Calibration

This tool can be used to evaluate data from several different NanoSPECT calibration measurements, including CT Geometrical, Multi-Pinhole SPECT, and Near Field Flood.

Getting There

The Calibration menu is available in the Tools Menu.

Function

Several calibration measurements are required to calibrate and maintain the NanoSPECT/CT imaging system. The Calibration menu provides options for analyzing CT Geometrical, Multi-Pinhole SPECT, and Near Field Flood measurements.

CT Geometrical

The gantry of the NanoSPECT has a reproducible wobble as it rotates around its axis. The CT Geometrical calibration measures this wobble and generates a file that is used to correct for the wobble in the reconstruction.

The calibration data is also used to assess the severity of the wobble and to insure that it does not exceed +/-0.5mm.

A CT Geometrical measurements consists of a set of projection data collected in a circular CT measurement of the Geometrical CT phantom. The orbits of the three metal balls embedded in the Plexiglas of the phantom are used to assess the wobble in the gantry. Example settings for this measurement include 55kVp, 360 projections, 1000ms. The protocol needed to run this measurement is called CT Geometrical and can be found in the Service Protocols section of the Nucline. After collecting the measurement data, it may be loaded into the CT Geometrical calibration panel using the "Open" button described below.

The CT Geometrical calibration panel is divided into five sections -- Projections, Orbits, 2nd Order Corrections, Results, and Control.

The Projections panel displays the projections collected in the calibration measurement as they are being analyzed. Each of the three metal fiducials is marked by a red, green, or blue circle. The progress of the measurement is also noted in this panel.

The Orbits panel displays the trajectory of the three metal fiducials as the projections are analyzed. The trajectories appear flat in the primary view; however, by drawing a box around a given trajectory it may be expanded to fill the entire panel, providing a more illustrative view.

To zoom in on a particular orbit, draw a box by holding down the left mouse button and dragging.

Upon release of the button, the display will show only the selected orbit. To return to the full view, just right-click.

The 2nd Order Corrections panel displays an important result of the calibration. This panel plots the axial and transaxial wobble of the gantry as it rotates. It is critical that these values be less than +/-0.5mm to enable successful correction of the gantry motion in the CT reconstruction.

The Results panel provides further calibration results, including a mean-square error (MSE) value that represents the deviation of the fiducials from their ideal trajectories. A one-word Quality assessment is provided as well as numerical values for other parameters including the radius of the source (Rs), radius of the detector (Rd), offsets of the source axially (Sz) and transaxially (Sy), and offsets of the detector axially (Dz) and transaxially (Dy).

The Control panel houses four buttons, including

Multi-Pinhole SPECT

Several calibrations are necessary to successfully implement multiplexed multiple-pinhole methods in small-animal SPECT. The MMP SPECT panel may be used to analyze four different small-animal SPECT calibrations, including linearity, collimator depth, pinhole offset, and geometrical. The panel is split into eight sections -- Templates, Control, Preview, Output, and a section for each of the four calibrations. There are also several helpful tools in the Menus.

To run the tool, first export all MMP calibrations (usually titled Service^SPECT_0*_CalibName) from the Data Browser into a folder. Hit the "Open" button in the Control panel and select all the files in the external calibration folder. The MMP Tool will read in the calibration data, recognizing which data corresponds to which calibration. The panels for the individual calibrations (see below) display the completeness of the data. Once all necessary data is present, the "Run" buttons are used in succession from the top calibration (Aluminum Grid) to the bottom calibration (Four-Point). Results for each calibration are presented in their respective panels; a description of possible errors is listed in Calibration Failures.

After successfully completing the analysis, the result CalibrationData_YYYYMMDD.txt file can be copied into C:\Nucline\Calibrations\CalibrationData.txt to update the calibration information used by the NanoSPECT.

Templates

The Templates section controls the input and output files used/generated by the calibration analysis.

Control

The Control panel has four buttons for manipulating the MMP Calibration window.

