Pointwise Encoding Time Reduction Research Articles

Quiet T1-Weighted Pointwise Encoding Time Reduction with Radial Acquisition for Assessing Myelination in the Pediatric Brain.

T1-weighted pointwise encoding time reduction with radial acquisition (PETRA) sequences require limited gradient activity and allow quiet scanning. We aimed to assess the usefulness of PETRA in pediatric brain imaging. We included consecutive pediatric patients who underwent both MPRAGE and PETRA. The contrast-to-noise and contrast ratios between WM and GM were compared in the cerebellar WM, internal capsule, and corpus callosum. The degree of myelination was rated by using 4-point scales at each of these locations plus the subcortical WM in the anterior frontal, anterior temporal, and posterior occipital lobes. Two radiologists made all assessments, and the intra- and interrater agreement was calculated by using intraclass correlation coefficients. Acoustic noise on MPRAGE and PETRA was measured. We included 56 patients 5 days to 14 years of age (mean age, 36.6 months) who underwent both MPRAGE and PETRA. The contrast-to-noise and contrast ratios for PETRA were significantly higher than those for MPRAGE (P < .05), excluding the signal ratio for cerebellar WM. Excellent intra- and interrater agreement were obtained for myelination at all locations except the cerebellar WM. The acoustic noise on PETRA (58.2 dB[A]) was much lower than that on MPRAGE (87.4 dB[A]). PETRA generally showed better objective imaging quality without a difference in subjective image-quality evaluation and produced much less acoustic noise compared with MPRAGE. We conclude that PETRA can substitute for MPRAGE in pediatric brain imaging.

Visualization of Middle Ear Ossicles in Elder Subjects with Ultra-short Echo Time MR Imaging.

Purpose:To evaluate the visualization of middle ear ossicles by ultra-short echo time magnetic resonance (MR) imaging at 3T in subjects over 50 years old.Materials and Methods:Sixty ears from 30 elder patients that underwent surgical or interventional treatment for neurovascular diseases were included (ages: 50–82, median age: 65; 10 men, 20 women). Patients received follow-up MR imaging including routine T1- and T2-weighted images, time-of-flight MR angiography, and ultra-short echo time imaging (PETRA, pointwise encoding time reduction with radial acquisition). All patients underwent computed tomography (CT) angiography before treatment. Thin-section source CT images were correlated with PETRA images. Scan parameters for PETRA were: TR 3.13, TE 0.07, flip angle 6 degrees, 0.83 × 0.83 × 0.83 mm resolution, 3 min 43 s scan time. Two radiologists retrospectively evaluated the visibility of each ossicular structure as positive or negative using PETRA images. The structures evaluated included the head of the malleus, manubrium of the malleus, body of the incus, long process of the incus, and the stapes. Signal intensity of the ossicles was classified as: between labyrinthine fluid and air, similar to labyrinthine fluid, between labyrinthine fluid and cerebellar parenchyma, or higher than cerebellar parenchyma.Results:In all ears, the body of the incus was visible. The head of the malleus was visualized in 36/60 ears. The manubrium of the malleus and long process of the incus was visualized in 1/60 and 4/60 ears, respectively. The stapes were not visualized in any ear. Signal intensity of the visible structures was between labyrinthine fluid and air in all ears.Conclusion:The body of the incus was consistently visualized with intensity between air and labyrinthine fluid on PETRA images in aged subjects. Poor visualization of the manubrium of the malleus, long process of the incus, and the stapes limits clinical significance of middle ear imaging with current PETRA methods.

Lung morphology assessment of cystic fibrosis using MRI with ultra-short echo time at submillimeter spatial resolution.

