Adult glioblastomas and brain metastases are the most common malignant tumors of the central nervous system and are associated with high recurrence and mortality rate [1,2]. The prevalence of these two malignancies has shown an increasing trend in recent decades. Distinguishing between solitary brain metastases (SBMs) and glioblastomas (GBMs) prior to treatment is of immense clinical significance owing to its implications for therapeutic decision-making and follow-up [3,4]. Nonsurgical therapy is preferred over traditional surgical resection for management of metastases. However, in the vast majority of cases with clinically and radiographically suspected GBMs, surgical resection followed by temozolomide-based chemoradiotherapy is the standard approach [5,6]. Additionally, both tumors typically present as heterogeneously enhanced masses surrounded by significant peritumoral edema [7]. The similar imaging findings of GBMs and metastases on conventional magnetic resonance imaging and post contrast T1-weighted images pose a great diagnostic challenge.

In recent years, several advanced imaging techniques have been utilized to differentiate SBMs from GBMs. For example, dynamic susceptibility contrast (DSC) is utilized for this purpose [8,9], although it is associated with the risks of contrast medium injection. Magnetic resonance spectroscopy (MRS) [10] reveals overlapping metabolites between these two entities. Diffusion-weighted imaging (DWI) [11] and diffusion tensor imaging (DTI) [12] have also been used to differentiate SBMs from GBMs; however, there are limitations in using these techniques to differentiate between SBMs and GBM, as both tumor entities are highly cellular and show restricted diffusion. Amide proton transfer-weighted (APTw) imaging, a particular type of CEST technique [13], is used to demonstrate the exchangeable mobile proteins and peptides at low concentrations, and a radio-frequency pulse is applied at the resonant frequency of 8.3 ppm to allow the exchange of amide protons with bulk water particles, which enables indirect signal intensity acquisition [14]. The APT value was shown to increase with the concentration of mobile proteins and peptides [15], which was unrelated to gadolinium enhancement. Arterial spin labeling (ASL) is a perfusion technique that does not require injection of exogenous contrast agents [16]. It has a high sensitivity for evaluating microvascular proliferation in the tumor using an endogenous intravascular tracer‑hydrogen nuclei in blood and allows for the measurement of CBF values in brain tissue [17]. A few previous few studies have demonstrated the usefulness of APTw or CBF values for identifying these two tumor entities based on tumor core and peritumoral parts [[18], [19], [20], [21]].

Given that both GBM and SBM are characterized by high cell proliferation and microvascular hyperplasia, we used APTw and ASL techniques sensitive to these two pathological changes to identify SBMs and GBMs in this study to improve the accuracy of diagnosis and treatment planning. The unique aspect of this study is that we investigated the use of combination of APTw and ASL imaging for differentiating SBMs from GBMs.