Pet or Mri to Measure Reading or Math Calculations

Abstract

Introduction

Positron emission tomography (PET) is a fully quantitative applied science for imaging metabolic pathways and dynamic processes in vivo. Attenuation correction of raw PET data is a prerequisite for quantification and is typically based on separate transmission measurements. In PET/CT attenuation correction, however, is performed routinely based on the available CT transmission data.

Objective

Recently, combined PET/magnetic resonance (MR) has been proposed as a viable alternative to PET/CT. Current concepts of PET/MRI do not include CT-like transmission sources and, therefore, alternative methods of PET attenuation correction must be found. This article reviews existing approaches to MR-based attenuation correction (MR-Air conditioning). Most groups have proposed MR-AC algorithms for encephalon PET studies and more recently also for torso PET/MR imaging. Near MR-Air-conditioning strategies require the utilise of complementary MR and transmission images, or morphology templates generated from transmission images. We review and talk over these algorithms and point out challenges for using MR-AC in clinical routine.

Discussion

MR-AC is piece of work-in-progress with potentially promising results from a template-based approach applicable to both brain and torso imaging. While efforts are ongoing in making clinically feasible MR-Air conditioning fully automatic, further studies are required to realize the potential benefits of MR-based motion bounty and partial volume correction of the PET data.

Introduction

Combined PET/CT has emerged as a powerful imaging modality for the diagnosis, staging and restaging of a multifariousness of cancers [one]. Less frequently peradventure, PET/CT is being used for cardiology and neurology examinations. In general, PET/CT examinations provide complementary and intrinsically coregistered CT and PET image volumes, whereby the CT transmission data are also used routinely for attenuation correction (AC) [2]. In general, CT-based AC (CT-AC) is based on a piecewise linear scaling algorithm that translates CT attenuation values into linear attenuation coefficients at 511 keV [3, 4]. Past using the CT images for the purpose of AC, lengthy PET transmission scanning with conventional rod- or point sources (TX-Air conditioning) has become obsolete in commercially bachelor PET/CT tomographs.

Recently, a combination of PET and MRI has been proposed every bit a promising alternative to existing dual modality PET/CT systems, and the first images of patients were presented in tardily 2006 [5]. The realization of PET/MR tomographs across pocket-sized-animal imaging prototypes [6, 7], withal, remains challenging. In particular, the lack of conventional or X-ray transmission sources mandates culling approaches to Ac of the complementary emission information [8].

Since electric current concepts of combined PET/MR tomographs practice not allow separate CT-similar transmission sources, PET attenuation coefficients demand to exist calculated from the available MR images. CT images are required at effective CT energies of lxx–80 keV and stand for the pixel-wise distribution of attenuation coefficients and, thus, yield a measure of the electron density in the image book. In contrast, MR images reflect the distribution of hydrogen nuclei (Fig. 1). Thus, MR-based Ac (MR-Air-conditioning) is far more challenging than CT-Air-conditioning since MR prototype voxel values correlate with the density of hydrogen nuclei in tissues and tissue relaxation properties rather than with the electron density-related mass attenuation coefficients of these tissues. Therefore, a direct mapping of CT-similar attenuation values from available MR images is challenging [9].

Fig. 1

Centric MR (a) and CT (b) images of the neck of the aforementioned patient showing the differences in advent of bone, air (trachea) and soft tissue. The inability to separate bone and air-filled structures clearly on the MR epitome renders piecewise linear scaling approaches to Air-conditioning incommunicable

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Although preclinical PET/MR prototype systems [x] accept been around since the early 1990's, MR-AC is nonetheless work in progress. While early preclinical PET/MR pattern concepts did not include ways for AC [half dozen, 7], a relatively uncomplicated 2-class Air conditioning scheme was suggested for the start clinical prototype [5]. The lack of feasible MR-AC methods today can be explained by the fact that attenuation is less disquisitional in modest animals than in patients, and, therefore, the issue of AC was of less importance in preclinical PET and PET/MRI.

