Imaging mass spectrometry data processing device

Abstract

A peak-waveform conversion processor detects a peak in a profile spectrum created based on data obtained at each measurement point in a sample's measurement area, and acquires a rod-like peak by performing centroid conversion processing on a waveform of the peak having a mountain shape. When an operator specifies a target compound to be observed, a mass difference calculation unit calculates a mass difference between a precise m/z of the target compound and an m/z of a rod-like peak at a position close to the precise m/z for each measurement point. A mass difference image creator creates an image showing a distribution of mass differences based on the calculated mass differences. A mass difference related information calculation unit acquires an index value such as an average value of a plurality of mass differences for each mass difference image, and creates a graph showing a frequency distribution of the mass differences.

Claims

1. An imaging mass spectrometry data processing device configured to process mass spectrometry data which is profile data obtained from a plurality of micro areas in a measurement area on a sample, the imaging mass spectrometry data processing device comprising: a) an input reception unit configured to receive a specification by a user of a compound or a mass-to-charge ratio value to be displayed in a mass spectrometry image; b) a mass difference calculation unit configured to calculate a difference, for each micro area, between a mass-to-charge ratio value corresponding to a compound specified by the user or a target mass-to-charge ratio value which is a mass-to-charge ratio value specified by the user, and an observed mass-to-charge ratio value which is acquired from a peak observed in a profile spectrum formed using the profile data and which is inferred to correspond to the target mass-to-charge ratio value, and c) a mass difference image creator configured to create and display an image showing a two-dimensional distribution of mass differences corresponding to the measurement area or a part of the measurement area based on information on the mass difference calculated by the mass difference calculation unit for each micro area.

2. The imaging mass spectrometry data processing device according to claim 1, further comprising: an index value calculation unit configured to calculate an index value representing an overall tendency of mass differences in a plurality of micro areas included in all or a part of the measurement area based on the mass differences in the plurality of micro areas.

3. The imaging mass spectrometry data processing device according to claim 1, further comprising: a graph creation unit configured to create a graph showing a tendency of a distribution of mass differences in a plurality of micro areas included in all or a part of the measurement area based on the mass differences in the plurality of micro areas.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic block diagram of an imaging mass spectrometer including a spectrometry data processing device according to an embodiment of the present invention.

(2) FIGS. 2A-2G are explanatory views of MS image creation processing in an imaging mass spectrometer of the present embodiment.

(3) FIG. 3 is a schematic diagram showing an example of mass difference information displayed in an imaging mass spectrometer of the present embodiment.

(4) FIG. 4 is a diagram showing an example of a relationship between a precision mass range corresponding to a target compound and an intensity integration range of a peak acquired by measurement.

(5) FIG. 5 is a diagram showing another example of a relationship between a precision mass range corresponding to a target compound and an intensity integration range of a peak acquired by measurement.

DESCRIPTION OF EMBODIMENTS

(6) Hereinafter, an imaging mass spectrometer including an imaging mass spectrometry data processing device according to an embodiment of the present invention will be described with reference to the accompanying drawings.

(7) FIG. 1 is a schematic configuration diagram of the imaging mass spectrometer according to the present embodiment. FIGS. 2A-2G are explanatory diagrams of characteristic data processing in the imaging mass spectrometer of the present embodiment.

(8) The imaging mass spectrometer of the present embodiment includes an imaging mass spectrometry section 1 that performs measurement on a sample, an optical microscopic image acquiring unit 2 that captures an optical micro image on the sample, a data processor 3, and an input unit 4 and a display unit 5 each serving as a user interface.

(9) The imaging mass spectrometry section 1 includes a matrix-assisted laser desorption/ionization (MALDI) ion trap time-of-flight mass spectrometer, for example, and performs mass spectrometry on many measurement points (micro areas) 102 in a two-dimensional measurement area 101 on a sample 100 such as a biological tissue section to acquire mass spectrometry data for each micro area, as shown in FIG. 2A. Here, the mass spectrometry data is mass spectrum data over a predetermined mass-to-charge ratio range, but may be MS.sup.n spectrum data for a specific precursor ion. The optical microscopic image acquiring unit 2 is formed by adding an image acquiring unit to an optical microscope and acquires a microscopic image of a two-dimensional area of the surface on a sample.

