Automated high throughput 3D imaging using desorption … · 2016-06-22 · Research Enabled...
Transcript of Automated high throughput 3D imaging using desorption … · 2016-06-22 · Research Enabled...
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INTRODUCTION Typically with imaging mass spectrometry, the overnight period is reserved for a single large acquisition or a few regions on a single slide, maximising the amount of information obtained at a time when the instrument would be otherwise be unused. An improved approach to maximising the amount of information and system utilisation during this period would be to analyse multiple samples on multiple slides. With advances in sprayer design, acquisition software, data directed methodologies and robotics, the overnight period can now be used to collect a full 3D imaging dataset or a large cohort of patient samples. Here we introduce a DESI system which combines DESI‐MS imaging with an automated slide loader (figure 1) and prototype software for high throughput mass spectrometry imaging.
Automated high throughput 3D imaging using desorption electrospray ionisation mass spectrometry
Emrys Jones1; Lukasz Migas2, Emmy Hoyes1, Richard Chapman1, Emmanuelle Claude1; James Langridge1; Steve Pringle1,Zoltan Takats3; Mike Morris1 1: Waters Corporation, Wilmslow, UK; 2: University of Manchester, Manchester, UK; 3: Imperial College London South Kensington Campus, London, UK.
METHODS All experiments were carried out on a Xevo G2‐XS Q‐ToF (Waters) equipped with a Prosolia (Indianapolis) 2D DESI stage. The automated slide loader and modified plate holder were provided by Prior Scientific (Cambridge, UK). Mouse brain and kidney were sectioned onto conventional glass slides; no further sample preparation was required. For the 3D imaging tissue sectioning was performed such as the voxels created were cubic, e.g. for 100 x 100 µm DESI images, sections at 100 µm distances from each other are included. Once optimised, the DESI parameters were unchanged for the duration of the studies. The current combination of sprayer and Xevo‐XS mass spectrometer allows for high intensity signal at accelerated scan rates, the example in figure 2 is data collected with a MS scan time of 0.186 s and stage movement of 200µm per second allowing five 40µm pixels to be collected per second. Faster rates will be described. The glass slides were scanned and regions of interest defined in HDI v1.4
(Waters) creating a MassLynx experiment file to be imported into a sample list
– a fully automated approach based on a rapid initial scan using Waters
Research Enabled Software (Wrens) is also introduced. Data visualisation is
performed in HDI v1.4 with the 3D reconstruction, processing and viewing
using custom written software and MATLAB (Mathworks).
CONCLUSION No requirement for a sample preparation step makes DESI
ideal for high throughput, automated imaging Single section tissue imaging at 10 minute time frames at
100 x 100 µm pixel size Automated slide loading and region of interest detection Full 3D image in <20 hours acquisition, suitable for
overnight data collection and processing Workflow for automated 3D reconstruction in place
Figure 4. Manual experiment set up/ automated data collection. All slides are scanned and
experiments defined by user before running multi slide experiment
Survey scan approach As previously shown the gentle sampling of DESI makes repeat analysis
possible, thus multiple experiments can be conducted in series. In order to
have a fully automated system it is possible to first carry out a coarse survey
scan to detect the tissue section, then image at the desired resolution only
over this area. In figure 5 this is demonstrated where a 90 second scan of the
full slide is used to direct the full analysis. Using this method provides a fully
automated method of analysing multiple sample slides.
Control and processing software
Imaging mass spectrometry experiments are typically carried out on regions of
a surface determined by the user. For an automated process, minimal user
interaction is desired, in an idealized situation the slides would be loaded into
the system and the experiment defined and run with no further input (fig 3).
There are many documented approaches to region identification for
microscopy, with DESI there is the ability to use a rapid mass spectrometry
mapping to find the location of the tissue.
RESULTS Speed of acquisition and tissue stability
In order to maximise the benefit of automatic slide loading the acquisition time
per sample should be in the region of minutes, not hours, and developments with
hardware and software has made this a realistic proposition. Figure 6
demonstrates the results of imaging the same tissue section at different MS scan
rates. Although total spectral intensity is lower, the data and image quality are
preserved, allowing a full image (14mm x 8mm at 100µm) in 12 minutes.