Every bit the recognition of combined PET/MR imaging is increasing we review the status of estimating PET attenuation maps from available MR images in clinical PET/MR imaging scenarios. We discuss potential pitfalls of MR-AC as well as a number of advantages inherent to MR-AC that could make it more useful than CT-Air-conditioning where applicable.

Methods

Table ane summarizes the main approaches to MR-AC for imaging patients. Interestingly, several studies on MR-AC appeared when such combined devices were not still considered seriously for clinical use. These early studies were focused on applications in the brain PET [eleven, 12]. With the considerations of clinical PET/MR prototypes several groups have proposed algorithms for extracranial MR-Air-conditioning besides [9, 13].

Tabular array 1 Overview of studies on MR-Air conditioning

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Brain imaging

Segmentation approaches

MR-AC for brain applications was first addressed past Le Goff-Rougetet et al., who proposed a method to summate PET AC factors from MR images in clinical examinations when both PET and MRI were required [11]. They argued that MR-AC helps simplify the clinical protocol and reduces the patient dose from standard PET transmission scanning. Their methodology, which they first practical to an FDG/water-filled, cylindrical Lucite phantom, is based on a coregistration of the MR images to the PET manual images using a surface matching technique. The coregistered MR images are and so segmented into three classes (Table 1). Air is considered but exterior the patient. Appropriate linear attenuation coefficient values (μ) at 511 keV are so assigned to these tissue classes.

El Fakhri et al. [14] also mentioned MR-AC, but they did non provide further details of their implementation or a operation evaluation. In a personal advice the authors stated that they acquired ii MR sequences for each field of study and performed a cluster identification on the joint histogram prior to assigning the corresponding attenuation values.

An alternative method for MR-Air-conditioning in brain PET was suggested past Zaidi et al. [12]. The authors had previously shown that the quality of PET neurology imaging was insufficient when standard PET Air conditioning methods were applied [15]. Therefore, they studied the use of MR-AC in brain PET (Tabular array 1). They present a workflow based on the availability of coregistered PET images, following standard (ellipse-fitted) AC, and MR images [12]. Using a partition method based on fuzzy logic the coregistered MR images are segmented into v tissue classes that are assigned attenuation coefficients at 511 keV. The unabridged procedure for MR-Air conditioning was reported to have 10 min on a Sun SPARC with minimal user intervention. Like Le Goff-Rougetet et al., the authors account for the caput holder earlier using the segmented MR-based attenuation map for MR-AC.

Atlas approaches

A viable alternative to multistep segmentation procedures [12, 16] is to use atlas co-registration (Fig. 2). For MR-AC, an atlas typically consists of a template MR image together with a respective attenuation label image. The template MR image tin be obtained equally an average of co-registered MR images from several patients. The label paradigm could represent a sectionalization into different tissue classes (e.g. air, bone and soft tissue) or a coregistered attenuation map from a PET transmission scan or a CT browse with continuous attenuation values. The template MR is warped to the patient-specific MR image volume. When applying the same spatial transformation to the atlas attenuation prototype a respective patient-specific attenuation map is generated.

Fig. ii

Principle of atlas-based MR-Ac. The atlas consists of a matching MR-CT image book that can be generated from a patient. An atlas of attenuation values at 511 keV is generated from matching CT images. The atlas MR prototype (superlative left) is coregistered to the MR epitome book of a specific patient (bottom left). This transformation is practical to the corresponding CT atlas, thus generating an attenuation image (i.e. pseudo-CT image) that approximately matches the patient anatomy

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Atlas-based approaches to MR-Air-conditioning were presented past Kops and Herzog [17] and Hofmann et al. [18]. Kops and Herzog generate a template of PET transmission images from the data from x patients that is matched to the PET manual template within SPM2 [nineteen]. The MR template within SPM2 (which is already aligned with the PET transmission template) is normalized to the MR image of the patient. The obtained transformation is and then applied to the template attenuation image to yield an attenuation paradigm for this patient. The same group has too employed MR-AC based on an MR division method past Dogdas et al. [16] following linear coregistration of the MR image to the measured PET manual image.

