First conceived as a strategy for storing collections of single images, PACS must now adapt to situations in which multiple images must be treated as a single unit, including 3D and 4D imaging and functional MRI.Along with new imaging techniques, new
First conceived as a strategy for storing collections of single images, PACS must now adapt to situations in which multiple images must be treated as a single unit, including 3D and 4D imaging and functional MRI.
Along with new imaging techniques, new clinical applications such as stroke detection and research applications such as the study of heart and brain function require changes to the traditional PACS concept, said Dr. Stephan Erberich, an assistant professor of radiology at the University of Southern California.
"These new imaging techniques require intensive postprocessing, which needs to be integrated into image workflow and PACS implementation," he said. "Interdisciplinary image distribution will become the high-water mark for the next 10 years in the PACS endeavor."
Integration of fMRI space and time data into PACS presents a number of issues, not the least of which is network utilization, according to a recent Erberich paper (Comput Med Imaging Graph 2003;27:229-240). Under DICOM, single 2D planes of the 4D data set must be transmitted one at a time. Since each brain slice includes a full 10 KB DICOM header, plus image data, network transmission quickly becomes inefficient.
"For the average clinical fMRI with 9000 images, the overall store/retrieve operation takes nine minutes, and 65 minutes for the average research fMRI with 64,800 images," Erberich said.
One solution is to not use DICOM. In this case, images are exported as raw MRI data, he said. This way, there is one file for one series with all raw 2D images stacked together. These must then be manually sent via FTP (file transfer protocol) to postprocessing workstations.
An intermediate PACS solution is to store all single fMRI images in PACS. The postprocessing workstation accesses the PACS and retrieves all single DICOM images of an experiment. Once the images are retrieved, the workstation converts the DICOM image data to appropriate format, such as Analyze or MINC, Erberich said.
Both of these approaches, however, involve manual setup and initiation of the postprocessing step.
There is an alternative. Recently, a DICOM working group released "Supplement 49," which defines multiframe MR images. Among other issues, the new extension solves PACS-related difficulties of slow image transmission.
The supplement defines an enhanced image storage service-object pair class that allows multiple images to be combined into one instance. A new unique identifier is created, allowing multiple images (slices) that describe a volume in time to be stored together as one object.
"The revised DICOM standard is the optimal solution for PACS-based fMRI," Erberich said.