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Original Report |
1 All authors: Department of Diagnostic Radiology, Eberhard-Karls-University of Tuebingen, Hoppe-Seyler-Str.3, D-72076 Tuebingen, Germany.
Received March 27, 2002;
accepted after revision October 28, 2002.
Address correspondence to C. W. König
(claudius.koenig{at}med.uni-tuebingen.de).
Abstract
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CONCLUSION. MR imaging–guided biopsy of the adrenal gland is feasible and safe. In all patients, appropriate specimens were obtained with full diagnostic yield and accuracy. MR fluoroscopy is particularly useful to establish an oblique paravertebral access without pleural transgression. For final needle placement, supplementary breath-hold multislice sequences are required in most cases.
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We used an open configuration clinical 0.2-T MR scanner (Magnetom Open, Siemens, Erlangen, Germany) equipped with 15 mT/m gradients and dedicated interventional accessories such as flexible ring-shaped receiver coils, a fiber optic light source, and a shielded liquid crystal display in-room monitor [1]. In one patient, diagnostic imaging was performed with the patient in a supine position and then in a prone position before biopsy. The other patients were primarily placed in a prone (n = 5) or semilateral (n = 1) position. Breath-averaged T1-weighted spin-echo and T2-weighted fast spin-echo sequences (11 slices, 6-mm thickness, 2-mm interslice gap) were applied for anatomic survey. Supplementary breath-hold sequences (expiration) were performed in a transverse and sagittal orientation, using T1-weighted fast low-angle shot (TE range, 7–12) and breath-hold T2-weighted fast spin-echo sequences (echo-train length, 17). Slice thickness was reduced to 5 mm if necessary. Additional sagittal images of the posterior pleural space obtained in deep inspiration were used for access planning. Contrast agent (gadopentetate dimeglumine) was applied in only one patient for diagnostic purposes before biopsy. The basic principles of MR imaging–guided biopsy have been described elsewhere [1, 3]. After sterile draping, local anesthetic administration and skin incision, we advanced an MR imaging–compatible needle subcutaneously, and the table was moved into the magnet. Cephalad navigation of the needle into the retroperitoneal space was performed with MR fluoroscopy or conventionally with multislice sequences. In the conventional mode, the cannula was placed in a stepwise fashion with repeated image updating with a set of three to five slices (fast low-angle shot or fast spin echo) centered parallel to the needle (scanning time, 7–16 sec). For fluoroscopic guidance, a single-slice FISP sequence (TR/TE, 17.8/8.1; flip angle, 90°; matrix, 48–64 x 128; rectangular field of view, 30–35 cm) in-plane with the desired needle path was continually measured with immediate image reconstruction and shown on the in-room liquid crystal display. An appropriate sagittal imaging plane was chosen, displaying the pleural recess and the tumor itself or the renal capsule close to the tumor to ensure that the kidney was not injured. The cannula was aligned with the target and advanced during continuous near real-time imaging. MR fluoroscopy was terminated after the needle was placed deeply in the retroperitoneal space, and final positioning was performed with repeated multislice sequences in most patients. Usually, the tumor was centrally targeted.
In two patients, specific parts of the mass were selectively sampled. One tumor revealed a target sign on contrast-enhanced CT; the other lesion was heterogeneous because of acute eccentric adrenal hemorrhage 2 days before biopsy (Figs. 1A, 1B, 1C, 1D). IV analgetics and sedatives (pethidine hydrochloride, midazolam) were administered when necessary. A coaxial biopsy system was used in six patients, consisting of a 16-gauge MR imaging–compatible trocar made of titanium alloy (CoaxNeedle, MRI Devices Daum, Schwerin, Germany; Puncture Needle, Biopsy Gun MRI, Somatex, Berlin, Germany) placed in front of the lesion, and a non-MR imaging–compatible spring-loaded biopsy gun (ASAP 18-gauge, Boston Scientific, Watertown, MA) for coaxial sampling of one or two biopsies. In one patient, a single pass with a noncoaxial titanium gun (Biopsy Gun MRI, 16-gauge, MRI Devices Daum) was performed. Samples were fixed in formaldehyde solution and sent to the pathologist. After needle withdrawal, T2-weighted fast spin-echo imaging was performed to reveal short-term complications.
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The other six diagnoses were adrenal metastasis (n = 3, each with an immunoprofile correctly identifying the primary tumor), pheochromocytoma (n =1), and adrenal tissue (n = 2). Diagnosis of adrenal adenoma was confirmed on follow-up CT in both patients with adrenal tissue at MR imaging biopsy. The diagnosis of pheochromocytoma was confirmed surgically. The tumor was classified as nonfunctioning and nonmalignant. This finding explained the unremarkable endocrinologic screening results before biopsy.
