Artifact reduction from metallic dental materials in T1‐weighted spin‐echo imaging at 3.0 tesla

Purpose:

To investigate and propose a method of artifact reduction arising from metallic dental materials by applying a slice‐encoding for metal artifact correction (SEMAC) technique on T1‐weighted spin‐echo (SE) imaging at 3 Tesla.

Materials and Methods:

The view angle tilting (VAT) technique was adapted to conventional T1‐weighted spin‐echo (SE) sequence to correct the in‐plane distortion, and the SEMAC technique was used for correcting the remaining through‐plane distortions. Fourier transform based B0 field simulations were performed to estimate the amount of field perturbation and a scout imaging method was developed which guide in selecting the number of slice‐encodings needed in SEMAC sequences. Phantoms of six different dental materials with various shapes and sizes that are used in practice (amalgam; titanium implant; gold and Ni‐Cr crowns; Ni‐Ti and stainless steel orthodontic wires) were imaged. In vivo images of two subjects were also acquired. The amounts of artifact reduction were quantified in phantom studies.

Results:

Compared with conventional SE imaging in phantom studies, in‐plane artifacts were reduced by up to 43% in the VAT SE images and 80% in the SEMAC images. Through‐plane artifacts were reduced by up to 65% in SEMAC images. In vivo SEMAC images also showed reduced artifacts.

Conclusion:

The SEMAC technique can mitigate artifact caused by metallic dental materials for T1w‐SE imaging. J. Magn. Reson. Imaging 2013;37:471–478. © 2012 Wiley Periodicals, Inc.     

Imaging near metal with a MAVRIC‐SEMAC hybrid

The recently developed multi‐acquisition with variable resonance image combination (MAVRIC) and slice‐encoding metal artifact correction (SEMAC) techniques can significantly reduce image artifacts commonly encountered near embedded metal hardware. These artifact reductions are enabled by applying alternative spectral and spatial‐encoding schemes to conventional spin‐echo imaging techniques. Here, the MAVRIC and SEMAC concepts are connected and discussed. The development of a hybrid technique that utilizes strengths of both methods is then introduced. The presented technique is shown capable of producing minimal artifact, high‐resolution images near total joint replacements in a clinical setting. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Imaging of the wrist at 1.5 Tesla using isotropic three-dimensional fast spin echo cube

Purpose:

To compare three‐dimensional fast spin echo Cube (3D‐FSE‐Cube) with conventional 2D‐FSE in MR imaging of the wrist.

Materials and Methods:

The wrists of 10 volunteers were imaged in a 1.5 Tesla MRI scanner using an eight‐channel wrist coil. The 3D‐FSE‐Cube images were acquired in the coronal plane with 0.5‐mm isotropic resolution. The 2D‐FSE images were acquired in both coronal and axial planes for comparison. An ROI was placed in fluid, cartilage, and muscle for SNR analysis. Comparable coronal and axial images were selected for each sequence, and paired images were randomized and graded for blurring, artifact, anatomic details, and overall image quality by three blinded musculoskeletal radiologists.

Results:

SNR of fluid, cartilage and muscle at prescribed locations were higher using 3D‐FSE‐Cube, without reaching statistical significance. Fluid–cartilage CNR was also higher with 3D‐FSE‐Cube, but not statistically significant. Blurring, artifact, anatomic details, and overall image quality were significantly better on coronal 3D‐FSE‐Cube images (P < 0.001), but significantly better on axial 2D‐FSE images compared with axial 3D‐FSE‐Cube reformats (P < 0.01).

Conclusion:

Isotropic data from 3D‐FSE‐Cube allows reformations in arbitrary scan planes, which may make multiple 2D acquisitions unnecessary, and improve depiction of complex wrist anatomy. However, axial reformations suffer from blurring, likely due to T2 decay during the long echo train, limiting overall image quality in this plane. J. Magn. Reson. Imaging 2011;33:908–915. © 2011 Wiley‐Liss, Inc.     

Accelerated slice encoding for metal artifact correction

Purpose:

To demonstrate accelerated imaging with both artifact reduction and different contrast mechanisms near metallic implants.

