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Computer Science

Hybrid Imaging: Instrumentation and Data Processing

Profile image of Shiyam SundarShiyam Sundar

Frontiers in Physics

https://doi.org/10.3389/FPHY.2018.00047
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30 pages

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Abstract

State-of-the-art patient management frequently requires the use of non-invasive imaging methods to assess the anatomy, function or molecular-biological conditions of patients or study subjects. Such imaging methods can be singular, providing either anatomical or molecular information, or they can be combined, thus, providing "anato-metabolic" information. Hybrid imaging denotes image acquisitions on systems that physically combine complementary imaging modalities for an improved diagnostic accuracy and confidence as well as for increased patient comfort. The physical combination of formerly independent imaging modalities was driven by leading innovators in the field of clinical research and benefited from technological advances that permitted the operation of PET and MR in close physical proximity, for example. This review covers milestones of the development of various hybrid imaging systems for use in clinical practice and small-animal research. Special attention is given to technological advances that helped the adoption of hybrid imaging, as well as to introducing methodological concepts that benefit from the availability of complementary anatomical and biological information, such as new types of image reconstruction and data correction schemes. The ultimate goal of hybrid imaging is to provide useful, complementary and quantitative information during patient work-up. Hybrid imaging also opens the door to multi-parametric assessment of diseases, which will help us better understand the causes of various diseases that currently contribute to a large fraction of healthcare costs.

