Attenuation Correction Maps Research Articles

Assessment of cardiac amyloidosis by using 18f-sodium fluoride PET/MR imaging

Purpose Cardiac amyloidosis exists in two predominant forms called acquired monoclonal immunoglobulin light-chain (AL) and transthyretin-related (ATTR). Their differentiation is a real diagnostic challenge for treatment management and prognosis of patient. Magnetic resonance (MR) imaging is increasingly used to aid in the diagnosis of cardiac amyloidosis on the basis of characteristic appearances on late Gadolinium enhancement (LGE) but cannot distinguish ATTR and Al forms. Several series have showed that scintigraphy using bone 99mTc-bisphosphonate tracers preferentially bind ATTR versus AL myocardial deposit. Our aim was to use 18F-Sodium Fluoride (18F-NaF) positron emission tomography (PET) bone tracer in hybrid PET/MR imaging to aid in both the diagnosis of cardiac amyloidosis and differentiation of ATTR and AL forms within a single scan. Methods Consecutive patients with biopsy-proven ATTR or AL cardiac amyloidosis and as many healthy control subjects were included. All patients underwent simultaneous PET/MR (BiographTM mMR, Siemens) scans after IV injection of 370MBq of 18F-NaF. The selected data were reconstructed using a 3D breath-hold Dixon MR attenuation correction map. MR protocol included LGE sequences, pre- and post-contrast T1 mapping. Maximal target-to-background ratio (TBRmax) was recorded, defined as maximal myocardial FDG uptake (SUVmax) corrected for mean right atrium blood pool activity. Mean TBRmax in ATTR, AL and controls were compared using a Student t-test. The institutional review board approved the study and all patients gave written informed consent. Results Eighteen patients (61.3 ± 10.5 yo, 12 M/6F) were prospectively recruited (6 ATTR, 3 AL and 9 controls). All amyloid patients had characteristic LGE appearances. Mean TBRmax were respectively 1.29 ± 0.31, 0.77 ± 0.06 and 0.68 ± 0.03 in ATTR, AL and control subjects. Mean TBRmax was significantly higher in ATTR than in AL patients ( P = 0.028) and in controls ( P = 0.0001). Mean TBRmax was significantly higher in AL patients than in controls ( P = 0.046). A TBRmax threshold of 0.85 appeared to differentiate all patients as having ATTR amyloidosis. There was no significant difference in terms of pre-contrast ( P = 0.48) and post-contrast T1 mapping ( P = 0.57) between ATTR and AL patients. Conclusion These results showed the potential of 18F-NaF PET/MR to diagnose cardiac amyloidosis and to differentiate ATTR and AL forms, confirming our preliminary published results (Trivieri et al. JACC 2016)

Individual refinement of attenuation correction maps for hybrid PET/MR based on multi-resolution regional learning

PET/MR is an emerging hybrid imaging modality. However, attenuation correction (AC) remains challenging for hybrid PET/MR in generating accurate PET images. Segmentation-based methods on special MR sequences are most widely recommended by vendors. However, their accuracy is usually not high. Individual refinement of available certified attenuation maps may be helpful for further clinical applications. In this study, we proposed a multi-resolution regional learning (MRRL) scheme to utilize the internal consistency of the patient data. The anatomical and AC MR sequences of the same subject were employed to guide the refinement of the provided AC maps. The developed algorithm was tested on 9 patients scanned consecutively with PET/MR and PET/CT (7 [18F]FDG and 2 [18F]FET). The preliminary results showed that MRRL can improve the accuracy of segmented attenuation maps and consequently the accuracy of PET reconstructions.

Single STE-MR Acquisition in MR-Based Attenuation Correction of Brain PET Imaging Employing a Fully Automated and Reproducible Level-Set Segmentation Approach.

