Ultrashort Echo Time Research Articles

Ultrashort echo time and zero echo time MRI at 7T.

Zero echo time (ZTE) and ultrashort echo time (UTE) pulse sequences for MRI offer unique advantages of being able to detect signal from rapidly decaying short-T2 tissue components. In this paper, we applied 3D ZTE and UTE pulse sequences at 7T to assess differences between these methods. We matched the ZTE and UTE pulse sequences closely in terms of readout trajectories and image contrast. Our ZTE used the water- and fat-suppressed solid-state proton projection imaging method to fill the center of k-space. Images from healthy volunteers obtained at 7T were compared qualitatively, as well as with SNR and CNR measurements for various ultrashort, short, and long-T2 tissues. We measured nearly identical contrast-to-noise and signal-to-noise ratios (CNR/SNR) in similar scan times between the two approaches for ultrashort, short, and long-T2 components in the brain, knee and ankle. In our protocol, we observed gradient fidelity artifacts in UTE, and our chosen flip angle and readout also resulted in shading artifacts in ZTE due to inadvertent spatial selectivity. These can be corrected by advanced reconstruction methods or with different chosen protocol parameters. The applied ZTE and UTE pulse sequences achieved similar contrast and SNR efficiency for volumetric imaging of ultrashort-T2 components. Key differences include that ZTE is limited to volumetric imaging, but has substantially reduced acoustic noise levels during the scan. Meanwhile, UTE has higher acoustic noise levels and greater sensitivity to gradient fidelity, but offers more flexibility in image contrast and volume selection.

Accuracy of UTE-MRI-based patient setup for brain cancer radiation therapy.

Radiation therapy simulations solely based on MRI have advantages compared to CT-based approaches. One feature readily available from computed tomography (CT) that would need to be reproduced with MR is the ability to compute digitally reconstructed radiographs (DRRs) for comparison against on-board radiographs commonly used for patient positioning. In this study, the authors generate MR-based bone images using a single ultrashort echo time (UTE) pulse sequence and quantify their 3D and 2D image registration accuracy to CT and radiographic images for treatments in the cranium. Seven brain cancer patients were scanned at 1.5 T using a radial UTE sequence. The sequence acquired two images at two different echo times. The two images were processed using an in-house software to generate the UTE bone images. The resultant bone images were rigidly registered to simulation CT data and the registration error was determined using manually annotated landmarks as references. DRRs were created based on UTE-MRI and registered to simulated on-board images (OBIs) and actual clinical 2D oblique images from ExacTrac™. UTE-MRI resulted in well visualized cranial, facial, and vertebral bones that quantitatively matched the bones in the CT images with geometric measurement errors of less than 1 mm. The registration error between DRRs generated from 3D UTE-MRI and the simulated 2D OBIs or the clinical oblique x-ray images was also less than 1 mm for all patients. UTE-MRI-based DRRs appear to be promising for daily patient setup of brain cancer radiotherapy with kV on-board imaging.

MRI-Based Attenuation Correction for PET/MRI Using Multiphase Level-Set Method.

Inaccuracy in MR image-based attenuation correction (MR-AC) leads to errors in quantification and the misinterpretation of lesions in brain PET/MRI studies. To resolve this problem, we proposed an improved ultrashort echo time MR-AC method that was based on a multiphase level-set algorithm with main magnetic field (B0) inhomogeneity correction. We also assessed the feasibility of this level-set-based MR-AC method (MR-AC(level)), compared with CT-AC and MR-AC provided by the manufacturer of the PET/MRI scanner (MR-AC(mMR)). Ten healthy volunteers and 20 Parkinson disease patients underwent(18)F-FDG and(18)F-fluorinated-N-3-fluoropropyl-2-β-carboxymethoxy-3-β-(4-iodophenyl)nortropane ((18)F-FP-CIT) PET scans, respectively, using both PET/MRI and PET/CT scanners. The level-set-based segmentation algorithm automatically delimited air, bone, and soft tissue from the ultrashort echo time MR images. For the comparison, MR-AC maps were coregistered to reference CT. PET sinogram data obtained from PET/CT studies were then reconstructed using the CT-AC, MR-AC(mMR), and MR-AC(level) maps. The accuracies of SUV, SUVr (SUV and its ratio to the cerebellum), and specific-to-nonspecific binding ratios obtained using MR-AC(level) and MR-AC(mMR) were compared with CT-AC using region-of-interest- and voxel-based analyses. There was remarkable improvement in the segmentation of air cavities and bones and the quantitative accuracy of PET measurement using the level set. Although the striatal and cerebellar activities in (18)F-FP-CIT PET and frontal activity in (18)F-FDG PET were significantly underestimated by the MR-AC(mMR), the MR-AC(level) provided PET images almost equivalent to the CT-AC images. PET quantification error was reduced by a factor of 3 using MR-AC(level) (SUV error < 10% in MR-AC(level) and < 30% in MR-AC(mMR) [version VB18P], and < 5% in MR-AC(level) and < 15% in MR-AC(mMR) [VB20P]). The results of this study indicate that our new multiphase level-set-based MR-AC method improves the quantitative accuracy of brain PET in PET/MRI studies.

