Quantitative Susceptibility Mapping (QSM) MRI in a ...
1
mean susceptibility (ppm) Quantitative Susceptibility Mapping (QSM) MRI in a Collagenase Rat Model of Intracerebral Hemorrhage (ICH) Kimmo Lehtimäki 1 , Artem Shatillo 1 , Elina Latonummi 1 , Antti Nurmi 1 1 Charles River Discovery Services, Kuopio, Finland 1 INTRODUCTION Intracerebral hemorrhage (ICH) is a significant cause of mortality throughout the world. Management of ICH in terms of clot lysis and iron scavenging after the initial insult is critical for the recovery and appropriate imaging methods to detect and follow the process are needed (Morgenstern et al., Stroke 2010). The objective of this study was to validate quantitative susceptibility mapping (QSM) methodology in a collagenase-induced ICH model in Wistar rats. Working hypothesis was that QSM could show window to observe the susceptibility changes both in the edematous processes and iron environment. 2 METHODS CONT’D 4 CONCLUSIONS • In vivo T2 mapping showed large hypointense lesions corresponding to actual acute hemorrhage at 6 hours which developed progressively to hyperintense lesions at later time points 1 and 3 days and started to resolve at 14 days. Diffusion values showed clear increase (while T2 remaining roughly at the same level) from day 1 to day 3, most likely reflecting reduced cytotoxic contribution in the lesion development. • QSM revealed large collagenase-induced ICH lesions with low susceptibility core and high susceptibility outer rim (high iron contribution) surrounded again by low susceptibility region outside the actual lesion. This “rim-around-rim” is assumed to reflect ongoing cytotoxic edema process whereas the low susceptibility in the core of the lesion relates to vasogenic edema and cell death. • Total lesion QSM results by histogram comparison show clear modulation of spread of susceptibility values from day 1 to day 3; namely, day 1 distribution contain significantly higher proportion of both oedema processes and iron content than day 3. • Based on these data, QSM method seems particularly suitable for the in vivo application in a rat model of ICH due to proper lesion size and the clear presence of iron. This combination of methodology and animal model may provide the window to study novel treatments of ICH. 3 RESULTS Figure 1. Longitudinal in vivo T2-MRI for lesion development at 6 hr, 24 hrs, 3 and 14 days post- ICH operation. Clearly visible haemorrhage at 6 hours is rapidly transformed into hyperintense lesion at 24 hours, coinciding the peak of the oedema process. Volume of the lesion reduces from initial insult as a function of time during the 14 days observation period. Figure 2. Ex vivo QSM results from a rat at day 1 post-ICH. Example of color coded and thresholded susceptibilities for a) paramagnetic regions within the lesion (>0.05 ppm, iron contribution) and diamagnetic b) regions (< -0.05 ppm, bottom panel) overlaid with original 3D Gradient echo T2* data. Figure 3. Group histogram results from ex vivo QSM experiments. a) Mean distributions (in 0.05 ppm histogram bins) for rats imaged at days 1 and 3; rats at 1 days post-ICH have significantly higher iron and oedema contribution as manifested by broader distribution. Significant histogram bins are shown in figure legend (t-test). b) Numerical beta-distribution simulation (“b” constant at 7.75, varying parameter “a”) for positive susceptibility part; width of the spread on the right (higher susceptibility) can be interpreted as higher iron content. c) quantified group beta- distribution constants showing resolving iron load from day 1 to day 3. 475.08 a) b) c) ** mean susceptibility (ppm) 2 METHODS All animal experiments were carried out according to the National Institute of Health (NIH) guidelines for the care and use of laboratory animals, and approved by the National Animal Experiment Board, Finland. Male Wistar rats, purchased from Charles River, Germany and weighing 230-300 g were used for the experiment. Animals were housed at a standard temperature (21 ± 1 °C) and in a light-controlled environment (lights on from 7 am to 8 pm) with ad libitum access to food and water. Intra cerebral hemorrhage (ICH) was produced in right hemisphere according to MacLellan with modifications (MacLellan et al. JCBFM (2008) 28, 516–525). Fifteen minutes before infusion, rats received buprenorphine 0.03 mg/kg s.c. During the operation the concentration of anesthetic was kept at 1.0-1.5 %. The rectal temperature was maintained at 37.0 ± 1.5 °C using a homeothermic blanket system. After attachment of stereotaxic apparatus the head skin on rat was shaved and disinfected with betadine solution. Right brain hemisphere was exposed through a small craniectomy to the skull using following coordinates AP +1.0, ML +2.8, DV -6.0 mm. The dura mater was carefully removed with fine forceps and a stereotaxic injection of collagenase IV (1.25 U/rat in 2 µL/rat or sterile saline for SHAM group) was made into the right