Combination of standard axial and thin‐section coronal diffusion‐weighted imaging facilitates the diagnosis of brainstem infarction

Abstract Background and Purpose Although diffusion‐weighted imaging (DWI) is a very sensitive technique for the detection of small ischemic lesions in the human brain, in particular in the brainstem it may fail to demonstrate acute ischemic infarction. In this study, we sought to evaluate the value of additional thin‐section coronal DWI for the detection of brainstem infarction. Methods In 155 consecutive patients (median age 69 [interquartile range, IQR 57–78] years, 95 [61.3%] males) with isolated brainstem infarction, MRI findings were analyzed, with emphasis on ischemic lesions on standard axial (5 mm) and thin‐section coronal (3 mm) DWI. Results On DWI, we identified ischemic lesions in the mesencephalon in 12 (7.7%), pons in 115 (74.2%), and medulla oblongata in 31 (20%) patients. In 3 (1.9%) cases—all of these with medulla oblongata infarction—the ischemic lesion was detected only on thin‐section coronal DWI. Overall, in 35 (22.6%) patients the ischemic lesion was more easily identified on thin‐section coronal DWI in comparison to standard axial DWI. In these, the ischemic lesions were significantly smaller (0.06 [IQR 0.05–0.11] cm3 vs. 0.25 [IQR 0.13–0.47] cm3; p < .001) in comparison to those patients whose ischemic lesion was more easily (6 [3.9%]) or at least similarly well identified (114 [73.5%]) on standard axial DWI. Conclusions Since thin‐section coronal DWI may facilitate the diagnosis of brainstem infarction, we suggest its inclusion in standard stroke MRI protocols.

others large vessel disease of the vertebral arteries or basilar artery, small vessel disease of small perforating arteries, and cardioembolism (de Ortiz et al., 2013).
Diffusion-weighted imaging (DWI) is a very sensitive technique for detection of small ischemic lesions in the human brain and in particular in the posterior fossa (Wardlaw et al., 2000). Consequently, DWI has become a reliable mean to identify and secure the diagnosis of acute brainstem infarction (Toi et al., 2003). Nevertheless, even DWI may fail to demonstrate ischemic lesions in a substantial proportion of patients with brainstem infarction (Oppenheim et al., 2000;Sylaja, Coutts, Krol, Hill, & Demchuk, 2008).
In this study, we sought to evaluate the additional value of combined standard axial and additional thin-section coronal DWI for the detection of brainstem infarction.

| Patients
In this retrospective single-center study, we identified all patients with isolated acute ischemic infarction in the brainstem from a MRI report database (2011)(2012)(2013)(2014)(2015). The study was approved by the local institutional review board (Medizinische Ethikkommission II der Medizinischen Fakultät Mannheim).

| MRI studies
Magnetic resonance imaging was performed on a 1.5-T or a 3-T MR system (Magnetom Sonata/Avanto/Trio, Siemens Medical Systems, Erlangen, Germany). A standardized protocol was used in all patients including standard axial and thin-section coronal DWI. Parameters of DWI are displayed in Table 1.

| MRI analysis
Localizations of hyperintense lesions in the brainstem were noted on standard axial and thin-section coronal DWI. The topography was determined according to the maps by Tatu et al. (1996) and categorized in (1) mesencephalon; (2) pons; and (3) medulla oblongata. The identifiability of ischemic lesions in standard axial and thin-section coronal DWI was independently evaluated by two raters (P.F. and A.F. with 2 and 10 years experience in neuroimaging, respectively) and categorized as (1) better delineation on axial DWI; (2) better delineation on coronal DWI; (3) equal delineation on axial and coronal DWI. Cases with discrepancies were rereviewed by both readers and discussed until a consensus was reached. Ischemic lesion size was measured on DWI by manually delineated ROI, summation of these areas in cm 2 on each section and multiplication with the slice thickness (plus interslice gap), to determine the volume in cm 3 by use of OsiriX (Pixmeo SARL, Bernex, Switzerland; Rosset, Spadola, & Ratib, 2004).  In 3 (1.9%) cases-all these with medulla oblongata infarction-the ischemic lesion was detected only on thin-section coronal DWI (for an example see Figure 1). Overall, in 35 (22.6%) patients the ischemic lesion was more easily identified on thin-section coronal DWI in comparison to standard axial DWI. In detail, ischemic lesions were better identifiable on thin-section coronal DWI in the mesencephalon in 3

| DISCUSSION
Due to the brainstem's small size and its densely packed composition, a very small ischemic lesion may result in relevant clinical symptoms.
Diffusion-weighted imaging is the gold standard for the detection of acute ischemic stroke (Wardlaw et al., 2000).  Sylaja et al., 2008). In order to overcome this limitation, additional thinsection axial DWI of the infratentorium has been suggested (Entwisle, Perchyonok, & Fitt, 2016;Sorimachi, Ito, Morita, & Fujii, 2008 This study has some limitations. First, this is a retrospective clinical study of moderate size. However, to our knowledge this is the first series investigating the additional value of thin-section coronal DWI for the diagnosis of acute brainstem infarction. Second, the study has been performed with different MRI scanners. However, DWI sequences have been customized for optimal comparability in daily clinical routine and consequently are generally comparable. Third, acute ischemic lesions on DWI were defined by a consensus reading and not by follow-up MRI.
In conclusion, combination of standard axial and thin-section coronal DWI possibly facilitates the diagnosis of brainstem infarction.
Consequently, we suggest the inclusion of thin-section coronal DWI in standard stroke MRI protocols for patients with suspected stroke in the posterior fossa.