Materials and Methods:

Slice‐encoding for metal artifact correction (SEMAC) is a modified spin echo sequence that uses view‐angle tilting and slice‐direction phase encoding to correct both in‐plane and through‐plane artifacts. Standard spin echo trains and short‐TI inversion recovery (STIR) allow efficient PD‐weighted imaging with optional fat suppression. A completely linear reconstruction allows incorporation of parallel imaging and partial Fourier imaging. The signal‐to‐noise ratio (SNR) effects of all reconstructions were quantified in one subject. Ten subjects with different metallic implants were scanned using SEMAC protocols, all with scan times below 11 minutes, as well as with standard spin echo methods.

Results:

The SNR using standard acceleration techniques is unaffected by the linear SEMAC reconstruction. In all cases with implants, accelerated SEMAC significantly reduced artifacts compared with standard imaging techniques, with no additional artifacts from acceleration techniques. The use of different contrast mechanisms allowed differentiation of fluid from other structures in several subjects.

Conclusion:

SEMAC imaging can be combined with standard echo‐train imaging, parallel imaging, partial‐Fourier imaging, and inversion recovery techniques to offer flexible image contrast with a dramatic reduction of metal‐induced artifacts in scan times under 11 minutes. J. Magn. Reson. Imaging 2010;31:987–996. ©2010 Wiley‐Liss, Inc.     

Reducing metallic artefacts in post-operative spinal imaging: slice encoding for metal artefact correction with dual-source parallel radiofrequency excitation MRI at 3.0?T

Objective:

To compare the effects of metal artefacts and acquisition time among slice encoding for metal artefact correction (SEMAC), SEMAC with dual-source parallel radiofrequency (SEMAC-DSPRF) transmission and fast spin echo (FSE) images using 3.0-T MRI.

Methods:

The signal-to-noise ratio (SNR) was calculated in a phantom study using a pedicle screw. A total of 16 patients who underwent spinal surgery using pedicle screws were included in the clinical study. T₁ weighted FSE, SEMAC and SEMAC-DSPRF images were obtained. Four imaging findings (visibility of the dural sac, neural foramens, bone–implant interface and overall artefacts) were evaluated by using five-point scales independently by two observers. The mean scan time was recorded.

Results:

The mean SNR was 71.2, 25.7 and 28.4 for FSE, SEMAC and SEMAC-DSPRF images, respectively. FSE images were ranked lower than SEMAC and SEMAC-DSPRF images, and ranking of SEMAC and SEMAC-DSPRF images did not differ statistically for all four imaging findings. The mean scan time was 9 min 51 s and 6 min 31 s for SEMAC and SEMAC-DSPRF images, respectively.

Conclusion:

SEMAC can reduce metallic artefacts and improve the visualisation of anatomical structures around metal implants. An additional DSPRF technique can reduce the acquisition time of SEMAC images without the loss of SNR and image quality.

Advances in knowledge:

This study demonstrates that the use of the DSPRF transmission technique can reduce the acquisition time of SEMAC images without loss of image quality in patients with metal implants.Metal implants are commonly used in surgical procedures to fixate fractures, replace damaged joints or immobilise joints. However, many kinds of local complications associated with metal implants can occur. They include mechanical aseptic loosening, prosthetic or periprosthetic fracture, dislocation, superficial and deep infections, heterotopic bone formation and osteolysis. Such complications are a common source of patient morbidity and often necessitate revision surgery.MRI is a commonly used imaging modality for musculoskeletal imaging because of its good, soft-tissue contrast. However, metal artefacts limit the usefulness of MRI for evaluation of disease in the presence of metal. Artefacts on MR images obtained in patients with metallic implants are produced by large differences between the magnetic properties of human tissues and those of the implanted metals. Differences in the magnetic susceptibilities of adjacent tissues and implants create local magnetic field inhomogeneities, altering the phase and frequency of local spins [1,2].Much less severe artefacts are produced by implants made of titanium alloy than ferromagnetic implants made of stainless steel. In addition, metal artefacts may generally be decreased either by choosing a small voxel size, using a short echo time (T_E) or short interecho interval, or by increasing the sampling bandwidth [3]. Recently, the three-dimensional MRI technique, slice encoding for metal artefact correction (SEMAC), was developed [4]. The SEMAC technique corrects metal artefacts via robust encoding of each excited slice against metal-induced field inhomogeneities. The robust encoding is achieved by extending a view angle tilting spin echo sequence with additional z-phase encoding [5]. Several studies have demonstrated that SEMAC effectively eliminated metal artefacts [4–7]. However, a higher z-phase encoding resolution requires more z-phase encoding steps to maintain the same z-phase encoding the field of view (FOV), which leads to a longer scan time [5].Recently, parallel radiofrequency (RF) transmission technology was commercialised. Parallel transmission describes the use of multiple RF transmit coils that project multiple, independent RF transmit signals. It can reduce dielectric resonance effects at high field strengths and enable control of the RF distribution to optimise RF deposition [8]. As a result, parallel transmission can reduce the required RF power, i.e. the specific energy absorption rate (SAR). In addition, parallel transmission can be used to shorten the duration of spatially selective RF pulses, i.e. image acquisition time, or to increase their spatial resolution definition while maintaining the pulse duration [8].Although SEMAC can reduce metal artefacts, long acquisition times may be a problem in its clinical application. If parallel RF transmission is combined with SEMAC to reduce acquisition time, clinical application of SEMAC could become more practicable.The purpose of this study is to compare the metal artefacts and acquisition time among SEMAC, SEMAC with dual-source parallel RF (DSPRF) transmission and fast spin echo (FSE) images with 3.0-T MRI.     