The aim of this study is to introduce a fully automatic and reproducible short echo-time (STE) magnetic resonance imaging (MRI) segmentation approach for MR-based attenuation correction of positron emission tomography (PET) data in head region. Single STE-MR imaging was followed by generating attenuation correction maps (μ-maps) through exploiting an automated clustering-based level-set segmentation approach to classify head images into three regions of cortical bone, air, and soft tissue. Quantitative assessment was performed by comparing the STE-derived region classes with the corresponding regions extracted from X-ray computed tomography (CT) images. The proposed segmentation method returned accuracy and specificity values of over 90% for cortical bone, air, and soft tissue regions. The MR- and CT-derived μ-maps were compared by quantitative histogram analysis. The results suggest that the proposed automated segmentation approach can reliably discriminate bony structures from the proximal air and soft tissue in single STE-MR images, which is suitable for generating MR-based μ-maps for attenuation correction of PET data.

SU‐F‐J‐112: Clinical Feasibility Test of An RF Pulse‐Based MRI Method for the Quantitative Fat‐Water Segmentation

Purpose:Quantitative fat‐water segmentation is important not only because of the clinical utility of fat‐suppressed MRI images in better detecting lesions of clinical significance (in the midst of bright fat signal) but also because of the possible physical need, in which CT‐like images based on the materials’ photon attenuation properties may have to be generated from MR images; particularly, as in the case of MR‐only radiation oncology environment to obtain radiation dose calculation or as in the case of hybrid PET/MR modality to obtain attenuation correction map for the quantitative PET reconstruction. The majority of such fat‐water quantitative segmentations have been performed by utilizing the Dixon's method and its variations, which have to enforce the proper settings (often predefined) of echo time (TE) in the pulse sequences. Therefore, such methods have been unable to be directly combined with those ultrashort TE (UTE) sequences that, taking the advantage of very low TE values (∼ 10's microsecond), might be beneficial to directly detect bones. Recently, an RF pulse‐based method (http://dx.doi.org/10.1016/j.mri.2015.11.006), termed as PROD pulse method, was introduced as a method of quantitative fat‐water segmentation that does not have to depend on predefined TE settings. Here, the clinical feasibility of this method is verified in brain tumor patients by combining the PROD pulse with several sequences.Methods:In a clinical 3T MRI, the PROD pulse was combined with turbo spin echo (e.g. TR=1500, TE=16 or 60, ETL=15) or turbo field echo (e.g. TR=5.6, TE=2.8, ETL=12) sequences without specifying TE values.Results:The fat‐water segmentation was possible without having to set specific TE values.Conclusion:The PROD pulse method is clinically feasible. Although not yet combined with UTE sequences in our laboratory, the method is potentially compatible with UTE sequences, and thus, might be useful to directly segment fat, water, bone and air.

Improvements in Segmentation Using Scatter and Photopeak Window Data for Attenuation Correction in Myocardial SPECT

Background: The method of Segmentation with Scatter and Photopeak window data in Attenuation Correction (SSPAC) recognizes the outlines of the body around chest and lungs using scatter window data after which an attenuation correction (μ) map can be constructed.We have developed a new extraction method, adding a masking process to SSPAC, because the extraction of outlines was otherwise incomplete. Methods and Results: The masking process extracted right and left lung fields from chest images.The quality of the masking process was confirmed by the results from a low count phantom.In a case study, automatic extraction by SSPAC had a 44% success rate for low count (stress condition) myocardial single-photon emission computed tomography (SPECT) images, but reached a success rate of 99% with the addition of the new masking process.Outline truncation and low counts can cause unsuccessful SSPAC. Conclusions: Our method for masking will contribute to a widespread use of SSPAC by improving the success rate in contouring.

Evaluation of single-photon emission computed tomography images obtained with and without copper filter by segmentation.

