Table 3.

iOUS articles included in our review.

Author	Title	Aims	Conclusions	Limitations
Sab B. (2020) [55]	Navigated 3D Ultrasound in Brain Metastasis Surgery: Analyzing the Differences in Object Appearances in Ultrasound and Magnetic Resonance Imaging	To analyse the differences in object appearances in ultrasound (US) and magnetic resonance imaging (MRI) in 35 cases of brain metastasis.	The use of i3D US enables clear visualization of tumour boundaries and real-time imaging updates to compensate for brain shift. This can be done to a significant degree even before the dura is opened.	(1) The quality of the precalibration, respectively, the co-registration of the US probes was not sufficiently assessed for each case.
Picarelli H. (2012) [56]	Intraoperative ultrasonography for presumed brain metastases: a case series study	To find if image-guided BM resection using intraoperative ultrasound (IOUS) can lead to better surgical results.	IOUS is a useful adjunct for the removal ofBMs, but additional studies comparing its efficacy to other intraoperative exams are necessary to determine its actual value and reliability.	(1) Small sample size.
Sweeney J. (2018) [22]	Efficacy of intraoperative ultrasonography in neurosurgical tumor resection	To assess the effectiveness of IOUS in identifying gross-total resection (GTR) for brain tumour patients, including both adults and children.	Incorporating IOUS as an imaging modality may facilitate achieving a more successful GTR of brain tumours in both adult and pediatric neurosurgical patients, providing a reliable approach.	(1) Heterogenous population.
Renner C. (2005) [57]	Evaluation of intra-operative ultrasound imaging in brain tumor resection: a prospective study	To evaluate IOUS as a tool of resection control after brain tumour surgery, including BMs.	The reliability of IOUS varies depending on the type of tumour. Its use is advantageous for the resection of metastases and select high-grade gliomas. However, IOUSwasless reliableconcerning volumetric accuracy than it iseffectivein navigation and resection control.	(1) Small sample size; (2) heterogenous population.
Lindner d. (2006) [58]	Application of intraoperative 3D ultrasound during navigated tumor resection	To prove the concept of 3D ultrasound on the basis of technical and human effects.	The introduction of 3D US has increased the value of neuronavigation substantially, making it possible to update several times during surgery and minimize the problem of brain shift for intraparenchymal tumour, including BMs.	(1) Small practitioner size; (2) subjective evaluation.
Arlt F. (2016) [59]	Intraoperative 3D contrast-enhanced ultrasound (CEUS): a prospective study of 50 patients with brain tumours	To examineCEand 3D reconstructed US in brain tumour (including BMs) surgery regarding the uptake of contrast agent pre- and post-tumour resection, imaging quality and in comparison, with postoperative MRi.	3D CEUS wasa reliable intraoperative imaging modality and could improve imaging quality both for glioma tumours and BMs.	(1) Small sample size; (2) heterogenous population; (3) GTR was not objectively defined.
Cepeda S. (2022) [60]	Advantages and Limitations of Intraoperative Ultrasound Strain Elastography Applied in Brain Tumor Surgery: A Single-Center Experience	To describe the main technical aspects, usefulness, and limitations of ioUS strain elastography applied in a large case series of brain tumours, including BMs	The tumour stiffness identified through IOUS strain elastography hada potential histopathological correlation, making this rapid and versatile technique highly promising for future exploration and exploitation in the coming years.	(1) Variability of evaluation due to artifacts after dural opening; (2) variability of the frequency and amplitude of the mechanical pulsations.
Cepeda S. (2020) [61]	Comparison of intraoperative ultrasound B-mode and strain elastography for the differentiation of glioblastomas from solitary brain metastases. An automated deep learning approach for image analysis.	To compare the discriminative capacity of intraoperative ultrasound B-mode and strain elastography to differentiate GBM from BMs.	Utilizing deep learning for automated processing of ultrasound images can lead to the development of highly accurate classification algorithms that can distinguish glioblastomas from metastases using intraoperative ultrasound.	(1) The sample size is relatively small. This aspect can cause an overfitting problem and the creation of an over-optimistic predictive model. (2) Variability of elasticity threshold values and the absence of an image quality control.