Figures (13)
N.A., data not available. TABLE 1 | PET, SPECT, CT, and MR specifications of selected dual and triple modality clinical systems commercially available.
N.A., data not available. TABLE 1 | PET, SPECT, CT, and MR specifications of selected dual and triple modality clinical systems commercially available.
FIGURE 1 | Images of commercially available hybrid imaging systems for clinical use. The figure shows three SPECT/CT, five PET/CT, three PET/MR and one triple modality SPECT/PET/CT systems. Systems include the first CTM-PET(/CT) system (Siemens mCT Flow, see section Organ-Specific System Design and Total-Body Systems); two SIPM-based PET systems (GE Signa, uMI 780), the first digital SIPM based PET system (Philips Vereos); a BGO-based system (GE Discovery IQ) and the first commercially available fully-integrated PET/MR (Siemens mMR). Images taken from vendor's web pages and published with the permission of the copyright holders (the respective vendors).
FIGURE 1 | Images of commercially available hybrid imaging systems for clinical use. The figure shows three SPECT/CT, five PET/CT, three PET/MR and one triple modality SPECT/PET/CT systems. Systems include the first CTM-PET(/CT) system (Siemens mCT Flow, see section Organ-Specific System Design and Total-Body Systems); two SIPM-based PET systems (GE Signa, uMI 780), the first digital SIPM based PET system (Philips Vereos); a BGO-based system (GE Discovery IQ) and the first commercially available fully-integrated PET/MR (Siemens mMR). Images taken from vendor's web pages and published with the permission of the copyright holders (the respective vendors).
Of note, the MI Labs, Mediso and Bruker companies also offer SPECT/CT and PET/CT systems with the same sub-system characteristics than the ones shown in the SPECT/PET/CT configurations. The Inveon SPECT/PET/CT systerr was discontinued and it is not commercially available anymore. MR solutions also offer and SPECT/MR system with multi-pinhole SPECT technology and similar MR capabilities than the ones shown in their PET/MR. N.A., data no available.
Of note, the MI Labs, Mediso and Bruker companies also offer SPECT/CT and PET/CT systems with the same sub-system characteristics than the ones shown in the SPECT/PET/CT configurations. The Inveon SPECT/PET/CT systerr was discontinued and it is not commercially available anymore. MR solutions also offer and SPECT/MR system with multi-pinhole SPECT technology and similar MR capabilities than the ones shown in their PET/MR. N.A., data no available.
FIGURE 2 | Selected commercially available, dual- and triple-modality preclinical systems. Images taken from vendor's web pages and published with the permission of the copyright holders (the respective vendors).
FIGURE 2 | Selected commercially available, dual- and triple-modality preclinical systems. Images taken from vendor's web pages and published with the permission of the copyright holders (the respective vendors).
TABLE 4 | Characteristics of photo-detectors used in medical imaging applications (adapted from Lecomte [79]).
TABLE 4 | Characteristics of photo-detectors used in medical imaging applications (adapted from Lecomte [79]).
TABLE 3 | Physical properties of some scintillators used in medical imaging applications (adapted from Lewellen [77]).
TABLE 3 | Physical properties of some scintillators used in medical imaging applications (adapted from Lewellen [77]).
TABLE 5 | Physical properties of some semiconductor detectors used in medical imaging applications (adapted from Peterson and Furenlid [80)). (LOR) of about 15 and 9cm, respectively. With this additional information, the SNR in the reconstructed PET images can be improved significantly [92]. Thanks to recent improvements in detector technology, new scintillator materials and fast digital SiPMs photodetectors [93], ToF resolution can be significantly improved; the latest developments in PET detector technology aim at timing resolutions of 100 ps FWHM, or less [92, 94].
TABLE 5 | Physical properties of some semiconductor detectors used in medical imaging applications (adapted from Peterson and Furenlid [80)). (LOR) of about 15 and 9cm, respectively. With this additional information, the SNR in the reconstructed PET images can be improved significantly [92]. Thanks to recent improvements in detector technology, new scintillator materials and fast digital SiPMs photodetectors [93], ToF resolution can be significantly improved; the latest developments in PET detector technology aim at timing resolutions of 100 ps FWHM, or less [92, 94].
FIGURE 3 | (A) Simplified view of the direct FBP image reconstruction technique: the projected data is filtered using different filter kernels and back-projected into the image domain. Multiple projections are required to obtain the final CT image. (B) Schematic view of the iterative reconstruction process: first, a forward projection of the initial estimated image is used to create the estimated projected data; then the estimated data is compared to the acquired raw data and a set of correction factors is derived. These correction factors are back-projected and used to update the initial estimated image. All three steps describe an iterative loop, which is repeated until a predefined condition is satisfied and the final image is generated. Similar reconstruction processes apply for SPECT and PET data.