Background:Measurement of accurate attenuation of photon flux in tissue is important to obtain reconstructed images using single-photon emission computed tomography (SPECT). Computed tomography (CT) scanner provides attenuation correction data for SPECT as well as anatomic information for diagnostic purposes. Segmentation is a process of dividing an image into regions having similar properties such as gray level, color, texture, brightness, and contrast. Image segmentation is an important tool for evaluation of medical images. X-ray beam used in CT scan is poly-energetic; therefore, we have used a copper filter to remove the low energy X-rays for obtaining correct attenuation factor. Images obtained with and without filters were quantitatively evaluated by segmentation method to avoid human error.Materials and Methods:Axial images of AAPM CT phantom were acquired with 3 mm copper filter (low intensity) and without copper filter (high intensity) using low-dose CT (140 kvp and 2.5 mA) of SPECT/CT system (Hawkeye, GE Healthcare). For segmentation Simulated Annealing Based Fuzzy c-means, algorithm is applied. Quantitative measurement of quality is done based on universal image quality index. Further, for the validation of attenuation correction map of filtered CT images, Jaszczak SPECT phantom was filled with 500 MBq of 99mTc and SPECT study was acquired. Low dose CT images were acquired for attenuation correction to be used for reconstruction of SPECT images. Another set of CT images were acquired after applying additional 3 mm copper filter. Two sets of axial SPECT images were reconstructed using attenuation map from both the CT images obtained without and with a filter.Results and Conclusions:When we applied Simulated Annealing Based Fuzzy c-means segmentation on both the CT images, the CT images with filter shows remarkable improvement and all the six section of the spheres in the Jaszczak SPECT phantom were clearly visualized.

Potential influence of Gadolinium contrast on image segmentation in MR-based attenuation correction with Dixon sequences in whole-body 18F-FDG PET/MR

To evaluate the influence of Gadolinium contrast agent on image segmentation in magnetic resonance (MR)-based attenuation correction (AC) with four-segment dual-echo time Dixon-sequences in whole-body [18F]-fluorodeoxyglucose positron emission tomography (PET)/MR imaging, and to analyze the consecutive effect on standardized uptake value (SUV). Hybrid imaging with an integrated PET/MR system was performed in 30 oncological patients. AC was based on MR imaging with a Dixon sequence with subsequent automated image segmentation. AC maps (µmaps) were acquired and reconstructed prior to (µmap-gd) and after (µmap+gd) Gd-contrast agent application. For quantification purposes, the SUV of organs and tumors based on both µmaps were compared. Tissue classification based on µmap-gd was correct in 29/30 patients; based on µmap+gd, the brain was falsely classified as fat in 12/30 patients with significant underestimation of SUV. In all cancerous lesions, tissue segmentation was correct. All concordant µmaps-gd/+gd resulted in no significant difference in SUV. In PET/MR, Gd-contrast agent potentially influences fat/water separation in Dixon-sequences of the head with above-average false tissue segmentation and an associated underestimation of SUV. Thus, MR-based AC should be acquired prior to Gd-contrast agent application. Additionally, integrating the MR-based AC maps into the reading-routine in PET/MR is recommended to avoid interpretation errors in cases where tissue segmentation fails.

An improved MR sequence for attenuation correction in PET/MR hybrid imaging

The aim of this study was to investigate the effects of MR parameters on tissue segmentation and determine the optimal MR sequence for attenuation correction in PET/MR hybrid imaging. Eight healthy volunteers were examined using a PET/MR hybrid scanner with six three-dimensional turbo-field-echo sequences for attenuation correction by modifying the echo time, k-space trajectory in the phase-encoding direction, and image contrast. MR images for attenuation correction were obtained from six MR sequences in each session; each volunteer underwent four sessions. Two radiologists assessed the attenuation correction maps generated from the MR images with respect to segmentation errors and ghost artifacts on a five-point scale, and the scores were decided by consensus. Segmentation accuracy and reproducibility were compared. Multiple regression analysis was performed to determine the effects of each MR parameter. The two three-dimensional turbo-field-echo sequences with an in-phase echo time and radial k-space sampling showed the highest total scores for segmentation accuracy, with a high reproducibility. In multiple regression analysis, the score with the shortest echo time (−3.44, P<0.0001) and Cartesian sampling in the anterior/posterior phase-encoding direction (−2.72, P=0.002) was significantly lower than that with in-phase echo time and Cartesian sampling in the right/left phase-encoding direction. Radial k-space sampling provided a significantly higher score (+5.08, P<0.0001) compared with Cartesian sampling. Furthermore, radial sampling improved intrasubject variations in the segmentation score (−8.28%, P=0.002). Image contrast had no significant effect on the total score or reproducibility. These results suggest that three-dimensional turbo-field-echo MR sequences with an in-phase echo time and radial k-space sampling provide improved MR-based attenuation correction maps.

