A Rating Scale for Brain Microbleeds

User guide for BOMBS (Brain Observer MicroBleed Scale): A rating scale for brain microbleeds The Brain Observer MicroBleed Scale is a classification system devised to improve levels of interrater agreement about the presence, number, size and location of brain microbleeds (BMBs). The use of a standard rating scale will hopefully minimise inter-observer variation, enable crosscomparison between research groups and facilitate meta-analysis of BMB studies. The BOMBS scale was developed after testing of an initial pilot scale by two observers highlighted several common difficulties in rating BMBs, leading to inter-observer variation. These difficulties were then incorporated into a revised scale. Brain microbleeds are most easily identified on haem-sensitive MRI sequences, also know as T2* or gradient echo (GRE). However, they may also be visible on T2-weighted images, especially if they are numerous. T2-weighted images are particularly useful for demonstrating flow voids in cortical vessels, which can mimic of BMBs (see later). If there are no BMBs evident on the GRE images, it is unlikely that they will be identified on other sequences, although all images should be reviewed as standard practice. The BOMBS rating scale is shown on the next page. This is followed by a more detailed description of the main sections, including a diagram of the 7 anatomical locations which must be viewed for each scan. Finally, there are examples of ‘certain’ and ‘uncertain’ BMBs and examples of common BMB mimics. The BOMBS rating scale This diagram was prepared by the BOMBS authors, but appears in the publication in Stroke (2009; 40:94099) and is copyright to Stroke. Description of BOMBS The top section of the scale (yellow) consists of a flow chart, highlighting the two main situations in which common problems should be reviewed: 1. If the lesion is in a location where ‘BMB mimics’ are common - basal ganglia (at site of deep MCA perforators), cortex and adjacent to petrous apex / orbit (see examples below) and 2. If there are 1-2 BMBs. This diagram was prepared by the BOMBS authors, but appears in the publication in Stroke (2009; 40:94099) and is copyright to Stroke. There are 7 locations which must be assessed on both sides of the brain. Descriptions of 3 of these (yellow) are given at the bottom of the scale as shown. The diagram on the next page demonstrates these locations. This diagram was prepared by the BOMBS authors, but appears in the publication in Stroke (2009; 40:94099) and is copyright to Stroke. Locations to be assessed in BOMBS (excluding cerebellum and brainstem) Lobar Cortex (grey matter) Lobar Deep white matter Cortex (grey matter) Deep white matter Deep Caudate (C), lentiform nucleus (LN), Deep thalamus (Th) Internal (IC) & external capsules (EC) C EC IC LN Th The following slides show MR images which have been chosen to demonstrate examples of ‘certain’ and ‘uncertain’ microbleeds. BOMBS improved inter-rater reliability when all brain locations were analyzed together, and particularly in lobar locations, which had been identified in our pilot study as a difficult part of the brain to rate (Table 2). Although the consideration of BMB mimics is widely recognized as being important, observer variation persists, even when mimics are carefully thought about during MR scan review. BOMBS had its main effect by differentiating ‘certain’ BMBs from ‘uncertain’ BMBs – uncertainty about BMBs may be an important problem, because it applied to between one third to one half of BMBs in this study (Table 2). BMB maximum diameters in prior research have varied from 2-5mm, to ≤7mm and ≤10mm. 1 Our findings should be regarded as a baseline measure of observer agreement for future studies using BOMBS. Further work on ways of improving observer agreement about BMBs is needed, and training observers to recognize certain and uncertain BMBs, as well as their mimics, is an obvious priority (Figure 3). BOMBS will also enable others to study agreement about BMB size, number, brain location, and diagnostic certainty, as well as exploring the influence of these factors on the diagnostic and prognostic utilities of BMBs. Note that deep BMBs are divided into 3 locations: caudate head/lentiform nucleus (C, LN), internal/external capsules (IC, EC) and thalamus (Th). Deep white matter BMBs which touch cortex are included in the category ‘cortex/grey-white matter junction’. Finally, BMBs should be rated as ‘certain’ and ‘uncertain’ for each side of the brain, in the appropriate row for rating size (<5mm, 5-10mm). Examples of BMBs and their common mimics are shown on the following 5 pages. to Stroke. ‘Certain’ BMBs Locations with BMBs: Cortex/grey-white matter junction (both sides) Internal capsule/external capsule (left) Thalamus (both sides) Note: Right thalamus BMB measures 5-10mm ‘Certain’ BMBs Locations: Cortex/grey-white matter junction (both sides) Deep white matter (both sides) Note: All BMBs (not all labelled) measure <5mm – however, there is also an old right parietal haemorrhage (long arrow) ‘Uncertain’ BMBs Here, there is a pale unilateral low signal lesion in the right internal capsule. In this case, no CT was available to check for asymmetric calcification. This lesion would be rated as ‘uncertain’. Here, the lesion is smaller and even paler than in the previous case and would therefore be rated as ‘uncertain’. Remember that when there are 1-2 BMBs, rating may be more difficult. Examples of common ‘BMB mimics’ 1. Basal ganglia calcification In this case, axial non-contrast CT (right) confirms the presence of bilateral basal calcification, mimicking BMBs on gradient echo MRI (left) 2. Cortical vessels In this case, the circled ‘BMB’ (magnified image on right) is due to a sulcal vessel seen in cross section - with a visible vessel leading up to it. 3. Partial volume artefact A. From petrous temporal bone Note the small apparent ‘pale’ BMBs in the left temporal lobe (arrows) – these are due to partial volume from the petrous temporal bone and therefore not BMBs. B. From orbit Similarly, this ‘pale BMB’ in the right medial frontal lobe (arrow) is due to partial volume artifact from the orbit, which lies immediately inferior to this slice. Conclusion Because the clinical implications of BMBs remain to be established, there is still an opportunity to improve the reliability of BMB assessment by the use (and further development) of the BOMBS rating scale, so that adequately-powered, well-designed studies will be able to answer the outstanding clinical concerns about their diagnostic and prognostic value, and whether presence of BMBs should influence the prescription of antiplatelet, anticoagulant, or thrombolytic drugs.