High-resolution proton density weighted three-dimensional fast spin echo (3D-FSE) of the knee with IDEAL at 1.5 Tesla: comparison with 3D-FSE and 2D-FSE--initial experience

Purpose:

To assess the feasibility of combining three‐dimensional fast spin echo (3D‐FSE) and Iterative‐decomposition‐of water‐and‐fat‐with‐echo asymmetry‐and‐least‐squares‐estimation (IDEAL) at 1.5 Tesla (T), generating a high‐resolution 3D isotropic proton density‐weighted image set with and without “fat‐suppression” (FS) in a single acquisition, and to compare with 2D‐FSE and 3D‐FSE (without IDEAL).

Materials and Methods:

Ten asymptomatic volunteers prospectively underwent sagittal 3D‐FSE‐IDEAL, 3D‐FSE, and 2D‐FSE sequences at 1.5T (slice thickness [ST]: 0.8 mm, 0.8 mm, and 3.5 mm, respectively). 3D‐FSE and 2D‐FSE were repeated with frequency‐selective FS. Fluid, cartilage, and muscle signal‐to‐noise ratio (SNR) and fluid‐cartilage contrast‐to‐noise ratio (CNR) were compared among sequences. Three blinded reviewers independently scored quality of menisci/cartilage depiction for all sequences. “Fat‐suppression” was qualitatively scored and compared among sequences.

Results:

3D‐FSE‐IDEAL fluid‐cartilage CNR was higher than in 2D‐FSE (P < 0.05), not different from 3D‐FSE (P = 0.31). There was no significant difference in fluid SNR among sequences. 2D‐FSE cartilage SNR was higher than in 3D FSE‐IDEAL (P < 0.05), not different to 3D‐FSE (P = 0.059). 2D‐FSE muscle SNR was higher than in 3D‐FSE‐IDEAL (P < 0.05) and 3D‐FSE (P < 0.05). Good or excellent depiction of menisci/cartilage was achieved using 3D‐FSE‐IDEAL in the acquired sagittal and reformatted planes. Excellent, homogeneous “fat‐suppression” was achieved using 3D‐FSE‐IDEAL, superior to FS‐3D‐FSE and FS‐2D‐FSE (P < 0.05).

Conclusion:

3D FSE‐IDEAL is a feasible approach to acquire multiplanar images of diagnostic quality, both with and without homogeneous “fat‐suppression” from a single acquisition. J. Magn. Reson. Imaging 2012;361‐369. © 2011 Wiley Periodicals, Inc.     

Fast spin-echo triple echo dixon: Initial clinical experience with a novel pulse sequence for simultaneous fat-suppressed and nonfat-suppressed T2-weighted spine magnetic resonance imaging

Purpose

To evaluate a prototype fast spin‐echo (FSE) triple‐echo Dixon (FTED) technique for T2‐weighted spine imaging with and without fat suppression compared to conventional T2‐weighted fast recovery (FR) FSE and short‐tau inversion recovery (STIR) imaging.

Materials and Methods

Sixty‐one patients were referred for spine magnetic resonance imaging (MRI) including sagittal FTED (time 2:26), STIR (time 2:42), and T2 FRFSE (time 2:55). Two observers compared STIR and FTED water images and T2 FRFSE and FTED T2 images for overall image quality, fat suppression, anatomic sharpness, motion, cerebrospinal fluid (CSF) flow artifact, susceptibility, and disease depiction.