We hypothesized that non-contrast-enhanced PETRA (pointwise encoding time reduction with radial acquisition) MR (magnetic resonance) sequencing could be an alternative to unenhanced computed tomography (CT) in assessing cystic fibrosis (CF) lung structural alterations, as well as compared agreements and concordances with those of conventional T1-weighted and T2-weighted sequences. Thirty consecutive CF patients completed both CT and MRI the same day. No contrast injection was used. Agreement in identifying structural alterations was evaluated at the segmental level using a kappa test. Intraclass correlation coefficients (ICC) and Bland-Altman analysis were used to assess concordances and reproducibility in Helbich-Bhalla disease severity scoring. Agreement between PETRA and CT was higher than that of T1- or T2-weighted sequences, notably in assessing the segmental presence of bronchiectasis (Kappa = 0.83; 0.51; 0.49, respectively). The concordance in Helbich-Bhalla scores was very good using PETRA (ICC = 0.97), independently from its magnitude (mean difference (MD) = -0.3 [-2.8; 2.2]), whereas scoring was underestimated using both conventional T1 and T2 sequences (MD = -3.6 [-7.4; 0.1]) and MD = -4.6 [-8.2; -1.0], respectively). Intra- and interobserver reproducibility were very good for all imaging modalities (ICC = 0.86-0.98). PETRA showed higher agreement in describing CF lung morphological changes than that of conventional sequences, whereas the Helbich-Bhalla scoring matched closely with that of CT. • Spatial resolution of lung MRI is limited using non-ultra-short TE MRI technique • Ultra-short echo time (UTE) technique enables submillimeter 3D-MRI of airways • 3D-UTE MRI shows very good concordance with CT in assessing cystic fibrosis • Radiation-free 3D-UTE MRI enables the Helbich-Bhalla scoring without a need for contrast injection.

Comprehensive non-contrast MR assessment of the iliofemoral arteries with visualization of vascular calcifications in TAVR candidates - initial experience

Background Transcatheter aortic valve replacement (TAVR) has emerged as an accepted therapeutic option for moderate and high-risk patients with severe aortic stenosis. In addition to vessel caliber, the presence and configuration of vascular calcifications by the iliofemoral access site is part of routine pre-interventional assessment with CTA and catheter angiography. Non-contrast MR angiography has been proven useful for evaluation of lower extremity peripheral vascular disease, however it cannot identify vascular calcifications using currently available sequences. Neutral contrast 3D and 3D point-wise encoding time reduction (PETRA) are novel MR techniques that can detect vascular calcifications and enable projection-like displays for review, similar to CT. We hypothesized that these techniques, associated with noncontrast Quiescent Interval Slice-Selective (QISS) MRA may be useful alternatives for pre-surgical planning in patients considered for TAVR.

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.

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.

Quiet Submillimeter MR Imaging of the Lung Is Feasible with a PETRA Sequence at 1.5 T.

To assess lung magnetic resonance (MR) imaging with a respiratory-gated pointwise encoding time reduction with radial acquisition (PETRA) sequence at 1.5 T and compare it with imaging with a standard volumetric interpolated breath-hold examination (VIBE) sequence, with extra focus on the visibility of bronchi and the signal intensity of lung parenchyma. The study was approved by the local ethics committee, and all subjects gave written informed consent. Twelve healthy volunteers were imaged with PETRA and VIBE sequences. Image quality was evaluated by using visual scoring, numbering of visible bronchi, and quantitative measurement of the apparent contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR). For preliminary clinical assessment, three young patients with cystic fibrosis underwent both MR imaging and computed tomography (CT). Comparisons were made by using the Wilcoxon signed-rank test for means and the McNemar test for ratios. Agreement between CT and MR imaging disease scores was assessed by using the κ test. PETRA imaging was performed with a voxel size of 0.86 mm(3). Overall image quality was good, with little motion artifact. Bronchi were visible consistently up to the fourth generation and in some cases up to the sixth generation. Mean CNR and SNR with PETRA were 32.4% ± 7.6 (standard deviation) and 322.2% ± 37.9, respectively, higher than those with VIBE (P < .001). Good agreement was found between CT and PETRA cystic fibrosis scores (κ = 1.0). PETRA enables silent, free-breathing, isotropic, and submillimeter imaging of the bronchi and lung parenchyma with high CNR and SNR and may be an alternative to CT for patients with cystic fibrosis.

