CT substitutes derived from MR images reconstructed with parallel imaging. Issue 8 (15th July 2014)

• Record Type: Journal Article
• Authors: Johansson, Adam; Garpebring, Anders; Asklund, Thomas; Nyholm, Tufve
• Is Part Of: Medical physics. Volume 41:Issue 8(2014)Part 1
• Page Start: n/a
• Page End: n/a
• Publication Date: 2014-07-15
• Subjects: Computed tomography -- Magnetic resonance imaging -- Reconstruction -- Probability theory, stochastic processes, and statistics; biomedical equipment -- biomedical MRI -- coils -- computerised tomography -- data acquisition -- gradient methods -- high‐speed techniques -- image matching -- image reconstruction -- image sequences -- medical image processing -- sampling methods; magnetic resonance imaging -- computed tomography substitute -- ultrashort echo time -- parallel imaging -- radial imaging; Involving electronic [emr] or nuclear [nmr] magnetic resonance, e.g. magnetic resonance imaging -- Computerised tomographs -- Biological material, e.g. blood, urine; Haemocytometers -- Methods or arrangements for processing data by operating upon the order or content of the data handled -- Data acquisition and logging -- Digital computing or data processing equipment or methods, specially adapted for specific applications -- Image data processing or generation, in general -- Coils -- Coils; Windings; Conductive connections; Computed tomography -- Medical image reconstruction -- Medical magnetic resonance imaging -- Image reconstruction -- Radiation therapy -- Positron emission tomography -- Tissues -- Germanium; Medical physics -- Periodicals; Medical physics; Geneeskunde; Natuurkunde; Toepassingen; Biophysics; Periodicals; Periodicals; Electronic journals; 610.153
• DOI: 10.1118/1.4886766
• Languages: English
• ISSNs: 0094-2405

Abstract

Abstract : Purpose: Computed tomography (CT) substitute images can be generated from ultrashort echo time (UTE) MRI sequences with radial k‐space sampling. These CT substitutes can be used as ordinary CT images for PET attenuation correction and radiotherapy dose calculations. Parallel imaging allows faster acquisition of magnetic resonance (MR) images by exploiting differences in receiver coil element sensitivities. This study investigates whether non‐Cartesian parallel imaging reconstruction can be used to improve CT substitutes generated from shorter examination times. Methods: The authors used gridding as well as two non‐Cartesian parallel imaging reconstruction methods, SPIRiT and CG‐SENSE, to reconstruct radial UTE and gradient echo (GE) data into images of the head for 23 patients. For each patient, images were reconstructed from the full dataset and from a number of subsampled datasets. The subsampled datasets simulated shorter acquisition times by containing fewer radial k‐space spokes (1000, 2000, 3000, 5000, and 10 000 spokes) than the full dataset (30 000 spokes). For each combination of patient, reconstruction method, and number of spokes, the reconstructed UTE and GE images were used to generate a CT substitute. Each CT substitute image was compared to a real CT image of the same patient. Results: The mean absolute deviation between the CT number in CT substitute and CT decreased when using SPIRiT as compared to gridding reconstruction. However, the reduction was small and the CT substitute algorithm was insensitive to moderate subsampling (≥5000 spokes) regardless of reconstruction method. For more severe subsampling (≤3000 spokes), corresponding to acquisition times less than a minute long, the CT substitute quality was deteriorated for all reconstruction methods but SPIRiT gave a reduction in the mean absolute deviation of down to 25 Hounsfield units compared to gridding. Conclusions: SPIRiT marginally improved the CT substitute quality for a given number of radial spokes as compared to gridding. However, the increased reconstruction time of non‐Cartesian parallel imaging reconstruction is difficult to motivate from this improvement. Because the CT substitute algorithm was insensitive to moderate subsampling, data for a CT substitute could be collected in as little as minute and reconstructed with gridding without deteriorating the CT substitute quality.