Hofmann et al. have suggested a revised atlas approach to MR-Air-conditioning [eighteen]. Here, the authors utilize a set of aligned MR-CT epitome volumes of 17 patients. Each of the available 17 MR prototype volumes from the MR-CT pairs is coregistered to the MR paradigm volume from the PET/MR report. The coregistration vectors are applied to the corresponding CT image volumes thus generating 17 CT image sets that are aligned to the MR image ready from the patient. Subsequently, a pattern recognition approach is used to friction match the MR epitome of the patient with the appropriate CT information from the MR-CT dataset that best matches the patient information. This voxel-based approach can merge fractional subvolumes from independent datasets into a unmarried CT volume that is used for MR-Air-conditioning of the patients. This atlas-based algorithm was validated on 3 clinical datasets comparing MR-AC to the gold standard CT-Air conditioning [18].

Trunk imaging

Due to the lack of prototype systems for whole-body PET/MRI studies of MR-Air conditioning, algorithms for extracranial applications are scarce. Beyer et al. set up a toolbox that facilitates cross-validation of MR-Ac and CT-AC using matching PET/CT and MR paradigm volumes [9] from ten patients. They studied ten patients who underwent routine trunk scans with arms upward on a combined PET/CT tomograph. Within i 24-hour interval of the PET/CT examination, complementary MR scans were caused. MR imaging was performed on a 1.five-T system with patients positioned with their arms downward. Single-station, transverse T1-weighted VIBE MR images were used to generate pseudo-CT images. Offset, the MR images were coregistered to the CT images using nonlinear curvature-regularized coregistration in conjunction with mutual information. 2d, the MR voxel value intensity distribution was matched to that of the coregistered CT image. MR-CT intensity transformation was performed in a three-pace process based on a nonproprietary histogram-matching algorithm. PET images were reconstructed on the PET/CT console following Air conditioning based on CT transmission images (PET_CTAC) and MR-based pseudo-CT images (PET_MRAC).

Although predominantly used for encephalon imaging, atlas-based methods can also be practical to whole-torso imaging. However, anatomic variability is high and it is unlikely that a full general spatial transformation captures all variables betwixt a template and patient-specific anatomy. Hofmann et al. [18] presented a motorcar learning arroyo that combines the data from an atlas registration with local information that is extracted from small prototype patches. Thus this method is less dependent on accurate template-to-patient registration. Validation was performed on two whole-body rabbit datasets and, in ongoing work, on five man torso datasets [13].

Results

MR-Air-conditioning tin can exist evaluated by comparing the PET images obtained post-obit standard TX-AC, CT-AC and MR-AC. We refer below to these images as PET_TXAC, PET_CTAC and PET_MRAC, respectively.

Both TX-Ac and CT-Air-conditioning have their shortcomings. Depending on the browse fourth dimension, TX scans accept relatively high noise levels that can be detrimental to Ac. CT scans on the other manus have very depression noise levels, but the mapping from CT Hounsfield units to 511 keV attenuation values can be wrong, particularly in the example of inorganic materials such every bit metal implants. Despite these problems, both TX-Air conditioning and CT-AC are commonly used and accepted. In accordance with the literature we present both PET_TXAC and PET_CTAC as the gilt standard against which PET_MRAC should be compared.

The comparing tin can exist done visually or quantitatively by means of relative differences of the reconstructed PET activity distributions. Differences can be assessed on a voxel-by-voxel basis, or perhaps more commonly for regions of interest (ROI). ROIs can exist defined automatically or manually past a man expert. For a study with n patients, where p ROIs are defined for each patient, it is impractical to quote all northward times p differences. Therefore, information technology is preferred to study either the maximum differences or the mean absolute divergence across all voxels. Some authors have quoted the mean differences, where the mean was taken from the positive or negative differences. This value only indicates the existence of an overall bias in the method; a value of zero for the differences would just indicate that activity was overestimated as often equally it was underestimated.

For a summary of the results of the about significant studies on MR-Air-conditioning see Tabular array one.