Results

On FTED images water and fat separation was perfect in 58 (.95) patients. Compared to STIR, the FTED water images demonstrated less motion in 57 (.93) of 61 patients (P < 0.05), better anatomic sharpness in 51 (.84) and patients (P < 0.05), and less CSF flow artifact in 7 (.11) P < 0.05) patients. There was no difference in fat suppression or chemical shift artifact. T2 FRFSE and FTED T2 images showed equivalent motion, CSF flow, and chemical shift artifact. Lesion depiction was equivalent on FTED water and STIR images and FTED T2 and T2 FRFSE images.

Conclusion

FTED efficiently provides both fat‐suppressed and nonfat‐suppressed T2‐weighted spine images with excellent image quality, equal disease depiction, and 56% reduction in scan time compared to conventional STIR and T2 FRFSE. J. Magn. Reson. Imaging 2011;33:390–400. © 2011 Wiley‐Liss, Inc.     

Feasibility of in ovo diffusion tractography in the chick embryo using a dual-cooling technique

Purpose:

To evaluate the capability of a dual‐cooling technique in suppressing motion artifact and to evaluate the feasibility of the noninvasive muscle fibers tracking using DTI during chick embryonic development.

Materials and Methods:

Fifteen eggs were divided into three groups of 5 eggs each (one group for each imaging sequence), and eggs were imaged every 48 h from incubation day 4; embryos were imaged in ovo using three sequences of varying duration (T1, T2, and DTI). For each sequence, three preprocessing methods were used: no‐cooling (NC), single‐cooling (SC), and dual‐cooling (DC). Two independent observers assessed images for motion artifact. The results of different preprocessing methods used for each sequence were compared by the χ² test. The Cohen kappa test was used to assess the interobserver variability.

Results:

For T1 imaging, motion artifact was adequately suppressed by both SC and DC methods (χ² test; P > 0.05). For T2 imaging, motion artifact was also sufficiently suppressed by both SC and DC methods (χ² test; P > 0.05) except incubation day 19 (χ² test; P < 0.001). For DTI, motion artifact was less with DC than SC after 8 days (χ² test; P < 0.05). Hindlimb muscle fibers of chick embryo could be serially evaluated with DTI from 8 days using dual‐cooling technique.

Conclusion:

The dual‐cooling technique enables DTI of chick embryo in ovo with minimal motion artifact, which permits muscle fiber tracking by DTI during chick embryonic development possible, and can improve the imaging quality of conventional MRI with long duration and those sensitive to motion. J. Magn. Reson. Imaging 2012;36:993–1001. © 2012 Wiley Periodicals, Inc.     

In vivo tibiofemoral cartilage-to-cartilage contact area of females with medial osteoarthritis under acute loading using MRI

Purpose:

To investigate the effect of acute loading on in vivo tibiofemoral contact area changes in both compartments, and to determine whether in vivo tibiofemoral contact area differs between subjects with medial knee osteoarthritis (OA) and healthy controls.

Materials and Methods:

Ten subjects with medial knee OA (KL3) and 11 control subjects (KL0) were tested. Coronal three‐dimensional spoiled gradient‐recalled (3D‐SPGR) and T₂‐weighted fast spin‐echo FSE magnetic resonance imaging (MRI) of the knee were acquired under both unloaded and loaded conditions. Tibiofemoral cartilage contact areas were measured using image‐based 3D models.

Results:

Tibiofemoral contact areas in both compartments significantly increased under loading (P < 0.001) and the increased contact area in the medial compartment was significantly larger than in the lateral compartment (P < 0.05). Medial compartment contact area was significantly larger in KL3 subjects than KL0 subjects, both at unloaded and loaded conditions (P < 0.05). Contact areas measured from 3D‐SPGR and T₂‐weighted FSE images were strongly correlated (r = 0.904).

Conclusion:

Females with medial OA increased tibiofemoral contact area in the medial compartment compared to healthy subjects under both unloaded and loaded conditions. The contact area data presented in this study may provide a quantitative reference for further cartilage contact biomechanics such as contact stress analysis and cartilage biomechanical function difference between osteoarthritic and healthy knees. J. Magn. Reson. Imaging 2011;. © 2011 Wiley Periodicals, Inc.     