Figure 8: Individual acquisition time of 40 sections used to create 3D lipid map of mouse kid‐
ney, three colour overlay of selected ions.
Figure 2. Results from DESI imaging studies demonstrate the ability to obtain high quality data at higher scan rates than previous studies. Data shown collected at 5 pixels per second.
Total acquisition time
The 40 sections, cut at 100µm distances through the kidney were imaged at 100 x 100, at 10 scans per second. The largest section was approximately 12mm x 10mm. The total acquisition time was 16 hours including the post acquisition data processing.
Figure 12: 3D visualisation of four separate lipid species within mouse kidney. The known biology of the sample is
well represented.
Figure 1. Schematic of the modified top plate for the Prosolia 2D stage to allow coupling to the Prior Scientific slide loader
View the above 3D dataset at Skfb.ly/NXnK or scan the QR
m/z 762.53 m/z 772.53
m/z 764.52
m/z 890.63
Figure 5. Use of survey scan to automatically define the region to be imaged at the desired analytical experimental conditions.
Figure 11. Isosurface output from auto-aligned data. Once the x-y offset and rotation is calculated for the first mass (typically the TIC) all others are processed using these transformation constants
Semi automatic approach For the minimal deviation from a standard work flow, each slide in the
proposed slide loader run is defined by the user creating an experiment
worksheet which can be added to a sequence list. A modification to the DESI
source control software controls the loading/unloading of slides in between
each acquisition (fig 4).
Figure 3: Different methods of multiple slide analysis, including optical region definition
example where edge and feature analysis algorithms are employed.
Figure 6. Comparison of DESI‐MS imaging of the same tissue section at varying mass spectrometry
scan rates. DESI stage speed was adjusted to ensure the same pixel dimensions, which are 100µm
x 100µm
Figure 7. Investigation into the stability of lipid signal from tissue on timescale of auto slide loader
experiment. The upper portion of a mouse kidney section was imaged and left on the DESI stage
for 14 hours. The lower portion was then imaged, the results demonstrate that without
normalisation the results are comparable.
Data processing
It is not only the acquisition of 3D imaging MS data that requires automation and
simplification if it is to become routine, the reconstruction and visualisation has
also in the past been very labour intensive.
For the data presented here, a number of automated steps have been developed
to go from a large amount of imaging MS data to easily visualised three
dimensional representations. Firstly, as shown in figure 9, a script runs through all
raw data extracting co‐ordinate and selected m/z integration ranges.
These can now be imported into a software package such as MATLAB to be
combined into a data cube for each m/z range. Previous 3D imaging experiments
have used fiducial markers incorporated into an embedding material to allow for
the alignment of subsequent sections. This requires a carefully designed
experiment and has proven problematic.
By using the data harvesting routine to create a total ion (or total lipid) map for
each section, object definition approaches can be used to find the centre of a
section and then run iterative rotations around this centre to maximise the
correlation with the previous (or a selected) section. The need for this correction
step, and an outline of the processes involved are shown in figure 10.
Figure 9. Data harvesting method for multiple samples. A C# script is
utilised to extract the co‐ordinates and intensities integrated between
selected mass windows.
Figure 10. a) The requirement for correction of data position and rotation. Error in sectioning and region of interest drawing means that images from subsequent sections will not
align when stacked. B) A MATLAB algorithm that automatically positions, aligns and rotates each z plane based on correlation with previous section during a rotation around the
object centre.
With rapid scanning, the degradation of lipid signal over a single section will not
be significant. However, while future systems may incorporate a cold cabinet for
the queued slides, the stability of such samples at ambient conditions over 10+
hours needs to be demonstrated for the current approach. This is investigated in
figure 7, where the two halves of a section were imaged 14 hours apart.
The effect of the alignment and rotational
correction can be seen in figure 11. The top two
isosurfaces have had the z axis stretched to
emphasise the extent of misalignment of the raw
data and the improvement made by this quick
optimisation process.
Once aligned a number of processes can be carried
out to enhance the clarity of the surfaces created,
such as smoothing and thresholding. These can
then be output as a common 3D model object to be
visualised by any means suitable. The examples in
figure 12 are combinations of isosurfaces that have
been imported into Blender (open source, Blender
Foundation) to be viewed with different material
properties such as colour and transparency. These
can then be shared by a number of viewing
platforms.