striatum. A total of 2 μL of collagenase solution was infused at a speed of 0.2 μL/min. The needle was left in place for another 5 min before being withdrawn. All surgical procedures were finished within 30 min or less. In vivo MRI acquisitions at 7T Bruker BioSpec MR system at 6 hours, 1, 3 and 14 days: • Absolute T2/volumetry MSME: TR/TE=2.5 s/10-120 ms, 1 mm slices and 117 µm in-plane resolution • 1/3 of the trace of the diffusion tensor was acquired using a DW SE sequence TR/TE 2200/28 ms, b- values 0, 1000 s/mm 2 . Slice positioning and resolution identical to T2 measurement • Region of interest analysis was performed for ipsilateral healthy hemisphere, lesion and contralateral hemisphere (in-house Matlab software; T2 values, volumetry and diffusion). Brain oedema was evaluated as percentage difference for hemispheres. Subset of animals (n=4/group at days 1 and 3) were subjected to transcardial saline+PFA perfusion and the brains in skull (incubated in 4 % PFA for two days) were placed into perfluoropolyether (GALDEN) filled tubes for ex vivo scanning. Ex vivo QSM MRI acquisitions at 11.7T Bruker BioSpec MR system at day 1 and 3 post-ICH: • 3D multi-echo GRE sequence with a TR = 250 ms, N echoes = 8, echo spacing = 4 ms (first 2.7 ms), flip 30° and isotropic resolution of 100 µm 3 • Field maps for original data were estimated by fitting the complex data over multiple echo times. Laplacian unwrapping and SHARP filtering (threshold=0.05, kernel diameter=9 voxels) were performed to remove residual wraps and background fields. • Susceptibility maps were calculated using threshold k-space division (TKD; threshold=2/3 and correction for susceptibility underestimation). • Manual ROI-delineation was applied for total visible lesion in the original 3D GRE data and transferred to QSM maps. QSM data were analyzed by histograms (areas normalized to 1 by division on sum of total histogram bin-events) using [-1 +1] range in forty-one 0.05 ppm width bins. • Positive side of individual’s susceptibility spread (>0.00 ppm, iron contribution) was additionally modelled by Beta-distribution. Fit-parameter “b” = 7.75 (average of all animals) was chosen for the model and thus, the fit parameter “a” describes the width of the spread (increasing value “a” reports higher content of iron). Normalized group histogram bin contribution
Transcript of Quantitative Susceptibility Mapping (QSM) MRI in a ...
mean susceptibility (ppm)
Quantitative Susceptibility Mapping (QSM) MRI in a Collagenase Rat Model of Intracerebral Hemorrhage (ICH)Kimmo Lehtimäki1, Artem Shatillo1, Elina Latonummi1, Antti Nurmi1
1Charles River Discovery Services, Kuopio, Finland
1 INTRODUCTIONIntracerebral hemorrhage (ICH) is a significant cause of mortality throughout the world.Management of ICH in terms of clot lysis and iron scavenging after the initial insult is critical forthe recovery and appropriate imaging methods to detect and follow the process are needed(Morgenstern et al., Stroke 2010). The objective of this study was to validate quantitativesusceptibility mapping (QSM) methodology in a collagenase-induced ICH model in Wistar rats.Working hypothesis was that QSM could show window to observe the susceptibility changes bothin the edematous processes and iron environment.
2 METHODS CONT’D 4 CONCLUSIONS• In vivo T2 mapping showed large hypointense lesions corresponding to
actual acute hemorrhage at 6 hours which developed progressively tohyperintense lesions at later time points 1 and 3 days and started to resolveat 14 days. Diffusion values showed clear increase (while T2 remainingroughly at the same level) from day 1 to day 3, most likely reflectingreduced cytotoxic contribution in the lesion development.
• QSM revealed large collagenase-induced ICH lesions with lowsusceptibility core and high susceptibility outer rim (high iron contribution)surrounded again by low susceptibility region outside the actual lesion. This“rim-around-rim” is assumed to reflect ongoing cytotoxic edema processwhereas the low susceptibility in the core of the lesion relates to vasogenicedema and cell death.
• Total lesion QSM results by histogram comparison show clear modulationof spread of susceptibility values from day 1 to day 3; namely, day 1distribution contain significantly higher proportion of both oedemaprocesses and iron content than day 3.
• Based on these data, QSM method seems particularly suitable for the invivo application in a rat model of ICH due to proper lesion size and theclear presence of iron. This combination of methodology and animal modelmay provide the window to study novel treatments of ICH.
3 RESULTS
Figure 1. Longitudinal in vivo T2-MRI for lesion development at 6 hr, 24 hrs, 3 and 14 days post-ICH operation. Clearly visible haemorrhage at 6 hours is rapidly transformed into hyperintenselesion at 24 hours, coinciding the peak of the oedema process. Volume of the lesion reduces frominitial insult as a function of time during the 14 days observation period.