CT und MRT der Hüftprothese

Metal-induced artifacts impair image quality of computed tomography (CT) and magnetic resonance imaging (MRI) in patients with hip prostheses. Due to new developments in metal artifact reduction both methods can now be used for evaluation of a painful hip prosthesis. Iterative reconstruction algorithms and dual-energy scans are among the newer CT techniques for artifact reduction, while slice-encoding for metal artifact correction (SEMAC) and multi-acquisition variable-resonance image combination (MAVRIC) have introduced substantial improvements for MRI. Loosening of the hip prosthesis, osteolysis from small wear particles and pseudotumors in metal-on-metal prostheses are specific pathologies in patients with total hip arthroplasty. Other causes of painful hip prostheses are infections, fractures, tendinopathies, tendon ruptures, muscle and nerve alterations and heterotopic ossifications.     

Cartilage morphology at 3.0T: Assessment of three‐dimensional magnetic resonance imaging techniques

Purpose:

To compare six new three‐dimensional (3D) magnetic resonance (MR) methods for evaluating knee cartilage at 3.0T.

Materials and Methods:

We compared: fast‐spin‐echo cube (FSE‐Cube), vastly undersampled isotropic projection reconstruction balanced steady‐state free precession (VIPR‐bSSFP), iterative decomposition of water and fat with echo asymmetry and least‐squares estimation combined with spoiled gradient echo (IDEAL‐SPGR) and gradient echo (IDEAL‐GRASS), multiecho in steady‐state acquisition (MENSA), and coherent oscillatory state acquisition for manipulation of image contrast (COSMIC). Five‐minute sequences were performed twice on 10 healthy volunteers and once on five osteoarthritis (OA) patients. Signal‐to‐noise ratio (SNR) and contrast‐to‐noise ratio (CNR) were measured from the volunteers. Images of the five volunteers and the five OA patients were ranked on tissue contrast, articular surface clarity, reformat quality, and lesion conspicuity. FSE‐Cube and VIPR‐bSSFP were compared to IDEAL‐SPGR for cartilage volume measurements.

Results:

FSE‐Cube had top rankings for lesion conspicuity, overall SNR, and CNR (P < 0.02). VIPR‐bSSFP had top rankings in tissue contrast and articular surface clarity. VIPR and FSE‐Cube tied for best in reformatting ability. FSE‐Cube and VIPR‐bSSFP compared favorably to IDEAL‐SPGR in accuracy and precision of cartilage volume measurements.

Conclusion:

FSE‐Cube and VIPR‐bSSFP produce high image quality with accurate volume measurement of knee cartilage. J. Magn. Reson. Imaging 2010;32:173–183. © 2010 Wiley‐Liss, Inc.     

Non‐contrast‐enhanced MR portography with time‐spatial labeling inversion pulses: Comparison of imaging with three‐dimensional half‐fourier fast spin‐echo and true steady‐state free‐precession sequences

Purpose

To compare and evaluate images acquired with two different MR angiography (MRA) sequences, three‐dimensional (3D) half‐Fourier fast spin‐echo (FSE) and 3D true steady‐state free‐precession (SSFP) combined with two time‐spatial labeling inversion pulses (T‐SLIPs), for selective and non‐contrast‐enhanced (non‐CE) visualization of the portal vein.

Materials and Methods

Twenty healthy volunteers were examined using half‐Fourier FSE and true SSFP sequences on a 1.5T MRI system with two T‐SLIPs, one placed on the liver and thorax, and the other on the lower abdomen. For quantitative analysis, vessel‐to‐liver contrast (Cv‐l) ratios of the main portal vein (MPV), right portal vein (RPV), and left portal vein (LPV) were measured. The quality of visualization was also evaluated.

Results

In both pulse sequences, selective visualization of the portal vein was successfully conducted in all 20 volunteers. Quantitative evaluation showed significantly better Cv‐l at the RPVs and LPVs in half‐Fourier FSE (P < 0.0001). At the MPV, Cv‐l was better in true SSFP, but was not statistically different. Visualization scores were significantly better only at branches of segments four and eight for half‐Fourier FSE (P = 0.001 and 0.03, respectively).