Automatic registration of misaligned CT attenuation correction maps in Rb-82 PET/CT improves detection of angiographically significant coronary artery disease

BackgroundWe aimed to evaluate the utility of fully automated software registration intended to improve CT attenuation correction (CTAC) map misalignments during cardiac 82Rb PET/CT myocardial perfusion imaging (MPI). Methods171 consecutive patients (108 males, mean age 69 years), undergoing both rest-stress 82Rb PET/CT MPI and invasive coronary angiography within 6 months (mean 14 days, range 0-170), were studied. List mode data were automatically processed in batch mode to generate transaxial attenuation corrected slices with four different CTAC alignment correction strategies: (i) no alignment correction (NONE); (ii) manual correction (MANUAL); (iii) automated 6-parameter rigid correction (AUTO); and (iv) targeted use of automated correction only where PET-CTAC alignment was initially judged as incorrect on either stress or rest scan (AUTO for misalignment only). Initial and final registration quality was graded (1-3) by an experienced radiologist (1: satisfactory alignment (<2 mm misalignment), 2: slight misalignment (2-5 mm in any direction), or 3: poor (>5 mm misalignment in any direction). Total perfusion deficit (TPD) and ischemic TPD (ITPD) were computed automatically, and their diagnostic accuracy to detect significant coronary artery disease with each realignment technique was assessed using receiver operating characteristic analysis. ResultsThe diagnostic accuracy of ITPD, expressed as area under curve, was .81 ± .03 with no alignment correction (NONE), .83 ± .03 with MANUAL correction, .85 ± .03 with AUTO correction (P < .05 vs. NONE and MANUAL), and .87 ± .03 with the targeted use of AUTO correction (P < .05 vs. NONE, MANUAL and AUTO). Both manual and software corrections increased the percentage of cases with satisfactory PET-CTAC map alignment (P < .05 for all) at rest (from 55% for NONE to 80% for MANUAL and 92% for AUTO) and at stress (from 51% for NONE to 78% for MANUAL and 84% for AUTO). ConclusionThe diagnostic accuracy of 82Rb PET/CT MPI with automated rigid alignment is improved compared to data with no CTAC scan alignment or with manual alignment. The optimal strategy for diagnostic performance is to apply automatic alignment only in cases which are visually identified as misaligned.

Positron-based attenuation correction for Positron Emission Tomography data using MCNP6 code

This paper presents the Monte Carlo simulation of the attenuation correction for Positron Emission Tomography (PET) data using MCNP6 code. Two attenuation correction maps have been generated, one for correcting the attenuation effect in a homogeneous phantom, which is a cylindrical volume of water and the other for correcting the attenuation effect in a heterogeneous phantom, which is a cylindrical volume of water within which, there are two small cylinders of bone-equivalent materials. These maps are derived from the data acquired as a result of transmission scans using a positron-emitting rod source. The attenuation map generated using this method does not need to be scaled because it is directly built for an energy of 511 keV. For each phantom, three types of simulations are done, one to estimate the radiotracer distribution in the phantom (emission scan) and two to estimate the distribution of attenuation coefficients in this phantom (transmission scans), the first with a blank field of view (FOV) and the second when the phantom exists in the FOV. From the transmission scans data, the attenuation map for each phantom is derived and after that it has been applied to the corresponding emission scan data during PET image reconstruction process to obtain the attenuation-corrected image. The images of the radiotracer distribution in each phantom reached in this study illustrate the quantitative and qualitative improvements in the image quality after attenuation correction than that before the attenuation correction.

Quality control for quantitative multicenter whole-body PET/MR studies: A NEMA image quality phantom study with three current PET/MR systems.