Encephalon imaging

Le Goff-Rougetet et al. [eleven] evaluated their MR-AC method for phantoms and a patient browse. Using ROIs within selected axial images of the phantom they found a maximum difference of 11% between PET_MRAC and PET_TXAC. The patient study revealed a maximum difference of 12%, primarily in the occipital cortex. In the written report by Le Goff-Rougetet et al. an expert placed ROIs into different regions of the brain of the coregistered MR images. However, ROIs were non placed in the lowest and highest slice of the MRI volume as their MR sectionalization method still needed refinement for these areas.

Figure 3 shows brain images of an FDG-PET written report past Zaidi et al. [12] comparing segmented MR-AC with a standard, transmission-guided Ac. The quality of the PET images following MR-Air conditioning appeared somewhat improved, which could be attributed to the lower noise levels in the MR-based attenuation maps (Fig. 3b). Analysis of the differences between the two methods of Ac was performed across 10 patient sets. Despite a tendency of the method to lead to activity overestimation, overall correlation of ROI activity values on PET_MRAC and PET_TXAC was good (r ^two= 0.91), indicating the feasibility of segmented MR-Air conditioning as suggested past the authors (Table 1).

Fig. 3

MR-Air-conditioning for brain PET [12]. Centric slices through an FDG-PET scan of a brain of a patient post-obit standard Air conditioning (a) and MR-Air conditioning (b) with the PET images (top) and the corresponding attenuation maps (bottom). Note the patient bed and head holder prior to MR-AC. The PET image following MR-AC appears visually similar with a slightly ameliorate indicate-to-dissonance ratio

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Kops and Herzog [17] validated their MR-AC algorithm in iv patients (Fig. 4). An analysis of ROIs fatigued on cortical and subcortical structures demonstrated that PET_MRAC differed from PET_TXAC past less than 10%. Maximum differences were observed over again in the occipital cortex and in the caudate nucleus. The same group also evaluated a template-based AC, which resulted in a 9% deviation from PET_TXAC.

Fig. 4

MR-Ac for brain PET: template-based Air-conditioning. a Attenuation map measured through a PET transmission scan. b Attenuation map obtained through template coregistration and addition of the head holder. c Voxel-past-voxel calculation of relative differences between PET attenuation corrected using attenuation maps a and b. (images from Kops and Herzog [12])

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Hofmann et al. [xviii] evaluated their method with three patient scans. Automatic ROI analysis of PET_MRAC and PET_CTAC yielded a hateful absolute divergence of 3% and a maximum divergence of x%, which is like to the previous MR-Ac approaches to brain PET (Fig. 5).

Fig. v

MR-AC for brain PET using the template-based method of Hofmann et al. [18]. Axial slices through patient information with mid-plane sections (elevation) and lower brain sections (bottom). T1-W spin-echo MR images (a), pseudo-CT images (b) as predicted from the template-based MR-Air-conditioning, and original CT images (c). Notation the visual similarity between the pseudo-CT and the original CT images. The T1-Westward spin-repeat MR images used here were caused with a comparatively large slice spacing of 4 mm, which may explicate the limited improvement in overall image quality, although the accuracy improvement within a given axial slice was high

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Torso imaging

Data on the utility of MR-Ac for torso or whole-body applications are still sparse. Beyer et al. [9] presented a set-upwardly that allows comparison of PET_MRAC and PET_CTAC using datasets from the same patients (Fig. 6). Still, in reality such complementary datasets are non bachelor from PET/MR studies and, therefore, this toolbox can exist used only as guidance in the evaluation of pitfalls in MR-Air conditioning. Nevertheless, the authors were able to demonstrate that histogram matching is a feasible technique to transform MR to pseudo-CT attenuation values if the MR image quality is loftier and MR images are gratuitous of distortions. In those cases where the MR image is distorted the PET images will be affected by MR-Ac. The study illustrates the demand for authentic patient positioning between MRI and PET scans without quantifying these effects further.