Figure 2. Ex vivo QSM results from a rat at day 1 post-ICH. Example of color coded and thresholdedsusceptibilities for a) paramagnetic regions within the lesion (>0.05 ppm, iron contribution) anddiamagnetic b) regions (< -0.05 ppm, bottom panel) overlaid with original 3D Gradient echo T2* data.
Figure 3. Group histogram results from ex vivo QSM experiments. a) Mean distributions (in 0.05ppm histogram bins) for rats imaged at days 1 and 3; rats at 1 days post-ICH have significantlyhigher iron and oedema contribution as manifested by broader distribution. Significant histogrambins are shown in figure legend (t-test). b) Numerical beta-distribution simulation (“b” constant at7.75, varying parameter “a”) for positive susceptibility part; width of the spread on the right(higher susceptibility) can be interpreted as higher iron content. c) quantified group beta-distribution constants showing resolving iron load from day 1 to day 3.
475.08
a) b)
c)
**
mean susceptibility (ppm)
2 METHODSAll animal experiments were carried out according to the National Institute of Health (NIH)guidelines for the care and use of laboratory animals, and approved by the National AnimalExperiment Board, Finland. Male Wistar rats, purchased from Charles River, Germany andweighing 230-300 g were used for the experiment. Animals were housed at a standardtemperature (21 ± 1 °C) and in a light-controlled environment (lights on from 7 am to 8 pm) withad libitum access to food and water.Intra cerebral hemorrhage (ICH) was produced in right hemisphere according to MacLellan withmodifications (MacLellan et al. JCBFM (2008) 28, 516–525). Fifteen minutes before infusion, ratsreceived buprenorphine 0.03 mg/kg s.c. During the operation the concentration of anesthetic waskept at 1.0-1.5 %. The rectal temperature was maintained at 37.0 ± 1.5 °C using a homeothermicblanket system. After attachment of stereotaxic apparatus the head skin on rat was shaved anddisinfected with betadine solution. Right brain hemisphere was exposed through a smallcraniectomy to the skull using following coordinates AP +1.0, ML +2.8, DV -6.0 mm. The duramater was carefully removed with fine forceps and a stereotaxic injection of collagenase IV (1.25U/rat in 2 µL/rat or sterile saline for SHAM group) was made into the right striatum. A total of 2 μLof collagenase solution was infused at a speed of 0.2 μL/min. The needle was left in place foranother 5 min before being withdrawn. All surgical procedures were finished within 30 min or less.
In vivo MRI acquisitions at 7T Bruker BioSpec MR system at 6 hours, 1, 3 and 14 days:• Absolute T2/volumetry MSME: TR/TE=2.5 s/10-120 ms, 1 mm slices and 117 µm in-plane resolution• 1/3 of the trace of the diffusion tensor was acquired using a DW SE sequence TR/TE 2200/28 ms, b-
values 0, 1000 s/mm2. Slice positioning and resolution identical to T2 measurement• Region of interest analysis was performed for ipsilateral healthy hemisphere, lesion and contralateral
hemisphere (in-house Matlab software; T2 values, volumetry and diffusion). Brain oedema wasevaluated as percentage difference for hemispheres.
Subset of animals (n=4/group at days 1 and 3) were subjected to transcardial saline+PFA perfusion andthe brains in skull (incubated in 4 % PFA for two days) were placed into perfluoropolyether (GALDEN)filled tubes for ex vivo scanning.Ex vivo QSM MRI acquisitions at 11.7T Bruker BioSpec MR system at day 1 and 3 post-ICH:• 3D multi-echo GRE sequence with a TR = 250 ms, N echoes = 8, echo spacing = 4 ms (first 2.7 ms),
flip 30° and isotropic resolution of 100 µm3
• Field maps for original data were estimated by fitting the complex data over multiple echo times.Laplacian unwrapping and SHARP filtering (threshold=0.05, kernel diameter=9 voxels) wereperformed to remove residual wraps and background fields.
• Susceptibility maps were calculated using threshold k-space division (TKD; threshold=2/3 andcorrection for susceptibility underestimation).
• Manual ROI-delineation was applied for total visible lesion in the original 3D GRE data andtransferred to QSM maps. QSM data were analyzed by histograms (areas normalized to 1 by divisionon sum of total histogram bin-events) using [-1 +1] range in forty-one 0.05 ppm width bins.
• Positive side of individual’s susceptibility spread (>0.00 ppm, iron contribution) was additionallymodelled by Beta-distribution. Fit-parameter “b” = 7.75 (average of all animals) was chosen for themodel and thus, the fit parameter “a” describes the width of the spread (increasing value “a” reportshigher content of iron).
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