Conclusion

Both 3D half‐Fourier FSE and true SSFP scans with T‐SLIPs enabled selective non‐CE visualization of the portal vein. Half‐Fourier FSE was considered appropriate for intrahepatic portal vein visualization, and true SSFP may be preferable when visualization of the MPV is required. J. Magn. Reson. Imaging 2009;29:1140–1146. © 2009 Wiley‐Liss, Inc.     

In vivo proton electron double resonance imaging of mice with fast spin echo pulse sequence

Purpose:

To develop and evaluate a two‐dimensional (2D) fast spin echo (FSE) pulse sequence for enhancing temporal resolution and reducing tissue heating for in vivo proton electron double resonance imaging (PEDRI) of mice.

Materials and Methods:

A four‐compartment phantom containing 2 mM TEMPONE was imaged at 20.1 mT using 2D FSE‐PEDRI and regular gradient echo (GRE)‐PEDRI pulse sequences. Control mice were infused with TEMPONE over ～1 min followed by time‐course imaging using the 2D FSE‐PEDRI sequence at intervals of 10–30 s between image acquisitions. The average signal intensity from the time‐course images was analyzed using a first‐order kinetics model.

Results:

Phantom experiments demonstrated that EPR power deposition can be greatly reduced using the FSE‐PEDRI pulse sequence compared with the conventional gradient echo pulse sequence. High temporal resolution was achieved at ～4 s per image acquisition using the FSE‐PEDRI sequence with a good image SNR in the range of 233–266 in the phantom study. The TEMPONE half‐life measured in vivo was ～72 s.

Conclusion:

Thus, the FSE‐PEDRI pulse sequence enables fast in vivo functional imaging of free radical probes in small animals greatly reducing EPR irradiation time with decreased power deposition and provides increased temporal resolution. J. Magn. Reson. Imaging 2012;471‐475. © 2011 Wiley Periodicals, Inc.     

Cross-sectional and in-plane coronary vessel wall imaging using a local inversion prepulse and spiral read-out: a comparison between 1.5 and 3 Tesla

Purpose:

To compare cross‐sectional and in‐plane coronary vessel wall imaging using a spiral readout at 1.5 and 3 Tesla (T).

Materials and Methods:

Free‐breathing coronary vessel wall imaging using a local inversion technique and spiral readout was implemented. Images were acquired in ten healthy adult subjects on a 3T clinical scanner using a 32‐element cardiac coil and repeated on a 1.5T clinical scanner using a 5‐element coil.

Results:

Cross‐sectional and in‐plane spiral vessel wall imaging was performed at both 1.5 and 3T. In cross‐sectional images, artifact scores were superior at 1.5T (P < 0.05) but no significant difference was found in image quality scores compared with 3T. Image quality (P < 0.01) and artifact scores (P < 0.01) were found to be superior for in‐plane images at 1.5T. Vessel wall sharpness in the in‐plane orientation was also found to be higher at 1.5T (P < 0.03).

Conclusion:

Although excellent in‐plane coronary vessel wall images can be acquired at 3T, the overall robustness may be affected by off‐resonance blurring due to increased B0 inhomogeneity compared with 1.5T. J. Magn. Reson. Imaging 2012;35:969–975. © 2011 Wiley Periodicals, Inc.     

Comparison of 2D and multislab 3D magnetic resonance techniques for measuring carotid wall volumes

Purpose

To compare a multislab three‐dimensional volume‐selective fast spin‐echo (FSE) magnetic resonance (MR) sequence with a routine two‐dimensional FSE sequence for quantification of carotid wall volume.

Materials and Methods

One hundred normal subjects (50 men, mean age 44.6 years) underwent carotid vessel wall MR using 2D and 3D techniques. Carotid artery total vessel volume, lumen volume, wall volume, and wall/outer wall (W/OW) ratio were measured over 20 contiguous slices. Two‐ (2D) and three‐dimensional (3D) results were compared.

Results

The mean difference between 2D and 3D datasets (as a percentage of the mean absolute value) was 1.7% for vessel volume, 4.9% for lumen volume, 4.7% for wall volume, and 5.8% for W/OW ratio. There was good correlation between 2D and 3D models for total vessel volume (R² = 0.93, P < 0.001), lumen area (R² = 0.92, P < 0.001), and wall volume (R² = 0.77, P < 0.001). The correlation for the W/OW ratio was weaker (R² = 0.30; P < 0.001). The signal‐to‐noise ratio (SNR) for the 3D technique was 2.1‐fold greater than for the 2D technique (P < 0.001). When using the 3D sequence, scan time was reduced by 63%.

