Linear array ultrasound: a dedicated tool for a dedicated application
Jan Coburger
Acta Neurochirurgica The European Journal of Neurosurgery ISSN 0001-6268 Acta Neurochir DOI 10.1007/s00701-015-2406-8
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Author's personal copy Acta Neurochir DOI 10.1007/s00701-015-2406-8
LETTER TO THE EDITOR - BRAIN TUMORS
Linear array ultrasound: a dedicated tool for a dedicated application Jan Coburger 1
Received: 13 March 2015 / Accepted: 17 March 2015 # Springer-Verlag Wien 2015
We appreciate the comment by Dr Moiyadi. As an experienced intraoperative ultrasound user, he adds additional clinical information to our article and provides an overview for the reader. Obviously, sector array ultrasound and linear array ultrasound are no competing technologies. At our center, we always have both types of probes draped if we perform a case. Thus, the surgeon can choose the transducer fitting for the respective situation. Our article provides a comparison of both probes solely for the purpose of residual tumor control. Using a strict intraoperative protocol, we harvested histological specimens to compare imaging results of linear array ultrasound, conventional sector array ultrasound, and iMRI. In this specific setting, linear array ultrasound was superior in the detection of residual tumor. In our experience, linear array intraoperative ultrasound is highly beneficial for residual tumor detection, and thus helps us to prevent multiple intraoperative iMRI scans. Only linear array ultrasound allows for high-frequency ultrasound (7– 30 MHz). The high frequency provides a high resolution, which renders a precise tissue differentiation possible. This feature allows for differentiation of skin tumors in dermatology or is used in breast-sparing surgery. However, an increase of frequency leads to a loss of penetration depth. Additionally, the aperture of linear array ultrasound was too large for intracranial use for a long time. Recently, small linear array transducers, as described in our article, became available and are used by many centers. The great potential is the high resolution directly below the tip of the transducer. Hence, the
* Jan Coburger
[email protected] 1
Department of Neurosurgery, Universität Ulm, Ludwig-Heilmeyerstr. 3, 89312 Günzburg, Germany
surgeon can literally scan the tissue during resection and detect thin margins of residual tumor. Actual low-frequency sector or curved array ultrasound transducers also allow for intracavital ultrasound. The distance to the tissue of interest can be artificially increased using gel pads, as described by Dr. Moiyadi. However, the physical resolution of ultrasound is mainly based on its frequency and thus will be lower in sector array ultrasound. As described previously, intracavital ultrasound reduces artifacts significantly. As nicely described by Selbekk et al., there are many potential artifacts the surgeon should be aware of [1]. Physically, all types of ultrasound devices are affected by these common ultrasound artifacts. Many of them can be reduced with a good imaging setup. A conventional ultrasound with an optimal setup might provide much better images than a high-frequency linear array ultrasound that was poorly adjusted. One of the key features of the intraoperative linear array probe is the hockey stick shape, which allows for an easy introduction in even small resection cavities. This might add to the benefit we found in our study. This shape, to the best of our knowledge, is technically only achievable with a linear array transducer. However, there are curved linear array and sector array transducers on the market that are smaller than the 3D sector matrix array transducer we used as a “conventional” probe in our setup. The main disadvantage of the small linear array transducer is that it provides a very detailed image of a small area of the brain. Especially if it is inserted in a resection cavity it is challenging to retrieve the spot with a small residual tumor. Thus, we recommend using the device in a navigation setup as it was performed in the actual study. Sector array ultrasound provides a much better overview and a better orientation concerning anatomical landmarks, especially at the beginning of surgery if no navigation is used. When the linear array ultrasound is referenced to the neuronavigation system, this overview can be provided by the preoperative or intraoperative MRI images. In this setup, we
Author's personal copy Acta Neurochir
often do not need a second probe, even if we approach deepseated lesions. Nevertheless, as Dr. Moiyadi stated, there are several indications for intraoperative ultrasound. Residual tumor detection in gliomas is just one of them. The type of applied probe should depend on the indication and the local setup. There are many indications where we do not want to miss “conventional” intraoperative ultrasound. For the surgical stage of residual tumor detection in gliomas, based on our histological results, we believe that the use of linear array ultrasound increases tumor detection. However, we can only emphasize the last paragraph of Dr. Moiyadi’s letter that only “various probes used together during
different stages of surgery … provide all the information … required”.
Conflicts of interest None.
References 1.
Selbekk T, Jakola AS, Solheim O, Johansen TF, Lindseth F, Reinertsen I, Unsgard G (2013) Ultrasound imaging in neurosurgery: approaches to minimize surgically induced image artefacts for improved resection control. Acta Neurochir (Wien) 155:973–980