Quantitative characterizations of ultrashort echo (UTE) images for supporting air–bone separation in the head

Accurate separation of air and bone is critical for creating synthetic CT from MRI to support Radiation Oncology workflow. This study compares two different ultrashort echo-time sequences in the separation of air from bone, and evaluates post-processing methods that correct intensity nonuniformity of images and account for intensity gradients at tissue boundaries to improve this discriminatory power. CT and MRI scans were acquired on 12 patients under an institution review board-approved prospective protocol. The two MRI sequences tested were ultra-short TE imaging using 3D radial acquisition (UTE), and using pointwise encoding time reduction with radial acquisition (PETRA). Gradient nonlinearity correction was applied to both MR image volumes after acquisition. MRI intensity nonuniformity was corrected by vendor-provided normalization methods, and then further corrected using the N4itk algorithm. To overcome the intensity-gradient at air–tissue boundaries, spatial dilations, from 0 to 4 mm, were applied to threshold-defined air regions from MR images. Receiver operating characteristic (ROC) analyses, by comparing predicted (defined by MR images) versus ‘true’ regions of air and bone (defined by CT images), were performed with and without residual bias field correction and local spatial expansion. The post-processing corrections increased the areas under the ROC curves (AUC) from 0.944 ± 0.012 to 0.976 ± 0.003 for UTE images, and from 0.850 ± 0.022 to 0.887 ± 0.012 for PETRA images, compared to without corrections. When expanding the threshold-defined air volumes, as expected, sensitivity of air identification decreased with an increase in specificity of bone discrimination, but in a non-linear fashion. A 1 mm air mask expansion yielded AUC increases of 1 and 4% for UTE and PETRA images, respectively. UTE images had significantly greater discriminatory power in separating air from bone than PETRA images. Post-processing strategies improved the discriminatory power of air from bone for both UTE and PETRA images, and reduced the difference between the two imaging sequences. Both post-processed UTE and PETRA images demonstrated sufficient power to discriminate air from bone to support synthetic CT generation from MRI data.

Quiet Submillimeter MR Imaging of the Lung Is Feasible with a PETRA Sequence at 1.5 T