Fig. vi

MR-Ac for torso applications [9]. From top to bottom: CT images from PET/CT studies are coregistered to the available MR images. CT-MR histogram matching yields images with pseudo-CT attenuation values that are used for MR-AC. PET images following CT-AC and MR-AC show major differences if the MR images inherit artefacts from suboptimal imaging protocols. If the MR image quality is proficient (thorax) and the coregistration is accurate, MR-Ac based on histogram matching appears feasible

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In an ongoing study, Hofmann et al. [xiii] aim to use an atlas-based arroyo to MR-Air-conditioning that is applicable to the human torso, and possibly whole-body imaging. Their method can predict bone structures on MR sequences that typically do non allow intensity-based segmentation of os. Figure 7 shows the initial results from atlas-based MR-Air conditioning for areas outside the brain.

Fig. 7

Upshot of ignoring cortical bone during CT-AC. a PET image reconstructed using the original CT image. b PET paradigm reconstructed using the same CT image with all os structures fix to the attenuation value of soft tissue, thus simulating a all-time-case scenario of MR-Ac where os attenuation is ignored. c Relative divergence (%) betwixt a and b showing that the largest effect is in the skeleton. Note voxels are set up to white in low uptake regions with SUV <0.2 in the original PET image

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Discussion

Various approaches for predicting the attenuation maps from MR images on PET/MRI examinations of patients are reviewed. While partition-based approaches work well for brain applications, torso imaging with PET/MRI may require more sophisticated methodologies such every bit atlas-based prototype transformations from MR images to pseudo-CT images. In general, MR-Air conditioning not simply needs to accost adequate transformation of MRI pixel value information to advisable PET attenuation values, but besides, in gild to be feasible in clinical routine. needs to account for additional pitfalls in torso and whole-body imaging. The pitfalls include the accurate representation of bone (typically not seen on MR images), potential truncation effects from patients extending beyond the transverse field-of-view (FOV) of the MR organisation and the presence of MR surface coils typically not seen on MR imaging.

The presence of bone

Equally bone structures are hard to separate on MR images a straightforward approach to MR-Air conditioning would exist to but ignore os. This is not a new approach, and was shown in early on studies on CT-AC to be of less impact than originally expected [twenty, 21] despite the fact that the fraction of cortical os varies in centric images.

Effigy 7 shows an example of CT-AC performed with and without consideration of os. This case illustrates the minimum bias expected from ignoring bone attenuation and considering this tissue class as soft tissue. In practice, an MR-AC algorithm that ignores bone tissue may likewise falsely attribute air as soft tissue (Fig. 1) and thus introduce a much higher bias. In a briefing abstruse Martínez-Möller et al. [22] recently reported a patient PET study candy with MR-AC without consideration of bone compared with CT-Air-conditioning. Based on an ROI analysis the authors reported mean differences of i.vii%, 7.3% and ii.9% for lung, bone and neck lesions, respectively. They concluded that MR-Air-conditioning without accounting for bone tissue does non lead to a clinical bias. However, farther studies are required to guess the bias on uptake in bone metastasis in case MR-Air conditioning is performed without because the presence of cortical bone.

MR imaging with ultrashort echo time (UTE)

Instead of performing advanced partitioning methods on standard MR images 1 may utilize dedicated MR sequences, such as ultrashort echo fourth dimension (UTE) sequences [23, 24] that yield betoken fifty-fifty from cortical bone (Fig. 8). Typically, the apply of just a single UTE epitome does non enable os to be separated from non-bone tissues. Nevertheless, when combined with a late echo image it is, in principle, possible to detect bone as that structure that yields signal on the brusque echo image, merely not on the late repeat image (Fig. 8). By using multiecho sequences [25] the two images can be acquired in 1 scan. While it seems promising for brain applications, it may not be acceptable every bit part of whole-trunk imaging protocols since the acquisition time is on the society of several minutes per bed position.

Fig. 8

MR images caused with a iii-D UTE sequence. a Sagittal brain section caused with short echo (0.07 ms left) and tardily repeat (1 ms right). b Angled view of a human foot. Note that bone yields a high signal as practice parts of the coil housing

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Required PET accuracy

Which of the presented MR-AC methods will be accustomed in clinical routine will depend not only on which MR-Air conditioning method yields the highest accuracy, but also the accuracy considered sufficient for clinical work. If, for case, variations of up to 10% in PET activity values are considered acceptable in clinical routine, then methods with college accuracy might be dismissed while giving preference to methods with other advantages such as robustness or computational speed. PET uptake values, such as the measured standard uptake values (SUV), are affected by many factors such every bit uptake fourth dimension, body composition, glucose load and others that are independent of the imaging device [26].