MR fluoroscopy was performed in all except one case (n = 6), starting with the second patient in our series. MR fluoroscopy was used in these six patients for interactive definition of skin entry site (fingerpointing). Needle angulation and advancement into the retroperitoneum were performed with MR fluoroscopic guidance in five patients, with particular focus on avoidance of the pleural recess. The only patient in whom MR fluoroscopy was deemed unnecessary for this purpose had a tumor far away from the pleura (Fig. 2). Entire fluoroscopic needle-tracking into the lesion was performed in two of five patients, one with the largest tumor in this series (10 x 5 cm) (Figs. 3A, 3B, 3C, 3D). MR fluoroscopy was additionally, but not exclusively, applied for needle navigation in the retroperitoneum in another two of five patients. In these patients, the final steps of cannula placement were guided with conventional-mode breath-hold imaging in a biplanar angulation for navigation in close vicinity to the kidney and spleen. In the remaining patient, MR fluoroscopy was used only to avoid the pleura. Further guidance was performed conventionally because of anatomic narrowness caused by splenomegaly.
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Generally, application of MR fluoroscopy for needle-tracking in the retroperitoneal fat was hampered by lateral deflection of the needle tip during respiratory motion. This deflection precluded the use of thin slices for needle-tracking. Selections of 6- to 8-mm slice thicknesses resulted in better signal-to-noise ratio and artifact-tracking but also raised the risk for unrecognized needle deviation due to partial volume effects. For the same reasons, adjacent structures, mainly the kidneys, were also occasionally displayed in the tracking slice and mimicked the risk of needle injury. In this case, biplanar breath-hold multislice imaging was indispensable to clarify anatomic relations.
The guiding procedure could be performed completely inside the magnet in six of seven patients. The only table movements necessary were for insertion of (non-MR imaging–compatible) biopsy guns in these patients. Time between the first images after needle insertion and the last image before needle withdrawal (needle time) varied from 13 min for the largest to 40 min for the smallest tumor, with an average of 25.8 min. Major complications did not occur.
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The favorable outcome achieved in our preliminary study revealing full diagnostic yield and accuracy without notable complications is in line with previous reports from CT-guided biopsy [5, 6, 7], suggesting that MR imaging–guided biopsies can be as safe and accurate as CT-guided procedures. This finding is not surprising because MR imaging guidance primarily affects the needle pass, but tissue sampling itself is essentially comparable to CT-guided procedures.
Key advantages of MR imaging versus CT guidance are the ability to establish a cephalad approach by sagittal monitoring of the needle course and to have permanent access to the patient during imaging (hands-on technique) without exposure of the radiologist to radiation. The true extension of the posterior costophrenic sulcus and the target moving with respiration can be depicted near real time. MR fluoroscopy was particularly useful to determine an appropriate needle angulation during transgression of the extraperitoneal back muscles because further needle angulation can be limited after the ribs are crossed in sturdy patients. MR fluoroscopy further aided to some degree in needle tracking in the retroperitoneal fat.
However, in close vicinity to the kidney or spleen, MR fluoroscopic guiding was not considered accurate enough; thus, stepwise guidance with conventional breath-hold multislice imaging was indispensable in these situations. Exclusive MR fluoroscopic guidance is justified only in patients with large tumors. Spatial resolution was considerably reduced to a 48 x 128 matrix to provide a frame rate of virtually one image per second, even improving needle conspicuity and anatomic survey. This type of MR fluoroscopy sequence proved to be sufficient for regions with intrinsically high lesion contrast like the adrenals embedded in fatty tissue. A similar technique based on T2-like refocused steady-state precession could also be applied if available [8]. Subsampling techniques continuously updating the lower frequency domains of k-space (keyhole, LoLo, [2, 9, 10]) seem to be less promising in areas moving with respiration.
Multiplanar imaging is another pivotal facility of MR imaging guidance. Precise alignment of the imaging plane to the angulated cannula aids in appropriate needle depiction particularly in close vicinity to vital structures like the kidney, spleen, and large vessels. Biplanar imaging is indispensable for accurate needle placement before biopsy to avoid missampling in small adrenal tumors. Chemical-shift techniques can, to some extent, also be applied in low-field systems, potentially obviating biopsy in case the tumor fits the benign criteria.
Conversely, some limitations of MR imaging guidance must also be considered. Needle-artifact size strongly depends on angulation toward the main magnetic field, which is vertically oriented in our magnet. Thus needle angulation away from the vertical line is not only favorable to avoid the pleural space but is crucial to maintain needle contrast. Biplanar imaging with repeated adjustment of slice orientation is time-consuming and outweighs time-savings achieved by the hands-on technique. Bony structures are less conspicuous on MR imaging than on CT, and they must be thoroughly identified in the planning sequences.
Irrespective of the imaging modality, biopsy of moving targets requires the patient's cooperation in breathing. With MR imaging guidance, respiratory motion can be tolerated to some extent using MR fluoroscopy, but precise needle-positioning finally requires breath-hold imaging. Thus, patients apparently unable to consistently suspend respiration should not be referred for MR imaging biopsy, and IV analgesics rather than sedatives should be applied to maintain the patient's consciousness. In this study, non-MR imaging–compatible disposable biopsy guns were predominantly used because of superior specimen quality compared with that collected with titanium tools [11]. The disposable guns can be easily handled in or near the low-field magnet because the ASAP gun contains a relatively low amount of steel. Obviously, precautions must be taken to prevent harm to the patient if ferromagnetic tools are introduced to the MR imaging suite.
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