Conclusion

Multislab volume selective 3D FSE carotid arterial wall imaging performs similarly to a conventional 2D technique, but with over twice the SNR and substantially reduced scan time. J. Magn. Reson. Imaging 2008;28:1476–1482. © 2008 Wiley‐Liss, Inc.     

Development of a novel optimized breathhold technique for myocardial T2 measurement in thalassemia

Purpose

To develop a reproducible fast spin‐echo (FSE) technique for accurate myocardial T2 measurement with application to iron overload assessment in thalassemia.

Materials and Methods

An FSE sequence was developed to permit acquisition of multiple TE images in one breathhold (BH‐FSE). A dynamic black‐blood scheme was introduced to better cancel blood signal. A nonselective refocusing train was also adopted to suppress stimulated echoes. The optimized technique was tested on phantoms and then applied to 10 normal volunteers and 10 thalassemia patients. Interstudy reproducibility was measured on all the 20 subjects.

Results

The mean difference in T2 values was 1.7% from phantom experiments between BH‐FSE and the conventional spin‐echo (SE) technique. High contrast BH‐FSE images were acquired from human subjects, with minimal stimulated echoes and effective blood suppression (P = 0.0005). The coefficient of variation for interstudy reproducibility was 4.3%. T2 values from thalassemia patients were substantially lower than those from the normal subjects (45.2 ± 26.1 msec vs. 56.9 ± 8.4ms, P = 0.02).

Conclusion

The dynamic black‐blood T2 sequence is a fast reproducible acquisition that compares favorably with conventional techniques, is robust to motion artifacts, and yields high blood‐myocardium contrast. This technique may provide a useful tool in thalassemia and other scenarios requiring myocardial T2 quantification. J. Magn. Reson. Imaging 2006. © 2006 Wiley‐Liss, Inc.

     

Improved blood suppression in three‐dimensional (3D) fast spin‐echo (FSE) vessel wall imaging using a combination of double inversion‐recovery (DIR) and diffusion sensitizing gradient (DSG) preparations

Purpose:

To provide improved blood suppression in three‐dimensional inner‐volume fast spin‐echo (3D IV‐FSE) carotid vessel wall imaging by using a hybrid preparation consisting of double inversion‐recovery (DIR) and diffusion sensitizing gradients (DSG).

Materials and Methods:

Multicontrast black‐blood MRI is widely used for vessel wall imaging and characterization of atherosclerotic plaque composition. Blood suppression is difficult when using 3D volumetric imaging techniques. DIR approaches do not provide robust blood suppression due to incomplete replacement of blood spins, and DSG approaches compromise vessel wall signal, reducing the lumen‐wall contrast‐to‐noise ratio efficiency (CNR_eff). In this work a hybrid DIR+DSG preparation is developed and optimized for blood suppression, vessel wall signal preservation, and vessel‐wall contrast in 3D IV‐FSE imaging. Cardiac gated T₁‐weighted carotid vessel wall images were acquired in five volunteers with 0.5 × 0.5 × 2.5 mm³ spatial resolution in 80 seconds.

Results:

Data from healthy volunteers indicate that the proposed method yields a statistically significant (P < 0.01) improvement in blood suppression and lumen‐wall CNR_eff compared to standard DIR and standard DSG methods alone.

Conclusion:

A combination of DIR and DSG preparations can provide improved blood suppression and lumen‐wall CNR_eff for 3D IV‐FSE vessel wall imaging. J. Magn. Reson. Imaging 2010; 31: 398–405. © 2010 Wiley‐Liss, Inc.     

Compressed‐Sensing multispectral imaging of the postoperative spine

Purpose:

To apply compressed sensing (CS) to in vivo multispectral imaging (MSI), which uses additional encoding to avoid magnetic resonance imaging (MRI) artifacts near metal, and demonstrate the feasibility of CS‐MSI in postoperative spinal imaging.

Materials and Methods:

Thirteen subjects referred for spinal MRI were examined using T2‐weighted MSI. A CS undersampling factor was first determined using a structural similarity index as a metric for image quality. Next, these fully sampled datasets were retrospectively undersampled using a variable‐density random sampling scheme and reconstructed using an iterative soft‐thresholding method. The fully and undersampled images were compared using a 5‐point scale. Prospectively undersampled CS‐MSI data were also acquired from two subjects to ensure that the prospective random sampling did not affect the image quality.