Integrated positron emission tomography/magnetic resonance (PET/MR) systems derive the PET attenuation correction (AC) from dedicated MR sequences. While MR-AC performs reasonably well in clinical patient imaging, it may fail for phantom-based quality control (QC). The authors assess the applicability of different protocols for PET QC in multicenter PET/MR imaging. The National Electrical Manufacturers Association NU 2 2007 image quality phantom was imaged on three combined PET/MR systems: a Philips Ingenuity TF PET/MR, a Siemens Biograph mMR, and a GE SIGNA PET/MR (prototype) system. The phantom was filled according to the EANM FDG-PET/CT guideline 1.0 and scanned for 5 min over 1 bed. Two MR-AC imaging protocols were tested: standard clinical procedures and a dedicated protocol for phantom tests. Depending on the system, the dedicated phantom protocol employs a two-class (water and air) segmentation of the MR data or a CT-based template. Differences in attenuation- and SUV recovery coefficients (RC) are reported. PET/CT-based simulations were performed to simulate the various artifacts seen in the AC maps (μ-map) and their impact on the accuracy of phantom-based QC. Clinical MR-AC protocols caused substantial errors and artifacts in the AC maps, resulting in underestimations of the reconstructed PET activity of up to 27%, depending on the PET/MR system. Using dedicated phantom MR-AC protocols, PET bias was reduced to -8%. Mean and max SUV RC met EARL multicenter PET performance specifications for most contrast objects, but only when using the dedicated phantom protocol. Simulations confirmed the bias in experimental data to be caused by incorrect AC maps resulting from the use of clinical MR-AC protocols. Phantom-based quality control of PET/MR systems in a multicenter, multivendor setting may be performed with sufficient accuracy, but only when dedicated phantom acquisition and processing protocols are used for attenuation correction.

Image artifacts from MR-based attenuation correction in dedicated PET/MR breast coil for PET/MR mammography.

We evaluated the artifacts in segmentation-based attenuation correction maps (μ-maps) of hybrid positron emission tomography/magnetic resonance (PET/MR) mammography using dedicated PET/MR breast coil in breast cancer patients.

Segmentation-based attenuation correction in positron emission tomography/magnetic resonance: erroneous tissue identification and its impact on positron emission tomography interpretation.

The objective of this study was to evaluate the frequency and characteristics of artifacts in segmentation-based attenuation correction maps (μ-maps) of positron emission tomography/magnetic resonance (PET/MR) and their impact on PET interpretation and the standardized uptake value (SUV) quantification in normal tissue and lesions. The study was approved by the local institutional review board. Attenuation maps of 100 patients with PET/MR and preceding PET/computed tomography examination were retrospectively inspected for artifacts (tracers: 2-deoxy-2-[¹⁸F]fluoro-D-glucose (¹⁸F-FDG), ¹¹C-Choline, ⁶⁸Ga-DOTATOC, ⁶⁸Ga-DOTATATE, ¹¹C-Methionine). The artifacts were subdivided into 9 different groups on the basis of their localization and appearance. The impact of μ-map artifacts in normal tissue and lesions on PET interpretation was evaluated qualitatively via visual analysis in synopsis with the non-attenuation-corrected (NAC) PET as well as quantitatively by comparing the SUV in artifact regions to reference regions. Attenuation map artifacts were found in 72% of the head/neck data sets, 61% of the thoracic data sets, 25% of the upper abdominal data sets, and 26% of the pelvic data sets. The most frequent localizations of the overall 276 artifacts were around metal implants (16%), in the lungs (19%), and outer body contours (31%). Twenty-one percent of all PET-avid lesions (38 of 184 lesions) were affected by artifacts in the majority without further consequences for visual PET interpretation. However, 9 PET-avid lung lesions were masked owing to μ-map artifacts and, thus, were only detectable on the NAC PET or additional MR imaging sequences. Quantitatively, μ-map artifacts led to significant SUV changes in areas with erroneous assignment of air instead of soft tissue (ie, metal artifacts) and of soft tissue instead of lung. Nevertheless, no change in diagnosis would have been caused by μ-map artifacts. Attenuation map artifacts that occur in a considerable percentage of PET/MR data sets have the potential to falsify PET quantification and visual PET interpretation. Nevertheless, on the basis of the present data, in the clinical interpretation setup, no changes in diagnosis due to μ-map artifacts may occur, especially when the μ-maps are checked for artifacts and PET/MR is read in synopsis with the NAC PET, if artifacts are present.

Effects of ferumoxytol on quantitative PET measurements in simultaneous PET/MR whole-body imaging: a pilot study in a baboon model.