MRI-Only Based Radiotherapy Treatment Planning for the Rat Brain on a Small Animal Radiation Research Platform (SARRP).

Computed tomography (CT) is the standard imaging modality in radiation therapy treatment planning (RTP). However, magnetic resonance (MR) imaging provides superior soft tissue contrast, increasing the precision of target volume selection. We present MR-only based RTP for a rat brain on a small animal radiation research platform (SARRP) using probabilistic voxel classification with multiple MR sequences. Six rat heads were imaged, each with one CT and five MR sequences. The MR sequences were: T1-weighted, T2-weighted, zero-echo time (ZTE), and two ultra-short echo time sequences with 20 μs (UTE1) and 2 ms (UTE2) echo times. CT data were manually segmented into air, soft tissue, and bone to obtain the RTP reference. Bias field corrected MR images were automatically segmented into the same tissue classes using a fuzzy c-means segmentation algorithm with multiple images as input. Similarities between segmented CT and automatic segmented MR (ASMR) images were evaluated using Dice coefficient. Three ASMR images with high similarity index were used for further RTP. Three beam arrangements were investigated. Dose distributions were compared by analysing dose volume histograms. The highest Dice coefficients were obtained for the ZTE-UTE2 combination and for the T1-UTE1-T2 combination when ZTE was unavailable. Both combinations, along with UTE1-UTE2, often used to generate ASMR images, were used for further RTP. Using 1 beam, MR based RTP underestimated the dose to be delivered to the target (range: 1.4%-7.6%). When more complex beam configurations were used, the calculated dose using the ZTE-UTE2 combination was the most accurate, with 0.7% deviation from CT, compared to 0.8% for T1-UTE1-T2 and 1.7% for UTE1-UTE2. The presented MR-only based workflow for RTP on a SARRP enables both accurate organ delineation and dose calculations using multiple MR sequences. This method can be useful in longitudinal studies where CT’s cumulative radiation dose might contribute to the total dose.

A female pelvic bone shape model for air/bone separation in support of synthetic CT generation for radiation therapy

Separating bone from air in MR data is one of the major challenges in using MR images to derive synthetic CT. The problem is further complicated when the anatomic regions filled with air are altered across scans due to air mobility, for instance, in pelvic regions, thereby the air regions estimated using an ultrashort echo time (UTE) sequence are invalid in other image series acquired for multispectral classification. This study aims to develop and investigate a female pelvic bone shape model to identify low intensity regions in MRI where air is unlikely to be present in support of synthetic CT generation without UTE imaging. CT scans of 30 patients were collected for the study, 17 of them also have corresponding MR scans. The shape model was built from the CT dataset, where the reference image was aligned to each of the training images using B-spline deformable registration. Principal component analysis was performed on B-spline coefficients for a compact model where shape variance was described by linear combination of principal modes. The model was applied to identify pelvic bone in MR images by deforming the corresponding MR data of the reference image to target MR images, where the search space of the deformation process was constrained within the subspace spanned by principal modes. The local minima in the search space were removed effectively by the shape model, thus supporting an efficient binary search for the optimal solution. We evaluated the model by its efficacy in identifying bone voxels and excluding air regions. The model was tested across the 17 patients that have corresponding MR scans using a leave-one-out cross validation. A simple model using the first leading principal mode only was found to achieve reasonable accuracy, where an averaged 87% of bone voxels were correctly identified. Finally dilation of the optimally fit bone mask by 5 mm was found to cover 96% of bone voxels while minimally impacting the overlap with air (below 0.4%).