(10) The data processor 3 performs predetermined processing after receiving mass spectrum data in each micro area collected by the imaging mass spectrometry section 1, and includes a data collection unit 31, a data storage unit 32, an input reception unit 33, a peak-waveform conversion processor 34, a mass difference calculation unit 35, a mass difference image creator 36, a mass difference related information calculation unit 37, an image creator 38, and a display processor 39, as functional blocks. The data storage unit 32 includes a profile data storage area 321 for storing raw data collected by measurement using the imaging mass spectrometry section 1 and a converted data storage area 322 for storing data processed by the peak waveform conversion processing described later.

(11) In general, the data processor 3 is in fact a personal computer (or a higher-performance workstation), and is configured to execute a function of each of the blocks by operating dedicated software installed in the computer on the computer. In that case, the input unit 4 is a pointing device such as a keyboard or a mouse, and the display unit 5 is a display monitor.

(12) Next, the measurement work for the sample by the imaging mass spectrometer of the present embodiment will be described.

(13) First, when an operator sets the sample 100 to be analyzed at a predetermined measurement position in the optical microscopic image acquiring unit 2 and performs a predetermined operation using the input unit 4, the optical microscopic image acquiring unit 2 captures an image of a surface of the sample 100 and displays the image on a screen of the display unit 5. The operator indicates a measurement area, which is all or a part of the sample 100, on the image using the input unit 4.

(14) The operator temporarily takes out the sample 100 from the device and attaches a matrix for MALDI to the surface of the sample 100. Then, the operator sets the sample 100 with the matrix attached at a predetermined measurement position in the imaging mass spectrometry section 1, and performs a predetermined operation using the input unit 4. This allows the imaging mass spectrometry section 1 to acquire mass spectrometry data over a predetermined mass-to-charge ratio range by performing mass spectrometry on each of the many micro areas in the measurement area indicated as described above on the sample 100. At this time, the data collection unit 31 performs so-called profile acquisition to collect profile data that is a continuous waveform in a mass-to-charge ratio direction within the mass-to-charge ratio range, and then stores the profile data in the profile data storage area 321 of the data storage unit 32. As a matter of course, the profile data storage area 321 stores a column of digitized data of samples acquired by sampling continuous profile waveforms at a predetermined sampling interval (sufficiently smaller than a peak width of each waveform).

(15) When a pattern on a sample surface (borders of different tissues, etc.) can be observed relatively clearly even with the matrix attached to the sample surface, the optical microscopic image acquiring unit 2 may capture an image after the matrix is preliminarily attached to the sample surface.

(16) After measurement of the sample 100 targeted is completed, the operator specifies a compound for which the two-dimensional intensity distribution in the sample 100 is to be determined (hereinafter referred to as a “target compound”) from the input unit 4. The input reception unit 33 receives this input information. When the target compound is specified, the input reception unit 33 acquires a precise mass-to-charge ratio value (typically a theoretical value of a mass-to-charge ratio) corresponding to the specified compound with reference to a compound database or the like stored preliminarily.

(17) The target compound can be specified by a method of directly inputting a compound name or selecting a compound from a compound list prepared preliminarily, for example. To specify a plurality of target compounds, although the target compounds may be specified one by one by the above method, a plurality of target compounds may be preliminarily listed to allow a plurality of target compounds listed in the list to be collectively specified by selecting the list. Instead of specifying the target compound, a mass-to-charge ratio value Ma (hereinafter referred to as a “target mass-to-charge ratio value”) for which a two-dimensional intensity distribution is to be determined may be directly specified.

(18) The user also specifies an allowable range ΔM of a mass-to-charge ratio assumed while specifying the target compound or the target mass-to-charge ratio value. However, when specifying a plurality of target compounds or target mass-to-charge ratio values, the operator does not necessarily specify an allowable range for each target compound or target mass-to-charge ratio, and thus the allowable range may be common to all the target compounds or the target mass-to-charge ratios, for example. Additionally, instead of specifying an allowable range using a numerical value of a unit of a mass-to-charge ratio such as “Da” or “u”, the allowable range may be specified using a ratio to a mass-to-charge ratio value to be the center, such as “ppm”. As a matter of course, other specification methods may be used. What is important is that some allowable range is set for each target compound or each target mass-to-charge ratio. Thus, regardless of whether a target compound or a target mass-to-charge ratio value is specified, information on the mass-to-charge ratio value Ma to be the center and the allowable range ΔM can be obtained for each target compound or each target mass-to-charge ratio value.