BackgroundSimultaneous PET/MR imaging depends on MR-derived attenuation maps (mu-maps) for accurate attenuation correction of PET data. Currently, these maps are derived from gradient-echo-based MR sequences, which are sensitive to susceptibility changes. Iron oxide magnetic nanoparticles have been used in the measurement of blood volume, tumor microvasculature, tumor-associated macrophages, and characterizing lymph nodes. Our aim in this study was to assess whether the susceptibility effects associated with iron oxide nanoparticles can potentially affect measured 18F-FDG PET standardized uptake values (SUV) through effects on MR-derived attenuation maps.MethodsThe study protocol was approved by the Institutional Animal Care and Use Committee. Using a Siemens Biograph mMR PET/MR scanner, we evaluated the effects of increasing concentrations of ferumoxytol and ferumoxytol aggregates on MR-derived mu-maps using an agarose phantom. In addition, we performed a baboon experiment evaluating the effects of a single i.v. ferumoxytol dose (10 mg/kg) on the liver, spleen, and pancreas 18F-FDG SUV at baseline (ferumoxytol-naïve), within the first hour and at 1, 3, 5, and 11 weeks.ResultsPhantom experiments showed mu-map artifacts starting at ferumoxytol aggregate concentrations of 10 to 20 mg/kg. The in vivo baboon data demonstrated a 53% decrease of observed 18F-FDG SUV compared to baseline within the first hour in the liver, persisting at least 11 weeks.ConclusionsA single ferumoxytol dose can affect measured SUV for at least 3 months, which should be taken into account when administrating ferumoxytol in patients needing sequential PET/MR scans.Advances in knowledge1. Ferumoxytol aggregates, but not ferumoxytol alone, produce significant artifacts in MR-derived attenuation correction maps at approximate clinical dose levels of 10 mg/kg.2. When performing simultaneous whole-body 18F-FDG PET/MR, a single dose of ferumoxytol can result in observed SUV decreases up to 53%, depending on the amount of ferumoxytol aggregates in the studied tissue.Implications for patient careAdministration of a single, clinically relevant, dose of ferumoxytol can potentially result in changes in observed SUV for a prolonged period of time in the setting of simultaneous PET/MR. These potential changes should be considered in particular when administering ferumoxytol to patients with expected future PET/MR studies, as ferumoxytol-induced SUV changes might interfere with therapy assessment.Electronic supplementary materialThe online version of this article (doi:10.1186/s40658-015-0109-0) contains supplementary material, which is available to authorized users.

A novel respiratory gating method for oncologic positron emission tomography based on bioimpedance approach.

Respiratory motion causes loss of image quality and inaccuracy of quantification in oncologic positron emission tomography (PET) imaging. This study introduces a bioimpedance-based gating method for compensation of respiratory motion artefacts. The bioimpedance-based respiratory gating method was studied parallel to a clinically used respiratory gating method [Real-time Position Management by Varian Medical Systems] in 4D PET/CT acquisition of 9 oncologic patients. The quantitative analysis consisted of the evaluation of tumour SUVpeak, SUVmax and volume. Additionally, target-to-background ratios as well as motion in cranial-caudal and anterior-posterior directions were measured. The evaluation was performed with amplitude- and time-based gating using averaged attenuation correction maps. Bioimpedance gating resulted in 17.7-18.9% increase in mean SUVpeak and 20.0-21.4% decrease in mean volume compared to non-gated images. The maximum motion measured from the bioimpedance-gated images was 19mm in cranial-caudal direction and 9mm in anterior-posterior direction. Bioimpedance-based respiratory gating compensates the adverse effects of motion in oncologic PET imaging.

Feasibility of simultaneous whole-brain imaging on an integrated PET-MRI system using an enhanced 2-point Dixon attenuation correction method.