Gradient-Modulated PETRA MRI.

Image blurring that results from off-resonance and fast T2* signal decay is a common issue in radial ultrashort echo time magnetic resonance imaging (MRI) sequences. One solution is to use a higher readout bandwidth, but this may be impractical for some techniques such as pointwise-encoding time reduction with radial acquisition (PETRA), which is a hybrid method of zero echo time and single-point imaging techniques. Specifically, PETRA has severe specific absorption rate (SAR) and radiofrequency (RF) pulse peak power limitations when using higher bandwidths in human measurements. In this study, we introduce gradient modulation (GM) to PETRA to reduce image-blurring artifacts while keeping SAR and RF peak power low. GM-PETRA tolerance to image blurring was evaluated in simulations and experiments by comparison with the conventional PETRA technique. We performed inner ear imaging of a healthy subject at 7 T. GM-PETRA showed significantly less image blurring as a result of off-resonance and fast T2* signal decay compared to PETRA. In in vivo imaging, GM-PETRA nicely captured complex structures of the inner ear such as the cochlea and semicircular canals. GM can improve PETRA image quality and mitigate SAR and RF peak power limitations without special hardware modification in clinical scanners.

Functional Proton MRI in Emphysematous Rats.

To demonstrate the feasibility of proton magnetic resonance imaging (MRI) ventilation-related maps in rodents for the evaluation of lung function in the presence of pancreatic porcine elastase (PPE)-induced emphysema. Twelve rats were equally divided into 3 groups: group 1 (no administration of PPE); group 2 (PPE selectively only in the left lung); and group 3 (PPE administered in both lungs). Magnetic resonance imaging (MRI) and computed tomographic (CT) data were acquired at baseline, at 2 weeks and 4 weeks after administration, after which the animals were euthanized. The MRI protocol comprised a golden angle 2-dimensional ultrashort echo time MRI sequence [echo time, 0.343 millisecond (ms); repetition time, 120 ms; 12 slides with thickness, 1 mm; acquisition time, 30 minutes], from which inspiration and expiration images were reconstructed after the extraction of a self-gating signal. Inspiration images were registered to images at expiration, and expansion maps were created by calculating the specific difference in signal intensity. The lungs were segmented, and the mean specific expansion (MSE) calculated as an established surrogate for fractional ventilation. Computed tomographic data provided lung density (peak of the Hounsfield unit histogram, HU_P), whereas histology provided the mean linear intercept for each lung. Two weeks after administration, the control group had a mean MSE in both lungs corresponding to 96% of the baseline. Group 2 had 85% of the baseline, and group 3 had 57%. Considering the PPE-treated lungs alone, a significant reduction in MSE of 27% at 2 weeks and 40% at 4 weeks was found with respect to nontreated lungs. Significant correlations between HU_P and MSE were found at all time points (baseline: r = 0.606, P = 0.0017; 2 weeks: r = 0.837, P ≤ 0.0001; 4 weeks: r = 0.765, P < 0.0001; all time points: r = 0.739, P < 0.0001). Mean linear intercept values significantly correlated both with MRI MSE (r = -0.770, P < 0.0001) and with CT HU_P (r = -0.882, P < 0.0001). The calculated ventilation-related maps showed a reduction of function in the PPE-treated lungs, both compared to the nontreated lungs and to the baseline values. Moreover, a good agreement between MRI-measured MSE, CT, and histology data quantitatively supports the presence of ventilation deficit in emphysematous lungs.In this work, we have demonstrated the feasibility of ventilation-related maps from non-contrast-enhanced H lung MRI, which were capable of tracking changes in lung function over time in emphysematous rats.

Advances in Imaging Cystic Fibrosis Lung Disease.

