 
5 technological advances [2, 14–16]:
1 Remote movement of the patient table
2 Up to 32 independent RF receive channels
3 Tim (Total imaging matrix) Matrix coils covering the whole body with high signal-to-noise ratio (SNR)
4 Hardware and software allowing iPAT (integrated Parallel Acquisition Technique) in all directions
5 High quality 3D T1-weighted VIBE (Volume Interpolated Breathhold Examination) The following sections describe how these elements may be combined into an effective screening protocol.

10-minute whole-body protocol

The patient is prepared by using the Head Matrix coil, the Neck coil, and two or four Body Matrix coils covering the entire torso. An intravenous line is established for contrast injection. This protocol is designed to screen for systemic metastases in the majority of patients. Specific needs of individuals may be addressed by adding sequences to this basic screening protocol, but the focus on keeping the overall protocol short must be maintained. One common example is to add a basic cardiac exam as part of the optional sequences. The T-SENSE sequence has a true temporal resolution of 
50 ms and can therefore be used to generate CINE-like image sets without requiring cardiac gating or even breath-holding.

Results

Figures 1 through 5 show a selection of typical images obtained using this protocol. Note the generally high image quality of all sequences used. Note in Figure 1 that the small liver lesions are easily visualized and accurately characterized as cysts. Note also the visualization of the small renal cancer in Figure 2. In particular, note the clear depiction of the small pulmonary metastases as seen in Figure 3, despite the fact that they are considered a challenge for MRI. Figures 4 and 5 do not demonstrate any pathology, but do demonstrate the high image quality of the screening protocol in the pelvis and head.

Fig. 1  Coronal HASTE (A) and axial In-Phase (B) pre-contrast, and axial VIBE (C) post-contrast abdominal images. A small liver lesion is visualized (arrows) and clearly characterized as a cyst.

Fig. 2  Coronal HASTE (A) and axial VIBE (B) post-contrast abdominal images. Small renal cancer showing mildly heterogenous enhancement identified in the inferior pole of the left kidney (arrows). Small cancers in the polar region can be missed in CT or MR examinations relying entirely on transverse images.

Fig. 3  Axial VIBE (A) and axial HASTE (B, C) post-contrast thoracic images. Small pulmonary metastases measuring less than 4 mm across are clearly visualized (arrows). These are clearly depicted despite the conventional wisdom indicating that such small lung metastases are difficult to see using MRI.

Fig. 4  Sagittal HASTE (A), axial HASTE (B), sagittal VIBE (C), and axial 
VIBE (D) post-contrast pelvic images.

Fig. 5  Sagittal HASTE (A), axial HASTE (B), and axial VIBE (C) post contrast cranial images.

Summary

This screening protocol fully utilizes the new Tim technology in order to effectively cover the whole body in less than 
10 minutes. Key features include the use of the high-SNR array coils, the Matrix coils, and multiple receiver channels that allow iPAT accelerated sequences. The coils themselves are physically compatible so they may all be placed on the patient and plugged in to the table simultaneously at the beginning of the exam, eliminating the need for coil or patient repositioning. Automatic table motion combined with the physical compatibility allows for rapid transition between anatomical regions. The sequences used have been demonstrated to be highly effective at detecting systemic metastases as well as a wide variety of other disease processes. In addition, the 3D VIBE can be reformatted in other orientations, particularly when the resolution is near-isotropic. This allows one breath-hold sequence to replace up to three without any loss of information. The short duration of this screening exam makes it a reasonable replacement for similar whole-body screening exams performed under CT. In addition this approach offers both improved safety (radiation and contrast agents) and diagnostic image quality in all regions except possibly in the lungs.

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