(19) The peak-waveform conversion processor 34 reads out profile data in a predetermined mass-to-charge ratio range near a mass-to-charge ratio value M specified for each micro area from the profile data storage area 321 and forms a profile spectrum (refer to FIGS. 2B and 2C). Then, the peak-waveform conversion processor 34 detects a peak having a mountain shape appearing in the profile spectrum for each micro area, and performs centroid conversion processing on the peak detected. For the centroid conversion processing, for example, a well-known algorithm described in Patent Literature 2 or the like may be used, and the position of the center of gravity and an area value of the peak having a mountain shape are typically calculated. Then, the position of the center of gravity, i.e., a mass-to-charge ratio value, is defined as the position of the rod-like peak, and the area value is defined as the height of the rod-like peak, i.e., the signal intensity value. This allows one peak having a mountain shape to be converted into one rod-like peak (so-called centroid peak), as shown in FIG. 2D.

(20) Instead of detecting a peak in a profile spectrum in a predetermined mass-to-charge ratio range near the mass-to-charge ratio value Ma specified, peaks may be detected in a profile spectrum of the entire mass-to-charge ratio range acquired by measurement, and then the centroid conversion processing may be performed on each of the peaks detected. When data constituting a mass spectrum including the rod-like peak acquired by performing the centroid conversion processing as described above is stored in the converted data storage area 322 of the data storage unit 32, mass spectrometry image creation processing described later can be performed without performing the centroid conversion processing on the same profile spectrum again.

(21) The image creator 38 calculates a mass-to-charge ratio range [Ma−ΔM to Ma+ΔM] from the mass-to-charge ratio value Ma and the allowable range ΔM for each target compound or for each target mass-to-charge ratio value in each micro area. Then, the image creator 35 determines whether or not a rod-like peak exists in the mass-to-charge ratio range [Ma−ΔM to Ma+ΔM], and when the rod-like peak exists in the mass-to-charge ratio range [Ma−ΔM to Ma+ΔM], the height (signal intensity value) Ic of the rod-like peak is regarded as a signal intensity value corresponding to the target compound in the micro area, as shown in FIG. 2E. In contrast, when the rod-like peak does not exist in the mass-to-charge ratio range [Ma−ΔM to Ma+ΔM] as shown in FIG. 2F, a signal intensity value corresponding to the target compound in the micro area is set to zero.

(22) Then, the image creator 38 determines a signal intensity value corresponding to each micro area by performing similar processing in each micro area. This allows a signal intensity value of each of the many micro areas 102 included in the measurement area 101 to be acquired for each target compound or each target mass-to-charge ratio value. Then, the signal intensity values are two-dimensionally disposed corresponding to positions of the micro areas 102 and display colors are applied to the respective signal intensity values according to a predetermined color scale to create a mass spectrometry image. The display processor 39 causes the mass spectrometry image created for each of the target compound and the target mass-to-charge ratio value to be displayed on the screen of the display unit 5 in the form of a list, for example.

(23) Then, the mass difference calculation unit 35 calculates a mass difference (absolute value) Δm between the mass-to-charge ratio value Ma specified by the operator and a mass-to-charge ratio value Mc based on measurement data for each micro area. The mass difference Δm to be obtained at this time is a difference between a precise mass-to-charge ratio value of the target compound specified by the operator and a mass-to-charge ratio value actually observed for the compound. Thus, a mass difference with respect to a mass-to-charge ratio value Mc obtained from a measured peak that is clearly different from the target compound is meaningless. Then, when there is no mass-to-charge ratio value Mc obtained from a measured peak within a predetermined mass-to-charge ratio range centered on the mass-to-charge ratio value Ma, the mass difference Δm may be uniformly set to a predetermined value (a maximum value of the mass difference). The predetermined mass-to-charge ratio range at this time may be expressed as [Ma−ΔM to Ma+ΔM] described above, or may be a range different from this.