Cystic fibrosis (CF) is a life shortening, multiorgan disease, in which morbidity and mortality are dominated by pulmonary pathologies, including mucous obstruction, chronic lung infections, progressive bronchiectasis, and declining lung function. Promising new therapies, including those that target the cystic fibrosis transmembrane conductance regulator (CFTR) protein, have shown impressive efficacy in treating CF patients with specific genotypes. While spirometry is the clinical gold standard for evaluating CF lung disease, it has known insensitivities to mild lung disease and statistical weaknesses in small cohorts. Imaging has begun to play an important role in evaluating the structural abnormalities associated with CF lung disease progression. X-ray computed tomography (CT) has been established as a modality with validated metrics to evaluate disease severity, even in young patients with mild disease. Magnetic resonance imaging (MRI), as a nonionizing alternative, has very strong potential for similar metrics and longitudinal use through the lifespan. In this brief review, recent advancements in imaging CF lung disease in pediatrics are discussed, specifically in the contexts of X-ray CT, ultra-short echo time MRI, and hyperpolarized-gas MRI, with an emphasis on how emerging techniques will likely impact the management of CF lung disease in the era of CFTR-modulator therapies, targeted at smaller subpopulations of CF with specific genotypes. In the future, techniques that provide sensitive, quantitative measurements of regional lung structure and function longitudinally will become increasingly important as outcome measures for clinical trials and for monitoring individual CF patients.

A Feasibility Study of Ultrashort Echo Time Imaging of Cortical Bone at 0.35 T

A Feasibility Study of Ultrashort Echo Time Imaging of Cortical Bone at 0.35 T

Qualitative and Quantitative Ultrashort Echo Time Imaging of Musculoskeletal Tissues.

Ultrashort echo time (UTE) sequences represent a group of clinically compatible techniques that are capable of using echo times < 1 ms. With these techniques, direct imaging of short T2/T2* tissues or tissue components can now be performed. Continuing modifications to the UTE techniques have allowed for faster and more robust sequences now comparable with conventional clinical sequences. UTE also allows for morphological imaging and quantitative evaluation in a manner not previously possible with conventional imaging sequences utilizing much longer echo times. Numerous potential clinical applications have emerged that are discussed in this review article.

Imaging modalities in cystic fibrosis

MRI has emerged as radiation-free imaging modality for assessment of cystic fibrosis lung disease. Here, we review the current status and new developments of MRI in cystic fibrosis. MRI is sensitive to detect abnormalities in lung structure and perfusion, even in preschool children with cystic fibrosis with early lung disease. Further, MRI is sensitive to detect changes associated with pulmonary exacerbations and response to antibiotic therapy in this challenging age group. MRI is sensitive to detect hallmarks of cystic fibrosis lung disease such as bronchial wall thickening, bronchiectasis, mucus plugging and abnormal lung perfusion. A morpho-functional MRI score has been established for semiquantitative assessment of these characteristic abnormalities over a broad range of disease severity. Recent studies demonstrated that MRI is sensitive to detect changes in mucus plugging and lung perfusion in response to antibiotic therapy for pulmonary exacerbations. These results suggest that MRI may be suitable for noninvasive monitoring and as a quantitative endpoint in clinical trials for cystic fibrosis. Technical advances including ultra-short echo time and Fourier decomposition imaging are expected to enhance morphological and functional MRI of cystic fibrosis lung disease without the need of contrast medium in the near future.

On the suppression of background signals originating from NMR hardware components. Application to zero echo time imaging and relaxation time analysis

Modern NMR imaging systems used for biomedical research are equipped with B0 gradient systems with strong maximum gradient strength and short switching time enabling 1H NMR measurements of samples with very short transverse relaxation times. However, background signal originating from non-optimized RF coils may hamper experiments with ultrashort delays between RF excitation and signal reception. We demonstrate that two simple means, outer volume suppression and the use of shaped B0 fields produced by higher-order shim coils, allow a considerable suppression of disturbing background signals. Thus, the quality of NMR images acquired at ultrashort or zero echo time is improved and systematic errors in quantitative data evaluation are avoided. Fields of application comprise MRI with ultrashort echo time or relaxation time analysis, for both biomedical research and characterizing porous media filled with liquids or gases.

Evaluation of optimized breath-hold and free-breathing 3D ultrashort echo time contrast agent-free MRI of the human lung.