Results:

A two‐fold outer reduction factor was deemed feasible for the spinal datasets. CS‐MSI images were shown to be equivalent or better than the original MSI images in all categories: nerve visualization: P = 0.00018; image artifact: P = 0.00031; image quality: P = 0.0030. No alteration of image quality and T2 contrast was observed from prospectively undersampled CS‐MSI.

Conclusion:

This study shows that the inherently sparse nature of MSI data allows modest undersampling followed by CS reconstruction with no loss of diagnostic quality. J. Magn. Reson. Imaging 2013;37:243–248. © 2012 Wiley Periodicals, Inc.     

Using the dGEMRIC technique to evaluate cartilage health in the presence of surgical hardware at 3T: comparison of inversion recovery and saturation recovery approaches

Objective

To evaluate the effect of metal artifact reduction techniques on dGEMRIC T₁ calculation with surgical hardware present.

Materials and methods

We examined the effect of stainless-steel and titanium hardware on dGEMRIC T₁ maps. We tested two strategies to reduce metal artifact in dGEMRIC: (1) saturation recovery (SR) instead of inversion recovery (IR) and (2) applying the metal artifact reduction sequence (MARS), in a gadolinium-doped agarose gel phantom and in vivo with titanium hardware. T₁ maps were obtained using custom curve-fitting software and phantom ROIs were defined to compare conditions (metal, MARS, IR, SR).

Results

A large area of artifact appeared in phantom IR images with metal when TI?≤?700 ms. IR maps with metal had additional artifact both in vivo and in the phantom (shifted null points, increased mean T₁ (+151 % IR ROI_artifact) and decreased mean inversion efficiency (f; 0.45 ROI_artifact, versus 2 for perfect inversion)) compared to the SR maps (ROI_artifact: +13 % T₁ SR, 0.95 versus 1 for perfect excitation), however, SR produced noisier T₁ maps than IR (phantom SNR: 118 SR, 212 IR). MARS subtly reduced the extent of artifact in the phantom (IR and SR).

Conclusions

dGEMRIC measurement in the presence of surgical hardware at 3T is possible with appropriately applied strategies. Measurements may work best in the presence of titanium and are severely limited with stainless steel. For regions near hardware where IR produces large artifacts making dGEMRIC analysis impossible, SR-MARS may allow dGEMRIC measurements. The position and size of the IR artifact is variable, and must be assessed for each implant/imaging set-up.     

DIfferential Subsampling with Cartesian Ordering (DISCO): a high spatio-temporal resolution Dixon imaging sequence for multiphasic contrast enhanced abdominal imaging

Purpose:

To develop and evaluate a multiphasic contrast‐enhanced MRI method called DIfferential Sub‐sampling with Cartesian Ordering (DISCO) for abdominal imaging.

Materials and Methods:

A three‐dimensional, variable density pseudo‐random k‐space segmentation scheme was developed and combined with a Dixon‐based fat‐water separation algorithm to generate high temporal resolution images with robust fat suppression and without compromise in spatial resolution or coverage. With institutional review board approval and informed consent, 11 consecutive patients referred for abdominal MRI at 3 Tesla (T) were imaged with both DISCO and a routine clinical three‐dimensional SPGR‐Dixon (LAVA FLEX) sequence. All images were graded by two radiologists using quality of fat suppression, severity of artifacts, and overall image quality as scoring criteria. For assessment of arterial phase capture efficiency, the number of temporal phases with angiographic phase and hepatic arterial phase was recorded.

Results:

There were no significant differences in quality of fat suppression, artifact severity or overall image quality between DISCO and LAVA FLEX images (P > 0.05, Wilcoxon signed rank test). The angiographic and arterial phases were captured in all 11 patients scanned using the DISCO acquisition (mean number of phases were two and three, respectively).

Conclusion:

DISCO effectively captures the fast dynamics of abdominal pathology such as hyperenhancing hepatic lesions with a high spatio‐temporal resolution. Typically, 1.1 × 1.5 × 3 mm spatial resolution over 60 slices was achieved with a temporal resolution of 4–5 s. J. Magn. Reson. Imaging 2012;35:1484–1492. © 2012 Wiley Periodicals, Inc.