HomeRadiologyVol. 279, No. 1 PreviousNext CommunicationsFree AccessErrataQuiet Submillimeter MR Imaging of the Lung Is Feasible with a PETRA Sequence at 1.5 TGaël Dournes, David Grodzki, Julie Macey, Pierre-Olivier Girodet, Michaël Fayon, Jean-François Chateil, Michel Montaudon, Patrick Berger, François LaurentGaël Dournes, David Grodzki, Julie Macey, Pierre-Olivier Girodet, Michaël Fayon, Jean-François Chateil, Michel Montaudon, Patrick Berger, François LaurentGaël DournesDavid GrodzkiJulie MaceyPierre-Olivier GirodetMichaël FayonJean-François ChateilMichel MontaudonPatrick BergerFrançois LaurentPublished Online:Mar 18 2016https://doi.org/10.1148/radiol.2016164006MoreSectionsPDF ToolsImage ViewerAdd to favoritesCiteTrack CitationsPermissionsReprints ShareShare onFacebookTwitterLinked In Originally published in:Radiology 2015;276(1):258–265 DOI: 10.1148/radiol.15141655Erratum in:Radiology 2016;279(1):328 DOI:10.1148/radiol.2016164006The following Acknowledgment should be listed: This study was achieved within the context of Laboratory of Excellence TRAIL, ANR-10-LABX-57. This has been updated online.Article HistoryPublished online: Mar 18 2016Published in print: Apr 2016 FiguresReferencesRelatedDetailsCited ByAssessment of lung density in pediatric patients using three-dimensional ultrashort echo-time and four-dimensional zero echo-time sequencesKonstantinos G.Zeimpekis, Christian J.Kellenberger, JuliaGeiger2022 | Japanese Journal of Radiology, Vol. 40, No. 7Encyclopedia of Respiratory MedicineBlandinePrevost, ChiaraSileo, NadiaNathan2022Three-dimensional magnetic resonance imaging ultrashort echo-time cones for assessing lung density in pediatric patientsKonstantinos G.Zeimpekis, JuliaGeiger, FlorianWiesinger, GasparDelso, Christian J.Kellenberger2021 | Pediatric Radiology, Vol. 51, No. 1Detecting small pulmonary nodules with spiral ultrashort echo time sequences in 1.5 T MRIYu-SenHuang, EmiNiisato, Mao-Yuan MarineSu, ThomasBenkert, Hsao-HsunHsu, Jin-YuanShih, Jin-ShingChen, Yeun-ChungChang2021 | Magnetic Resonance Materials in Physics, Biology and Medicine, Vol. 34, No. 3A multimodal approach to detect and monitor early lung disease in cystic fibrosisPedroMondéjar-López, AlexanderHorsley, FelixRatjen, SilviaBertolo, Helenede Vicente, ÒscarAsensio de la Cruz2021 | Expert Review of Respiratory Medicine, Vol. 15, No. 6The lower respiratory airway wall in children in health and diseaseMichaelFayon, FabienBeaufils2021 | ERJ Open Research, Vol. 7, No. 3Respiratory Diseases of the Newborn InfantYohanSoreze, ChiaraSileo, AuroreCoulomb l'Hermine, MarieLegendre, NadiaNathan2021The Value of PETRA in Pulmonary Nodules of <3 cm Among Patients With Lung CancerHuiFeng, GaofengShi, HuiLiu, YuDu, NingZhang, YaningWang2021 | Frontiers in Oncology, Vol. 11Pediatric Body MRIPierluigiCiet, Mark C.Liszewski, Edward Y.Lee2020Congenital Cystic Lung DiseaseKumikoNozawa2020The radiological diagnosis of bronchiectasis: what's in a name?Harm A.W.M.Tiddens, Jennifer J.Meerburg, Menno M.van der Eerden, PierluigiCiet2020 | European Respiratory Review, Vol. 29, No. 1563D printed phantoms mimicking cortical bone for the assessment of ultrashort echo time magnetic resonance imagingRobbaRai, DavidManton, Michael G.Jameson, SonalJosan, Michael B.Barton, Lois C.Holloway, Gary P.Liney2018 | Medical Physics, Vol. 45, No. 2Allergic Bronchopulmonary Aspergillosis in Cystic Fibrosis: MR Imaging of Airway Mucus Contrasts as a Tool for DiagnosisGaël Dournes, Patrick Berger, John RefaitJulie Macey, Stephanie Bui, Laurence Delhaes, Michel Montaudon, Olivier Corneloup, Jean-François Chateil, Roger Marthan, Michaël Fayon, François Laurent, 22 May 2017 | Radiology, Vol. 285, No. 1IRM du parenchyme pulmonaire : vers une application clinique ?G.Dournes, J.Macey, E.Blanchard, P.Berger, F.Laurent2017 | Revue de Pneumologie Clinique, Vol. 73, No. 1Accompanying This ArticleQuiet Submillimeter MR Imaging of the Lung Is Feasible with a PETRA Sequence at 1.5 TMar 13 2015RadiologyRecommended Articles 129Xe Pulmonary MRI for Individuals with Post-acute COVID-19 SyndromeRadiology2022Volume: 305Issue: 2pp. 477-478Using Lung MRI and Elastic Registration to Assess Pulmonary FibrosisRadiology2019Volume: 291Issue: 2pp. 493-494Imaging Dynamic Expiration: Feasibility of MRI Spirometry Using Hyperpolarized Xenon GasRadiology: Cardiothoracic Imaging2021Volume: 3Issue: 4MRI Shows Lung Perfusion Changes after Vaping and SmokingRadiology2022Volume: 304Issue: 1pp. 195-204Hyperpolarized 129Xe MR Spectroscopy in the Lung Shows 1-year Reduced Function in Idiopathic Pulmonary FibrosisRadiology2022Volume: 305Issue: 3pp. 688-696See More RSNA Education Exhibits MR Imaging of The Lung Pathology: Why and How?Digital Posters2018Deep Learning Techniques for Automated Segmentation of Diffuse Lung Disease Opacities on CT ImagesDigital Posters2018Black-Blood Lung MRI - Correlations of High-resolution MRI and Ultra-high-resolution CT Images Digital Posters2020 RSNA Case Collection Congenital Lobar OverinflationRSNA Case Collection2020Congenital lobar overinflationRSNA Case Collection2021Postoperative lung herniaRSNA Case Collection2020 Vol. 279, No. 1 Metrics Altmetric Score PDF download