FIGURE 3 | (A) Simplified view of the direct FBP image reconstruction technique: the projected data is filtered using different filter kernels and back-projected into the image domain. Multiple projections are required to obtain the final CT image. (B) Schematic view of the iterative reconstruction process: first, a forward projection of the initial estimated image is used to create the estimated projected data; then the estimated data is compared to the acquired raw data and a set of correction factors is derived. These correction factors are back-projected and used to update the initial estimated image. All three steps describe an iterative loop, which is repeated until a predefined condition is satisfied and the final image is generated. Similar reconstruction processes apply for SPECT and PET data.
FIGURE 4 | Illustration of the partial volume effect (PVE) in medical imaging. (A) A circular source of uniform activity in a warm background region yields a measured image in which part of signal emanating from the source is observed outside (spill-out) while part of the background signal is observed within the lesion (spill-in). (B) The tracer distribution is sampled on a voxel grid and the contours of the voxels do not match the actual contours of the tracer distribution (Tissue-fraction).
FIGURE 4 | Illustration of the partial volume effect (PVE) in medical imaging. (A) A circular source of uniform activity in a warm background region yields a measured image in which part of signal emanating from the source is observed outside (spill-out) while part of the background signal is observed within the lesion (spill-in). (B) The tracer distribution is sampled on a voxel grid and the contours of the voxels do not match the actual contours of the tracer distribution (Tissue-fraction).
FIGURE 5 | Effect of respiratory and cardiac motion on PET image quality. (A) Blurring effect due to cardiac motion in a patient scanned with 13N-NHS. (B) Misalignment artifact between the PET-emission data and the corresponding MR-AC map. Misalignment artifacts like these are common in cardiac PET-imaging where they are known to cause false-positive findings in up to 40% of all studies [180]. Reprinted from Rausch et al. [181] with permission from Elsevier.
FIGURE 5 | Effect of respiratory and cardiac motion on PET image quality. (A) Blurring effect due to cardiac motion in a patient scanned with 13N-NHS. (B) Misalignment artifact between the PET-emission data and the corresponding MR-AC map. Misalignment artifacts like these are common in cardiac PET-imaging where they are known to cause false-positive findings in up to 40% of all studies [180]. Reprinted from Rausch et al. [181] with permission from Elsevier.
IGURE 6 | Illustration of popular MR-based AC methods using the same axial slice of AC maps of a patient processed with different AC methods: (A) Reference ‘T-based AC; Standard methods: (B) standard DIXON-based MR-AC implemented in the Siemens mMR system (SW v. VB20), (C) UTE-based standard MR-AC for rain examinations implemented in the Siemens mMR system (SW v. VB20); Template-based methods: (D) Koesters et al. [208], (E) Anazodo et al. [209], =) Izquierdo-Garcia et al. [210], (G) Burgos et al. [211], (H) Merida et al. [212]; MLAA-based methods: (I) Benoit et al. [213]; Segmentation-based methods: (J) vabello et al. [214], (K) Juttukonda et al [215], (L) Ladefoged et al. [216]. Figure adapted from Ladefoged et al. [217] (Courtesy of Claes N. Ladefoged, Rigshospitale Oopenhagen, Denmark; Modified from the original image published under the creative commons license http://creativecommons.org/licenses/by-nc-nd/4.0/). Figure ublished with the permission of the copyright holders (Claes N. Ladefoged).
IGURE 6 | Illustration of popular MR-based AC methods using the same axial slice of AC maps of a patient processed with different AC methods: (A) Reference ‘T-based AC; Standard methods: (B) standard DIXON-based MR-AC implemented in the Siemens mMR system (SW v. VB20), (C) UTE-based standard MR-AC for rain examinations implemented in the Siemens mMR system (SW v. VB20); Template-based methods: (D) Koesters et al. [208], (E) Anazodo et al. [209], =) Izquierdo-Garcia et al. [210], (G) Burgos et al. [211], (H) Merida et al. [212]; MLAA-based methods: (I) Benoit et al. [213]; Segmentation-based methods: (J) vabello et al. [214], (K) Juttukonda et al [215], (L) Ladefoged et al. [216]. Figure adapted from Ladefoged et al. [217] (Courtesy of Claes N. Ladefoged, Rigshospitale Oopenhagen, Denmark; Modified from the original image published under the creative commons license http://creativecommons.org/licenses/by-nc-nd/4.0/). Figure ublished with the permission of the copyright holders (Claes N. Ladefoged).
FIGURE 7 | Schematic view of the IDIF concept. (A) 1- PET coronal view showing the carotid arteries, 2- T1-MPRAGE image, 3- TOF MR angiography image of neck region used to segment the carotid arteries, 4- PET coronal view co-registered with the segmented carotid arteries. (B) Illustration of the 2-tissue compartment model commonly used for FDG tracer kinetics. Here, Cp represents the blood activity and Cj, Co the activity concentration of each tissue over time t. K1, k2, etc., are the rate constants that define the rate of tracer movement between compartments. This figure was adapted from Sari et al. [228], with permission of SAGE Publications, Ltd.
FIGURE 7 | Schematic view of the IDIF concept. (A) 1- PET coronal view showing the carotid arteries, 2- T1-MPRAGE image, 3- TOF MR angiography image of neck region used to segment the carotid arteries, 4- PET coronal view co-registered with the segmented carotid arteries. (B) Illustration of the 2-tissue compartment model commonly used for FDG tracer kinetics. Here, Cp represents the blood activity and Cj, Co the activity concentration of each tissue over time t. K1, k2, etc., are the rate constants that define the rate of tracer movement between compartments. This figure was adapted from Sari et al. [228], with permission of SAGE Publications, Ltd.

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