To evaluate an optimized stack of radials ultrashort echo time (UTE) 3D magnetic resonance imaging (MRI) sequence for breath-hold and free-breathing imaging of the human lung. A 3D stack of ultrashort echo time radials trajectory was optimized for coronal and axial lower-resolution breath-hold and higher-resolution free-breathing scans using Bloch simulations. The sequence was evaluated in 10 volunteers, without the use of contrast agents. Signal-to-noise ratio (SNR) mean and 95% confidence interval (CI) were determined from separate signal and noise images in a semiautomated fashion. The four scanning schemes were evaluated for significant differences in image quality using Student's t-test. Ten clinical patients were scanned with the sequence and findings were compared with concomitant computed tomography (CT) in nine patients. Breath-hold 3D spokes images were compared with 3D stack of radials in five volunteers. A Mann-Whitney U-test was performed to test significance in both cases. Breath-hold imaging of the entire lung in volunteers was performed with SNR (mean = 42.5 [CI]: 35.5-49.5; mean = 34.3 [CI]: 28.6-40) in lung parenchyma for coronal and axial scans, respectively, which can be used as a quick scout scan. Longer respiratory triggered free-breathing scan enabled high-resolution UTE scanning with mean SNR of 14.2 ([CI]: 12.9-15.5) and 9.2 ([CI]: 8.2-10.2) for coronal and axial scans, respectively. Axial free-breathing scans showed significantly higher image quality (P = 0.008) than the three other scanning schemes. The mean score for comparison with CT was 1.67 (score 0: n = 0; 1: n = 3; 2: n = 6). There was no significant difference between CT and MRI (P = 0.25). 3D stack of radials images were significantly better than 3D spokes images (P < 0.001). The optimized 3D stack of radials trajectory was shown to provide high-quality MR images of the lung parenchyma without the use of MRI contrast agents. The sequence may offer the possibility of breath-hold imaging and provides greater flexibility in trading off slice thickness and parallel imaging for scan time.

Dynamic contrast-enhanced quantitative susceptibility mapping with ultrashort echo time MRI for evaluating renal function.

Dynamic contrast-enhanced (DCE) MRI can provide key insight into renal function. DCE MRI is typically achieved through an injection of a gadolinium (Gd)-based contrast agent, which has desirable T1 quenching and tracer kinetics. However, significant T2* blooming effects and signal voids can arise when Gd becomes very concentrated, especially in the renal medulla and pelvis. One MRI sequence designed to alleviate T2* effects is the ultrashort echo time (UTE) sequence. In the present study, we observed T2* blooming in the inner medulla of the mouse kidney, despite using UTE at an echo time of 20 microseconds and a low dose of 0.03 mmol/kg Gd. We applied quantitative susceptibility mapping (QSM) and resolved the signal void into a positive susceptibility signal. The susceptibility values [in parts per million (ppm)] were converted into molar concentrations of Gd using a calibration curve. We determined the concentrating mechanism (referred to as the concentrating index) as a ratio of maximum Gd concentration in the inner medulla to the renal artery. The concentrating index was assessed longitudinally over a 17-wk course (3, 5, 7, 9, 13, 17 wk of age). We conclude that the UTE-based DCE method is limited in resolving extreme T2* content caused by the kidney's strong concentrating mechanism. QSM was able to resolve and confirm the source of the blooming effect to be the large positive susceptibility of concentrated Gd. UTE with QSM can complement traditional magnitude UTE and offer a powerful tool to study renal pathophysiology.

Ultrashort echo (UTE) versus pointwise encoding time reduction with radial acquisition (PETRA) sequences at 3 Tesla for knee meniscus: A comparative study

