Design of a Calibration Phantom for Measuring the Temporal Resolution of a Tomographic Imaging Device

[+] Author and Article Information
Alexander H. Slocum, Stephen E. Jones, Rajiv Gupta

Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139; Department of Radiology, Massachusetts General Hospital, Boston, MA 02114




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J. Med. Devices 1(3), 225-232 (Aug 13, 2007) (8 pages) doi:10.1115/1.2785189 History: Received May 15, 2007; Revised August 13, 2007

This paper describes the design and development of a calibration phantom to be used to aid in the calculation of the temporal resolution of tomographic imaging devices. Current practice for characterizing the dynamic response of a tomographic imaging device, such as a computed tomography or magnetic resonance imaging machine, uses image acquisition time as a surrogate for temporal resolution. At present, no standard method for describing the temporal resolution of a tomographic imaging device exists. Similar to the spatial modulation transfer function (MTF) used for characterizing spatial resolution, the concept of temporal MTF (t-MTF) can be used to enable characterization of temporal resolution. A scanner’s t-MTF represents the percentage amplitude modulation transfer in the image as a function of the input frequency. The calibration phantom uses slotted disks, each mounted to the rotating ring gear of a planetary gear assembly. The sun gears of each planetary gear set are driven from a common shaft to create differential speed sectors, allowing for about two decades of input frequencies to be obtained using a single motor and driveshaft. Preliminary results show a monotonic decline in the modulation transfer as the input frequency is increased. As expected, there is more modulation transfer at lower frequency and less modulation transfer at high frequency. Analogous to the spatial resolution, one can define the frequency for which there is 10% modulation transfer as the temporal resolution of a scanner.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

CT machine scanning bore. The X and Y axes are shown, with the sweet spot at their intersection.

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Figure 3

Cross section of single planetary gear assembly

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Figure 4

TRP Iteration II, single gear assembly front and rear views

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Figure 5

(a) Overall view of assembly cross section. (b) Close-up view of assembly cross section.

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Figure 6

Bevel gears used to transmit torque from the motor, which must be outside the scanning area, to the planetary gear sets’ common driveshaft

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Figure 7

Overall view of the second iteration prototype

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Figure 8

Drive system and microcontroller

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Figure 9

Detail of motor mounting and coupling

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Figure 10

The frequency simulation disk driven by the 34 tooth sun gear

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Figure 11

The frequency simulation disk driven by the 16 tooth sun gear

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Figure 12

Cross-section image of the TRP generated with the clinical scanner

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Figure 13

Modulation data. Each frequency produced by the TRP was registered by the Sensation 64 scanner and was assigned a grayscale value (measured in HU).

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Figure 14

The t-MTF graph for the Sensation 64 scanner. Gantry was operated at a speed of 1rotation∕s. The temporal resolution, defined as the frequency at which there is 10% modulation transfer, is approximately 0.8Hz.




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