The Calibrations

Four different MMP calibrations may be analyzed with this tool. For each calibration, Preview, Details, and Run buttons are provided. Preview allows the user to view the images for that particular calibration in the Preview panel. Details provides information about each calibration. Run performs the analysis of that measurement. Additionally, the orbits from the 4-point calibration may be displayed using the Orbits button.

Aluminum Grid Measurement / Linearity Check

Data for the Aluminum Grid Measurement is collected with a device containing 16 carefully-positioned holes. The collimated beam source holder (see NanoSPECT/CT documentation) is placed in each hole. The resulting 16 point images are used to analyze the linearity of the detector.

Collimator Depth

The collimator depth phantom is used to collect data for the collimator depth calibration. This calibration was once used to determine the distance from the detector plane to the pinhole plane, however, it is now out-of-date and is turned off by default. It may be turned back on using the "Enable ColDepth" option found in the MMP Menus.

Pinhole Offset (Collimated Beam)

The Pinhole Offset calibration evaluates the deviation between the center of the pinhole plane and the center of the detector plane.

Geometrical Calibration (4-point)

The 4-point calibration provides a wealth of geometrical information for the NanoSPECT/CT. This geometrical information is then used by special algorithms to determine the forward model necessary for successful SPECT reconstruction.

Calibration failures!

In each calibration panel, there are also fields relaying information about the completeness of the data for each head and some quality measure (either Resolution, Linearity, or Quality). The table below includes information about the limits for passing a particular calibration and how to interpret error messages. In each case, "x" designates a head number. For example, in the 4-point calibration, a R1 (as shown in the screen shot) would indicate that the Radius of Rotation for Head 1 was outside the acceptable limits.

Calibration Error Message Interpretation Limits
Intrinsic Resolution Rx Intrinsic detector resolution >2.4 mm
Intrinsic Resolution Px Pixel Size <0.97mm or >1.03mm
Intrinsic Resolution Lx Quality of linearity >30 (A.U.)
Collimator Depth Hx Detector plane to aperture plane distance >130.1mm or <134.1mm
Collimated Beam Hx Absolute distance between aperture center and detector center (y or z) >1.5mm
4-Point Ax Absolute aperture offset (transaxial or axial) >1.0mm
4-Point Dx Absolute detector offset (transaxial or axial) >1.5mm
4-Point Rx Radius of rotation >45.9mm or <43.9mm
4-Point Q Overall quality of the calibration >3.0 (A.U.)
4-Point B Horizontal bed offset >3.0mm

Preview

The Preview panel displays calibration data for a particular calibration (selected using that calibration's Preview button). This panel is useful for checking head order in the 4-point calibration or determining a missing point in the Linearity Check calibration. The slider bar at the bottom allows movement between heads.

Aluminum Grid Preview
Collimated Beam Preview
Four-Point Preview

Output

The Output panel provides a log of the steps being performed. For example, the Output section will inform the user that files have been found, provide the numerical results from the analysis, and help identify potential errors.

MMP Menus

Several useful fixes and features may be found in the MMP Menus of File, Tools, and Help.

MMP Menu Item Function Keyboard Shortcut
File Open Data... Identical to the "Open" button in the Control panel, this opens a browser to allow the loading of calibration data files. Ctrl-O
File Basic Configuration The ID of the NanoSPECT and # of heads (typically 2 or 4) are set in this panel, pictured below. Ctrl-B
File Enable ColDepth By default, the collimator depth phantom is not used in the MMP Calibration analysis. However, it may be enabled with this checkbox.
File Exit Used to exit the MMP Calibration tool. Ctrl+Q
File Enable ColDepth By default, the collimator depth phantom is not used in the MMP Calibration analysis. However, it may be enabled with this checkbox.
Tools Merge 4-point files In the presence of only one single-pinhole aperture, 4-point data are collected one head at a time. This tool merges those data into a single file.
Tools Convert Calib File On some Nucline versions, a slightly modified version of the calibration data is needed. This tool converts the calibration file accordingly.