PurposeThe purposes of this study were to (1) correlate the ultrashort echo time (TE) signal intensity of the pointwise encoding time reduction with radial acquisition (PETRA) sequence with that of the ultrashort echo (UTE) sequence using in vivo meniscal ultrashort TE imaging of the knee with a 3-Tesla (3T) clinical magnetic resonance imaging (MRI) scanner and (2) compare the two ultrashort TE sequences in three groups of patients with normal, degenerated, and torn knee menisci. Materials and methodsFollowing institutional review board approval, we analyzed 47 knee MRIs of 46 patients who presented with knee pain and underwent knee MRIs, including both the prototype 3D PETRA sequence knee MRI (TE: 70μs) and the prototype 3D UTE sequence (TE: 70μs) using a 3T MRI scanner (MAGNETOM Trio, Siemens, Erlangen, Germany). The study group was classified into three subgroups: (1) normal meniscus on conventional MRI, with no positive meniscus-related physical examination on medical records; (2) meniscal degeneration; and (3) meniscal tear. For quantitative assessment, the mean signal intensities inside user-drawn regions of interest (ROIs) for each image set were drawn on the medical menisci as well as on the bone marrow of medical femoral condyle. For statistical analyses, the Pearson correlation test was used for correlation of the ultrashort TE signal intensity on the UTE and the PETRA sequences, and one-way ANOVA with post-hoc analysis using the Scheffe test was conducted to compare groups. ResultsThe correlation test showed moderate correlation between the mean signal intensity values of the two ultrashort TE sequences (Pearson's coefficient: 0.4817; P<0.05; 95% CI: 0.3113–0.6221). The normalized mean signal intensity values were lower for patients with meniscal degeneration and tear on both the PETRA and the UTE images. The PETRA images showed the significantly difference between normal and tear groups and between degeneration and normal groups (P<0.05) whereas the UTE images showed significantly difference between normal and tear groups (P<0.05). ConclusionBoth the PETRA sequence and the UTE sequence can visualize the short T2 tissue. We demonstrated that there was significantly lower signal intensity on the ultrashort TE UTE and the PETRA images of patients with meniscal tear.

Region specific optimization of continuous linear attenuation coefficients based on UTE (RESOLUTE): application to PET/MR brain imaging

The reconstruction of PET brain data in a PET/MR hybrid scanner is challenging in the absence of transmission sources, where MR images are used for MR-based attenuation correction (MR-AC). The main challenge of MR-AC is to separate bone and air, as neither have a signal in traditional MR images, and to assign the correct linear attenuation coefficient to bone. The ultra-short echo time (UTE) MR sequence was proposed as a basis for MR-AC as this sequence shows a small signal in bone. The purpose of this study was to develop a new clinically feasible MR-AC method with patient specific continuous-valued linear attenuation coefficients in bone that provides accurate reconstructed PET image data.A total of 164 [18F]FDG PET/MR patients were included in this study, of which 10 were used for training. MR-AC was based on either standard CT (reference), UTE or our method (RESOLUTE). The reconstructed PET images were evaluated in the whole brain, as well as regionally in the brain using a ROI-based analysis. Our method segments air, brain, cerebral spinal fluid, and soft tissue voxels on the unprocessed UTE TE images, and uses a mapping of values to CT Hounsfield Units (HU) to measure the density in bone voxels.The average error of our method in the brain was 0.1% and less than 1.2% in any region of the brain. On average 95% of the brain was within ±10% of PETCT, compared to 72% when using UTE.The proposed method is clinically feasible, reducing both the global and local errors on the reconstructed PET images, as well as limiting the number and extent of the outliers.

Continuous MR bone density measurement using water- and fat-suppressed projection imaging (WASPI) for PET attenuation correction in PET-MR

Due to the lack of signal from solid bone in normal MR sequences for the purpose of MR-based attenuation correction, investigators have proposed using the ultrashort echo time (UTE) pulse sequence, which yields signal from bone. However, the UTE-based segmentation approach might not fully capture the intra- and inter-subject bone density variation, which will inevitably lead to bias in reconstructed PET images. In this work, we investigated using the water- and fat-suppressed proton projection imaging (WASPI) sequence to obtain accurate and continuous attenuation for bones. This approach is capable of accounting for intra- and inter-subject bone attenuation variations. Using data acquired from a phantom, we have found that that attenuation correction based on the WASPI sequence is more accurate and precise when compared to either conventional MR attenuation correction or UTE-based segmentation approaches.

Three-dimensional accurate detection of lung emphysema in rats using ultra-short and zero echo time MRI.