Arterial spin labeled carotid MR angiography: A phantom study examining the impact of technical and hemodynamic factors.

To quantify the accuracy of three-dimensional (3D) radial arterial spin labeled (ASL) magnetic resonance angiography (MRA) using vascular models of carotid stenosis. Eight vascular models were imaged at 1.5 Tesla using pulsatile flow waveforms at rates found in the internal carotid arteries (100-400 mL/min). The impacts of the 3D ASL imaging readout (fast low angle shot (FLASH) versus balanced steady-state free precession (bSSFP)), ultrashort echo time imaging using a pointwise encoding time reduction with radial acquisition (PETRA), and model stenosis severity on the accuracy of vascular model display at the location of stenosis were quantified. Accuracy was computed vis-à-vis a reference bSSFP volume acquired under no flow. Comparisons were made with standard-of-care contrast-enhanced MRA (CEMRA) and Cartesian time-of-flight (TOF) MRA protocols. For 50% and 70% stenoses, CEMRA was most accurate (respective accuracies of 81.7% and 78.6%), followed by ASL FLASH (75.7% and 71.8%), ASL PETRA (69.6% and 70.6%), 3D TOF (66.6% and 57.1%), ASL bSSFP (68.7% and 51.2%), and 2D TOF (65.1% and 50.6%). Flow phantom imaging studies show that ASL MRA can improve the display of hemodynamically significant carotid arterial stenosis compared with TOF MRA, with FLASH and ultrashort echo time readouts being most accurate.

Short T2 tissue imaging with the Pointwise Encoding Time reduction with Radial Acquisition (PETRA) sequence: The additional value of fat saturation and subtraction in the meniscus

PurposeThe purposes of this study were (1) to compare single-echo PETRA with dual-echo PETRA using in vivo MR imaging, (2) to compare non-fat-saturated PETRA with fat-saturated PETRA using a 3-T clinical MR scanner, and (3) to determine the effect of the adequate sequence and post-processing method. Materials and methodsTwenty-two patients underwent dual-echo 3D PETRA sequence knee MR imagining (TE of 70μs and 2.3ms) with and without fat-saturation using a 3T clinical MR scanner (Magnetom Trio, Siemens, Erlangen, Germany). The study population was classified into two groups: (1) normal meniscus on conventional MR images with no related physical examination on medical records and (2) meniscal degeneration or tear. We reformatted four image sets: (1) ultrashort TE signal without fat-saturation, (2) ultrashort TE signal with fat-saturation, (3) weighted-subtraction image of dual-echo PETRA images without fat-saturation, and (4) weighted-subtraction image with fat-saturation. For the weighted-subtraction images, the ultrashort TE image was rescaled relative to the second echo image with weighting factors from 0.6 based on SNR and CNR analyses. For quantitative assessment, the mean signal intensities inside user-drawn regions of interest (ROIs) were drawn and recorded. For statistical analyses, the t-test was used to compare the two groups and effect size was used for comparison of the discrimination power. ResultsIn all image sets, the mean signal intensity values were lower in patients with meniscal degeneration/tear compared to controls on both fat-saturated and non-fat-saturated MR images. The single-echo ultrashort TE images showed higher effect sizes than the weighted-subtraction image of dual-echo images. ConclusionWe demonstrated that there was significantly lower signal intensity on ultrashort TE PETRA images in the patients with meniscal pathologies. In addition, the single-echo of the ultrashort TE PETRA images echo time could be a more sensitive indicator between normal and pathologic meniscus conditions in patients.