Validation of MR-AC methods

In the absence of simultaneous PET/MR systems, validation of whole-body MR-Air-conditioning is inherently hard; movement of the patient between MR and PET(/CT) examinations is unavoidable. Therefore, even if the attenuation map could be predicted with a high accuracy from the MR prototype, patient movement between the MR browse and a TX or CT scan may still cause the MR-predicted attenuation map to be different from the reference scan attenuation values, thereby also causing a difference between PET_MRAC and PET_CTAC. Several authors [ix, 13] have suggested compensating for patient motility between scans by performing nonrigid MR-CT coregistration. Information technology should, however, be noted that non all motion-related misalignments can be corrected. For example, pockets of gas in the intestinal region may vary significantly betwixt scans, or non even appear on one of the two complementary studies. In addition, the validation of nonlinear coregistration algorithms remains an open event [27] that requires addressing if coregistration is to become an integral part of routine MR-AC.

Truncated field-of-view

In clinical PET/MR imaging patients may well extend beyond the transverse FOV of the MRI arrangement. Thus, the artillery and even the trunk of the patient may non exist fully covered past the MR image. Nevertheless, the contribution of the truncated beefcake to overall attenuation needs to be accounted for. The very aforementioned trouble was described for PET/CT applications where truncated attenuation maps were shown to yield significant image distortion and bias near the surface area of truncation [28–31]. Recently, Delso et al. discussed the effect of MR truncation on MR-Air conditioning [32]. Their study showed that when the arms were exterior the FOV the PET activity after Air conditioning was biased by up to fourteen% in that area [32]. Using simple image processing techniques they were able to recover the arms in the truncated image and thus reduce the quantification bias to 2%.

An alternative solution would be to employ the uncorrected PET image to estimate the patient cross-section in those areas outside the measured FOV where no MR data is available. The feasibility of such an arroyo notwithstanding needs to exist validated. In imaging scenarios with highly specific tracers the artillery may exist difficult to segment automatically in the uncorrected PET images. Yet another approach would be to predict the body cross-section through atlas matching exterior the FOV of the MR image. In theory, these approaches could even be combined such that the atlas co-registration is performed based on the MR image where the MR image is available, and elsewhere based on the uncorrected PET image.

MR coils

The fact that the MR coils are located inside the FOV of the PET system is a claiming that has not withal been addressed by any of the studies on MR-AC. For encephalon scans, the caput coil is rigid and its attenuation values tin can be estimated from a baseline CT scan. Later for any PET/MR study only the relative position of the head coil inside the PET/MR system would be required. For extracranial examinations the situation is far more difficult. Surface coils are required to avoid suboptimal signal generation (Fig. 6). Surface coils contain elastic components and hence cannot be located easily with respect to the gradient coil or the patient. MR sequences with UTE could possibly help detect surface ringlet landmarks and thus help account for their attenuation.

User intervention

Ideally, for application in clinical PET/MRI scenarios MR-Ac should be fully automated in order to limit user interaction and, subsequently, examination and processing times. Despite claims of some groups that their method for MR-AC is "robust", problems remain that crave "some manual intervention of the operator" [12]. Thus, automation of MR-Ac remains a challenge, particularly in patients with a large deviation from normal anatomy.

Potential benefits of MR-AC

In PET/CT the PET epitome is acquired over several minutes, while the CT scan is a thing of seconds and is oftentimes acquired during a single breath-agree. As a issue, patient movement typically causes local misalignment between the PET and CT images and may lead to serious artefacts for Air-conditioning, for case near the diaphragm [31]. Some authors have recommended 4-D PET/CT acquisition and AC [33, 34]; however, this may involve a substantially college patient radiations dose.