Emphysema is a life-threatening pathology that causes irreversible destruction of alveolar walls. In vivo imaging techniques play a fundamental role in the early non-invasive pre-clinical and clinical detection and longitudinal follow-up of this pathology. In the present study, we aimed to evaluate the feasibility of using high resolution radial three-dimensional (3D) zero echo time (ZTE) and 3D ultra-short echo time (UTE) MRI to accurately detect lung pathomorphological changes in a rodent model of emphysema.Porcine pancreas elastase (PPE) was intratracheally administered to the rats to produce the emphysematous changes. 3D ZTE MRI, low and high definition 3D UTE MRI and micro-computed tomography images were acquired 4 weeks after the PPE challenge. Signal-to-noise ratios (SNRs) were measured in PPE-treated and control rats. T2* values were computed from low definition 3D UTE MRI. Histomorphometric measurements were made after euthanizing the animals. Both ZTE and UTE MR images showed a significant decrease in the SNR measured in PPE-treated lungs compared with controls, due to the pathomorphological changes taking place in the challenged lungs. A significant decrease in T2* values in PPE-challenged animals compared with controls was measured using UTE MRI. Histomorphometric measurements showed a significant increase in the mean linear intercept in PPE-treated lungs. UTE yielded significantly higher SNR compared with ZTE (14% and 30% higher in PPE-treated and non-PPE-treated lungs, respectively).This study showed that optimized 3D radial UTE and ZTE MRI can provide lung images of excellent quality, with high isotropic spatial resolution (400 µm) and SNR in parenchymal tissue (>25) and negligible motion artifacts in freely breathing animals. These techniques were shown to be useful non-invasive instruments to accurately and reliably detect the pathomorphological alterations taking place in emphysematous lungs, without incurring the risks of cumulative radiation exposure typical of micro-computed tomography.

Quantifying temperature-dependent T1 changes in cortical bone using ultrashort echo-time MRI.

To demonstrate the feasibility of using ultrashort echo-time MRI to quantify T1 changes in cortical bone due to heating. Variable flip-angle T1 mapping combined with 3D ultrashort echo-time imaging was used to measure T1 in cortical bone. A calibration experiment was performed to detect T1 changes with temperature in ex vivo cortical bone samples from a bovine femur. Ultrasound heating experiments were performed using an interstitial applicator in ex vivo bovine femur specimens, and heat-induced T1 changes were quantified. The calibration experiment demonstrated that T1 increases with temperature in cortical bone. We observed a linear relationship between temperature and T1 with a linear coefficient between 0.67 and 0.84 ms/°C over a range of 25-70°C. The ultrasound heating experiments showed increased T1 changes in the heated regions, and the relationship between the temperature changes and T1 changes was similar to that of the calibration. We demonstrated a temperature dependence of T1 in ex vivo cortical bone using a variable flip-angle ultrashort echo-time T1 mapping method.

Respiratory motion correction of PET using MR-constrained PET-PET registration.

BackgroundRespiratory motion in positron emission tomography (PET) is an unavoidable source of error in the measurement of tracer uptake, lesion position and lesion size. The introduction of PET-MR dual modality scanners opens a new avenue for addressing this issue. Motion models offer a way to estimate motion using a reduced number of parameters. This can be beneficial for estimating motion from PET, which can otherwise be difficult due to the high level of noise of the data.MethodWe propose a novel technique that makes use of a respiratory motion model, formed from initial MR scan data. The motion model is used to constrain PET-PET registrations between a reference PET gate and the gates to be corrected. For evaluation, PET with added FDG-avid lesions was simulated from real, segmented, ultrashort echo time MR data obtained from four volunteers. Respiratory motion was included in the simulations using motion fields derived from real dynamic 3D MR volumes obtained from the same volunteers.ResultsPerformance was compared to an MR-derived motion model driven method (which requires constant use of the MR scanner) and to unconstrained PET-PET registration of the PET gates. Without motion correction, a median drop in uncorrected lesion {mathrm {SUV}}_{mathrm {peak}} intensity to 78.4 pm 18.6 ,,% and an increase in median head-foot lesion width, specified by a minimum bounding box, to 179 pm 63.7,, % was observed relative to the corresponding measures in motion-free simulations. The proposed method corrected these values to 86.9 pm 13.6,, % (p<0.001) and 100 pm 29.12,, % (p<0.001) respectively, with notably improved performance close to the diaphragm and in the liver. Median lesion displacement across all lesions was observed to be 6.6 pm 5.4,mathrm {mm} without motion correction, which was reduced to 3.5 pm 1.8,mathrm {mm} (p<0.001) with motion correction.DiscussionThis paper presents a novel technique for respiratory motion correction of PET data in PET-MR imaging. After an initial 30 second MR scan, the proposed technique does not require use of the MR scanner for motion correction purposes, making it suitable for MR-intensive studies or sequential PET-MR. The accuracy of the proposed technique was similar to both comparative methods, but robustness was improved compared to the PET-PET technique, particularly in regions with higher noise such as the liver.