Purpose: To evaluate a potential approach for improved attenuation correction (AC) of PET in simultaneous PET and MRI brain imaging, a straightforward approach that adds bone information missing on Dixon AC was explored.Methods: Bone information derived from individual T1-weighted MRI data using segmentation tools in SPM8, were added to the standard Dixon AC map. Percent relative difference between PET reconstructed with Dixon+bone and with Dixon AC maps were compared across brain regions of 13 oncology patients. The clinical potential of the improved Dixon AC was investigated by comparing relative perfusion (rCBF) measured with arterial spin labeling to relative glucose uptake (rPETdxbone) measured simultaneously with 18F-flurodexoyglucose in several regions across the brain.Results: A gradual increase in PET signal from center to the edge of the brain was observed in PET reconstructed with Dixon+bone. A 5–20% reduction in regional PET signals were observed in data corrected with standard Dixon AC maps. These regional underestimations of PET were either reduced or removed when Dixon+bone AC was applied. The mean relative correlation coefficient between rCBF and rPETdxbone was r = 0.53 (p < 0.001). Marked regional variations in rCBF-to-rPET correlation were observed, with the highest associations in the caudate and cingulate and the lowest in limbic structures. All findings were well matched to observations from previous studies conducted with PET data reconstructed with computed tomography derived AC maps.Conclusion: Adding bone information derived from T1-weighted MRI to Dixon AC maps can improve underestimation of PET activity in hybrid PET-MRI neuroimaging.

Evaluation of external beam hardening filters on image quality of computed tomography and single photon emission computed tomography/computed tomography.

This study was undertaken to evaluate the effect of external metal filters on the image quality of computed tomography (CT) and single photon emission computed tomography (SPECT)/CT images. Images of Jaszack phantom filled with water and containing iodine contrast filled syringes were acquired using CT (120 kV, 2.5 mA) component of SPECT/CT system, ensuring fixation of filter on X-ray collimator. Different thickness of filters of Al and Cu (1 mm, 2 mm, 3 mm, and 4 mm) and filter combinations Cu 1 mm, Cu 2 mm, Cu 3 mm each in combination with Al (1 mm, 2 mm, 3 mm, and 4 mm), respectively, were used. All image sets were visually analyzed for streak artifacts and contrast to noise ratio (CNR) was derived. Similar acquisition was done using Philips CT quality control (QC) phantom and CNR were calculated for its lexan, perspex, and teflon inserts. Attenuation corrected SPECT/CT images of Jaszack phantom filled with 444–555 MBq (12–15 mCi) of 99mTc were obtained by applying attenuation correction map generated by hardened X-ray beam for different filter combination, on SPECT data. Uniformity, root mean square (rms) and contrast were calculated in all image sets. Less streak artifacts at iodine water interface were observed in images acquired using external filters as compared to those without a filter. CNR for syringes, spheres, and inserts of Philips CT QC phantom was almost similar to Al 2 mm, Al 3 mm, and without the use of filters. CNR decreased with increasing copper thickness and other filter combinations. Uniformity and rms were lower, and value of contrast was higher for SPECT/CT images when CT was acquired with Al 2 mm and 3 mm filter than for images acquired without a filter. The study suggests that for Infinia Hawkeye 4, SPECT/CT system, Al 2 mm, and 3 mm are the optimum filters for improving image quality of SPECT/CT images of Jaszack or Philips CT QC phantom keeping other parameters of CT constant.

A 16-channel MR coil for simultaneous PET/MR imaging in breast cancer.

To implement and evaluate a dedicated receiver array coil for simultaneous positron emission tomography/magnetic resonance (PET/MR) imaging in breast cancer. A 16-channel receiver coil design was optimized for simultaneous PET/MR imaging. To assess MR performance, the signal-to-noise ratio, parallel imaging capability and image quality was evaluated in phantoms, volunteers and patients and compared to clinical standard protocols. For PET evaluation, quantitative (18) F-FDG PET images of phantoms and seven patients (14 lesions) were compared to images without the coil. In PET image reconstruction, a CT-based template of the coil was combined with the MR-acquired attenuation correction (AC) map of the phantom/patient. MR image quality was comparable to clinical MR-only examinations. PET evaluation in phantoms showed regionally varying underestimation of the standardised uptake value (SUV; mean 22 %) due to attenuation caused by the coil. This was improved by implementing the CT-based coil template in the AC (<2 % SUV underestimation). Patient data indicated that including the coil in the AC increased the SUV values in the lesions (21 ± 9 %). Using a dedicated PET/MR breast coil, state-of-the-art MRI was possible. In PET, accurate quantification and image homogeneity could be achieved if a CT-template of this coil was included in the AC for PET image reconstruction. • State-of-the-art breast MRI using a dedicated PET/MR breast coil is feasible. • A multi-channel design facilitates shorter MR acquisition times via parallel imaging. • An MR coil inside a simultaneous PET/MR system causes PET photon attenuation. • Including a coil CT-template in PET image reconstruction results in recovering accurate quantification.