Prada F. (2019) [62]	Intraoperative strain elastosonography in brain tumor surgery.	To describe the implementation of elastosonography in oncological neurosurgery for lesion discrimination and characterization.	Elastosonography allowed surgeons to understand the mechanical properties of the brain and lesions in examination and permitteda better discrimination between different tissues, compared to B-mode.	(2) Variability of evaluation due to variability of elasticity threshold values.
Unsgaard G. (2005) [63]	Ability of navigated 3D ultrasound to delineate gliomas and metastases—comparison of image interpretations with histopathology	To test the ability of a 3D US-based intraoperative imaging and navigation system to delineate gliomas and metastases in a clinical setting.	Reformatted images derived from 3D US volumes provided a clear delineation of BMs, aiding in pre-resection planning. Navigated 3D US wasequally as reliable as navigated 3D MRI in outlining metastases.	(1) Small practitioner size; (2) subjective assessment of US.
Tonnier V.M. (2001) [64]	Comparison of intraoperative MR imaging and 3D-navigated ultrasonography in the detection and resection control of lesions.	Compared two intraoperative imaging modalities:low-field MR imaging and a prototype of a 3D-navigated US in terms of imaging quality in lesion detection and intraoperative resection control.	Based on these preliminary results, intraoperative MR imaging remains superior to intraoperative ultrasonography in terms of resection control in glioma surgery and BMs surgery.	(1) Small practitioner size; (2) Subjective assessment of US for the detection of residual tumours.
Prada F. (2014) [65]	Intraoperative Contrast-Enhanced Ultrasound for BrainTumor Surgery	To provide the dynamic and continuous CEUS evaluation of a variety of brain lesions.	The findings indicate that brain metastases exhibit a nodular, heterogeneous appearance, with central necrotic areas and visible small/large vessels.	(1) Subjective evaluation of CEUS.
Wen He (2008) [66]	Intraoperative contrast-enhanced ultrasound for brain tumors	To investigate the feasibility and value of CEUS in resection for brain tumours, including BMs.	CEUS has the potential to serve as a highly valuable imaging technique, not only in defining the boundary between the tumour and the healthy brain tissue prior to resection, but also in detecting any residual tumour tissue after the initial resection.	(1) Subjective evaluation of CEUS; (2) heterogenous population.
Prada F. (2022) [67]	Multiparametric Intraoperative Ultrasound in Oncological Neurosurgery: A Pictorial Essay	To define the aspect of Brain tumours in iOUS, including BMs.	BMs are typically identifiable in: (1) B-mode hyperechoic lesions with a granular or heterogeneous appearance, including features such as a peripheral ring and central necrosis or nodule and cystic areas. The most common B-mode pattern consists of solid components and cystic or necrotic regions with well-defined borders. (2) CEUS, which reveals rapid contrast enhancement, with a fast arterial phase and contrast enhancement peak. The CE is intense and persistent. The arterial supply often follows a centripetal pattern with several macro vessels within the lesion. (3) Elastography: BMs may be stiffer (e.g., kidney, colon) or softer (e.g., lung, endometrial) than normal brain tissue, with a low mean elasticity value due in part to central necrosis.	(1) Subjective assessment of US; (2) heterogeneous pattern of BMs depending on primary tumour.
Erdogan N. (2005) [68]	Ultrasound guidance in intracranial tumor resection: correlation with postoperative magnetic resonance findings	To assess the agreement between intraoperative ultrasonography and postoperative CE MRI in detecting tumour residue.	IOUS can be a valuable adjunct. However, surgical manipulation should be minimized. In cases where tumours have preoperatively-detected cystic components near CSF-containing spaces, careful evaluation with IOUS is necessary to detect any residual cystic components. Additionally, a low-thickness echogenic rim should not be relied upon as a definitive sign of the absence of residue.	(1) Small practitioner size; (2) subjective assessment of US for the detection of the tumour.