As an MRI examination typically takes much longer than a CT examination, patients conceivably spend an fifty-fifty longer time in an PET/MRI arrangement than in a PET/CT. Consequently, patient movement is likely to cause even more astringent artefacts in PET/MR than in PET/CT. Here, the utilise of periodic MRI navigator signals in conjunction with a 4-D model of the human trunk may help to correct for motion-induced image degradation in PET/MR data following 4-D MR-AC, which would be a major reward over CT-Air conditioning.

Additional potential benefits of simultaneous PET/MR acquisition

As early as 1991, Leahy et al. [35] suggested that PET reconstruction could be improved by using anatomical MR images from the same patient every bit prior information. This remains a field of active research and the potential of the method can be seen in Fig. nine. While information technology is commonplace today that well-nigh all neurology patients who receive a PET scan also receive an MR browse, MR-guided PET reconstruction has not yet made the transition from inquiry into clinical routine. Aside from logistical problems of automatically retrieving the matching MR image from the PACS, 1 of the reasons for this might be that misregistrations, which are unavoidable in retrospective PET–MR coregistration, quickly lead to deterioration of epitome quality [36]. In combined PET/MR tomographs, the coregistration accuracy is improved and may help promote the concept of MR-guided PET prototype reconstruction.

Fig. 9

Comparison of a standard filtered dorsum projection (FBP) that was reconstructed PET image, and a PET image that was constructed using a maximum a posteriori (MAP) approach with an MR image from the aforementioned patient used as prior information (images with permission from Nuyts et al. [37])

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Fifty-fifty if the PET paradigm is reconstructed independently of the MR image, it is even so possible to use the MR image of the patient equally an assistance for improved quantification. In item MR-guided partial volume correction (PVC) was suggested as early as in 1990 [38, 39]. Once again PET and MR images from combined PET/MRI examinations may facilitate improvements in MR-based PET quantification through the utilise of MR-based PVC.

Conclusion

With the onset of a enquiry interest in combined PET/MR imaging several studies have appeared on the employ of MR for AC of the PET data. MR-Air-conditioning is not every bit straightforward as CT-AC that allows the estimation of 511 keV–attenuation maps from CT transmission images. In the absence of CT-like transmission sources in PET/MR culling solutions to MR-AC include the employ of complex division tools that were shown to work for encephalon applications. In extra-cranial PET/MR other approaches that include atlas-matching announced more than promising. While MR-Ac is work-in-progress further advantages of MR-AC over CT-AC become credible, which include the additional utilise of MR for retrospective movement correction or partial volume correction of the PET.

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Disharmonize of involvement statement

Bernd Pichler is a consultant for Siemens Medical Solutions. Thomas Beyer is an employee of cmi-experts.

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Affiliations

1. Max Planck Institute for Biological Cybernetics, Spemannstraße 38, 72076, Tuebingen, Federal republic of germany

   Matthias Hofmann & Bernhard Schölkopf

2. Laboratory for Preclinical Imaging and Imaging Technology of the Werner Siemens-Foundation, Section of Radiology, University of Tuebingen, Roentgenweg eleven, 72076, Tuebingen, Deutschland

   Matthias Hofmann & Bernd Pichler

3. Wolfson Medical Vision Laboratory, Department of Technology, Academy of Oxford, Oxford, OX1 3PJ, Great britain

   Matthias Hofmann

4. Department of Nuclear Medicine, University Hospital Duisburg-Essen, Hufelandstr 55, 45122, Essen, Germany

   Thomas Beyer

5. cmi-experts GmbH, Pestalozzistr, 8032, Zurich, Switzerland

   Thomas Beyer

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Correspondence to Matthias Hofmann.

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Hofmann, M., Pichler, B., Schölkopf, B. et al. Towards quantitative PET/MRI: a review of MR-based attenuation correction techniques. Eur J Nucl Med Mol Imaging 36, 93–104 (2009). https://doi.org/10.1007/s00259-008-1007-7

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• Published: 23 Dec 2008

• Issue Date: March 2009

• DOI : https://doi.org/x.1007/s00259-008-1007-vii

Keywords

• PET/MRI
• PET quantification
• Attenuation correction

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