A robust MR-based attenuation map generation in short-TE MR images of the head employing hybrid spatial fuzzy C-means clustering and intensity inhomogeneity correction.

Deriving an accurate attenuation correction map (μ-map) from magnetic resonance (MR) volumes has become an important problem in hybrid PET/MR imaging. Recently, short echo-time (STE) MR imaging technique incorporating fuzzy C-means (FCM) tissue classification and 2-point Dixon image acquisition has been introduced as a feasible approach for segmentation of the bone from air and soft tissue. However, this method imposes additional imaging and the performance of the standard FCM algorithm, suffering from the lack of spatial information, becomes impaired in the presence of inherent noise and intensity inhomogeneity. Here, we exploit a spatial fuzzy C-means (SFCM) segmentation algorithm in combination with a robust intensity inhomogeneity correction method on single STE-MR images, to differentiate various tissue classes. MR images of five subjects were acquired on a clinical 1.5T MRI System, MAGNETOM Avanto, using a FLASH 3D pulse sequence with TE=1.1ms, TR=12ms, flip angle=18°, voxel size=1.2×1.2×2mm3. The proposed segmentation approach consists of four main steps: (1) Intensity-inhomogeneity correction, to separate air and bone in the regions with high inhomogeneity, like the nasal areas; (2) applying SFCM to segment the images into four clusters including air, a part of soft tissue and bone, and two other soft tissue classes. Upon this step, the air cluster would be accurately separated; (3) employing shape factor analysis to remove the eyes with close signal intensity to that of bone; and (4) μ-map generation by downsampling and assigning attenuation coefficients to the corresponding segmented tissues. Quantitative evaluation indicated sensitivity of over 95% for air and soft tissue and about 81% in the bony region, and specificity of over 95% for all tissue classes. The proposed STE-MR imaging in combination with the segmentation technique could be potentially exploited as an efficient approach to generate MR-based attenuation correction maps in clinical PET/MR applications, where bone and air coexist.

A fully automated and reproducible level-set segmentation approach for generation of MR-based attenuation correction map of PET images in the brain employing single STE-MR imaging modality.

Generating MR-based attenuation correction map (μ-map) for quantitative reconstruction of PET images still remains a challenge in hybrid PET/MRI systems, mainly because cortical bone structures are indistinguishable from proximal air cavities in conventional MR images. Recently, development of short echo-time (STE) MR imaging sequences, has shown promise in differentiating cortical bone from air. However, on STE-MR images, the bone appears with discontinuous boundaries. Therefore, segmentation techniques based on intensity classification, such as thresholding or fuzzy C-means, fail to homogeneously delineate bone boundaries, especially in the presence of intrinsic noise and intensity inhomogeneity. Consequently, they cannot be fully automatized, must be fine-tuned on the case-by-case basis, and require additional morphological operations for segmentation refinement. To overcome the mentioned problems, in this study, we introduce a new fully automatic and reproducible STE-MR segmentation approach exploiting level-set in a clustering-based intensity inhomogeneity correction framework to reliably delineate bone from soft tissue and air. MR images were acquired on a clinical 1.5T MRI System, MAGNETOM Avanto, using a FLASH 3D pulse sequence with TE=1.1ms, TR=12ms, flip angle=18°, voxel size=1.2×1.2×2mm3. For segmentation of the STE-MR images into three regions, consisting of bone, air and soft tissue, a region-based level-set segmentation algorithm was applied. In this technique, k-means clustering is applied to estimate the intensity properties of each region for bias field correction simultaneously with the level-set segmentation. This algorithm incorporates both intensity and spatial information to define continuous boundaries. The quantitative assessment outcomes of the segmentation performance yielded an average of 89%, 82%, 91%, and 73% for the accuracy, sensitivity, specificity and dice scores in bone segmentation, respectively. The results suggest that the proposed fully automatic segmentation approach can reliably discriminate bony structures from the neighboring air and soft tissue in STE-MR images, which is suitable for generating accurate μ-maps in clinical PET/MR applications.