Projection MR imaging of peripheral arterial calcifications.

Both CT and MR angiography are accurate for the evaluation of luminal abnormalities in peripheral arterial disease (PAD). However, only CT (requiring exposure to potentially hazardous ionizing radiation) provides a reliable means to detect vascular calcifications. In this study, we demonstrate the feasibility of detecting peripheral arterial calcifications with MRI. The institutional investigational review board approved the study. Seven patients with PAD and iliofemoral arterial calcifications shown by prior CT angiography (CTA) were studied. The imaging techniques included: 1) dual-echo three-dimensional (3D) gradient recalled echo (GRE) pulse sequence using flow compensation and in-phase echo times (TE); and 2) prototype version of 3D point-wise encoding time reduction with radial acquisition (PETRA), which enables imaging with an ultra-short TE. With both techniques after grayscale inversion, vascular calcifications appeared bright, as did cortical bone, and were readily distinguished from the vessel lumen and surrounding soft tissues. The location and conformation of the calcifications corresponded with CT. The second echo GRE provided the highest contrast-to-noise ratios; whereas PETRA was best suited for the creation of thick projection images. In this pilot study, MRI was able to detect vascular calcifications. Projection imaging using PETRA provided a similar appearance to, and allowed direct comparison with, CT.

Quiet T1‐weighted imaging using PETRA: Initial clinical evaluation in intracranial tumor patients

To compare the lesion contrast and signal to noise ratio (SNR) obtained with T1-weighted pointwise encoding time reduction with radial acquisition (PETRA) to those of Magnetization-Prepared RApid Gradient-Echo (MPRAGE) for contrast-enhanced imaging of primary and metastatic intracranial tumors, and to investigate whether PETRA is able to reduce acoustic noise for improved patient comfort. Fifteen patients with intracranial tumors underwent 3 Tesla MRI including inversion-prepared PETRA and MPRAGE. The two sequences had comparable scan times, spatial resolution and spatial coverage. "Tumor conspicuity" was rated qualitatively by two radiologists, while enhancing lesion-to-white matter contrast to noise ratio (CNR) and white-matter SNR were analyzed quantitatively using paired t-tests. The acoustic noise generated by each sequence was measured. Qualitative rating of "tumor conspicuity" by two radiologists resulted in nearly identical average scores for the two sequences. Quantitative analyses revealed that (i) there was no significant difference between the mean CNR values of the two sequences (P = 0.57), (ii) the mean SNR of PETRA was significantly higher than that of MPRAGE (P < 0.01), and (iii) the mean sound level of PETRA was significantly lower than that of MPRAGE (P < 0.01). Inversion-prepared PETRA was found to be viable as a quiet alternative to MPRAGE for contrast-enhanced T1-weighted studies of intracranial tumors.

Ultrashort echo time imaging using pointwise encoding time reduction with radial acquisition (PETRA)

Sequences with ultrashort echo times enable new applications of MRI, including bone, tendon, ligament, and dental imaging. In this article, a sequence is presented that achieves the shortest possible encoding time for each k-space point, limited by pulse length, hardware switching times, and gradient performance of the scanner. In pointwise encoding time reduction with radial acquisition (PETRA), outer k-space is filled with radial half-projections, whereas the centre is measured single pointwise on a Cartesian trajectory. This hybrid sequence combines the features of single point imaging with radial projection imaging. No hardware changes are required. Using this method, 3D images with an isotropic resolution of 1 mm can be obtained in less than 3 minutes. The differences between PETRA and the ultrashort echo time (UTE) sequence are evaluated by simulation and phantom measurements. Advantages of pointwise encoding time reduction with radial acquisition are shown for tissue with a T(2) below 1 ms. The signal to noise ratio and Contrast-to-noise ratio (CNR) performance, as well as possible limitations of the approach, are investigated. In-vivo head, knee, ankle, and wrist examples are presented to prove the feasibility of the sequence. In summary, fast imaging with ultrashort echo time is enabled by PETRA and may help to establish new routine clinical applications of ultrashort echo time sequences.