(24) The mass difference image creator 36 creates a mass difference image 200 showing a two-dimensional distribution of the mass difference Δm by applying a display color according to the color scale to a value of the mass difference Δm calculated for each micro area. Instead of the color scale, a grayscale may be used. The mass difference image 200 shows accuracy of the mass-to-charge ratio for the target compound specified by the operator, and is created as many as target compounds or target mass-to-charge ratio values specified by the operator.

(25) The mass difference related information calculation unit 37 acquires a predetermined index value for grasping a tendency of mass differences of the entire one mass difference image based on a value of the mass difference Δm for each of micro areas corresponding to the one mass difference image and creates a graph. Examples available as the index value includes an average value, a mode value, a maximum value, a minimum value, a variance, a standard deviation, and the like, of the mass difference Δm. Examples available as the graph includes a histogram showing a relationship between a mass difference range in which a value of the mass difference Δm is divided for each predetermined width and a discrete value (i.e., a frequency) of micro areas, and a pie chart or a band graph in which a frequency for each mass difference range is represented by an overall composition ratio. The index value and graph described above can be obtained for each mass difference image, i.e., for each target compound and each target mass-to-charge ratio value specified by the operator.

(26) The display processor 39 causes the mass difference image created by the mass difference image creator 36 and the index value and the graph obtained by the mass difference related information calculation unit 37 to be displayed together on the screen of the display unit 5. These may be displayed in association with the mass spectrometry image of the target compound, or may be displayed corresponding to selection instructed by the operator for a mass spectrometry image displayed on the screen. When only a mass difference image is displayed on the screen and the operator instructs detailed information about the displayed mass difference image to be further displayed, the index value or graph corresponding to the mass difference image may be displayed.

(27) FIG. 3 shows an example of a screen for displaying a graph and an index value. Here, a histogram of mass differences is displayed. In general, when measurement is performed properly, a mass difference is not so large, and thus a frequency increases in a mass difference range where the mass difference Δm is close to zero. However, the mass difference Δm here increases in frequency at a position away from zero. This suggests that one mass difference image includes a portion having a locally large mass difference, and thus that another compound having a mass-to-charge ratio very close to that of the target compound may locally exists. The operator substantially cannot recognize such a thing from a mass spectrometry image, but can easily recognize it by checking a mass difference image or a graph such as a histogram.

(28) Although the device of the above embodiment allows the peak-waveform conversion processor 34 to perform centroid conversion processing on a peak having a mountain shape to convert the peak into a rod-like peak, a waveform processing method other than the centroid conversion processing may be used as long as a peak having a mountain shape can be converted into a rod-like peak or a narrow peak other than the rod-like peak, having a peak width sufficiently smaller than that of the peak having a mountain shape. Reconstructing data constituting the peak by using deconvolution using a predetermined distribution function such as the Gaussian function enables calculating a peak having a peak width corresponding to about accuracy of the mass spectrometer. As a matter of course, another waveform processing method may be used.

(29) To calculate the mass difference Δm, a mass-to-charge ratio value corresponding to a peak top of a peak having a mountain shape may be used instead of a mass-to-charge ratio value Mc of a peak subjected to the centroid conversion processing.

(30) The embodiment described above is only an example of the present invention. Thus, even when alteration, modification, or addition is appropriately applied to the embodiment within the scope of the spirit of the present invention, the embodiment is clearly included in the scope of claims of the present application.

REFERENCE SIGNS LIST

(31) 1 . . . Imaging Mass Spectrometry Unit 2 . . . Optical Microscopic Image Acquiring Unit 3 . . . Data Processing Unit 31 . . . Data Collector 32 . . . Data Storage Section 321 . . . Profile Data Storage Area 322 . . . Converted Data Storage Area 33 . . . Input Reception Unit 34 . . . Peak-waveform Conversion Processor 35 . . . Mass Difference Calculation Unit 36 . . . Mass Difference Image Creator 37 . . . Mass Difference Related Information Calculation Unit 38 . . . Image Creator 39 . . . Display Processor 4 . . . Input Unit 5 . . . Display Unit