Fast three-point dixon MR imaging using low-resolution images for phase correction: a comparison with chemical shift selective fat suppression for pediatric musculoskeletal imaging

Physeal Abnormalities in Children With High-risk Neuroblastoma Intensively Treated With/Without 13-Cis-Retinoic Acid

BACKGROUND

This study aimed to investigate the presence of physeal abnormality and its effect on growth in children with high-risk neuroblastoma treated by intensive multimodal treatment with/without 13-cis-retinoic acid (13-CRA).

METHODS

Fifteen patients diagnosed with high-risk neuroblastomas at the age of 1 to 10 years, who received treatment such as high-dose chemotherapy and autologous stem cell transplantation with/without 13-CRA, and with complete data during their >2-year follow-up were retrospectively reviewed. The physeal abnormalities were investigated by whole-body magnetic resonance imaging, serially performed every 3 to 6 months. The patients' height growth was also investigated and compared with that of age-and-sex-matched patients with brain tumors who also underwent high-dose chemotherapy and autologous stem cell transplantation.

RESULTS

Six of 15 patients presented multifocal physeal abnormalities during follow-up, and all lesions occurred in patients with 13-CRA use. The lesions in 3 patients completely resolved spontaneously without any adverse effect on growth, but some lesions in the other 3 patients progressed to disturb the bony growth. Height growth of matched patients with brain tumors were not significantly different, and none of the matched controls showed definite bony deformity during the follow-up.

CONCLUSIONS

Some children who were treated for high-risk neuroblastomas experienced multifocal physeal insults, probably due to the use of 13-CRA. Most lesions resolved spontaneously, but some led to bony deformity. If the lesions are not followed by premature physeal closure, there seems to be no further adverse effect of 13-CRA on leg length growth. Routine periodic screening for physeal status is needed for the patients with high-risk neuroblastomas using 13-CRA.

LEVEL OF EVIDENCE

Level IV-prognostic study.

Efficiency of Fat Suppression in T1-Weighted Inner Ear Magnetic Resonance Imaging: Multipoint Dixon Method Versus Hybrid Techniques

Background

Temporal bone is a region where fat suppression is difficult due to the inhomogeneity of various structures with different molecular properties.

Introduction:

We aimed to determine the most effective fat suppression sequence in order to increase the visibility of the inner ear region.

Materials and Methods

The hybrid techniques and T1-Weighted mDIXON images of 40 patients with Magnetic Resonance (MR) imaging of the inner ear were prospectively compared by two experienced radiologists in terms of fat suppression efficacy. In all fat-suppressed sequences, the Signal to Noise Ratio (SNR), the spinal cord signal intensity / mean fat signal intensity ratio and spinal cord signal to noise ratio were calculated. The suppression efficacy of MR techniques for fat areas in the inner ear was visually graded.

Results

Qualitative assessment of image quality due to fat suppression in the inner ear was made; the Dixon technique performed significantly better than SPAIR and SPIR techniques (p<0.0001). The mean signal intensity of the inner ear fat and SNR for the Dixon technique were significantly lower than that for SPIR and SPAIR techniques (p<0.0001). Inter-observer agreement regarding the assessment of the inner ear fat, mean signal intensity values and mean SNR values for fat suppression techniques was significant.

Conclusion

The Dixon technique exhibited higher image quality and fat suppression efficiency than the hybrid techniques in the MR imaging of the inner ear.

Improved nerve conspicuity with water-weighting and denoising in two-point Dixon magnetic resonance neurography

BACKGROUND

T₂-weighted, two-point Dixon fast-spin-echo (FSE) is an effective technique for magnetic resonance neurography (MRN) that can provide quantitative assessment of muscle denervation. Low signal-to-noise ratio and inadequate fat suppression, however, can impede accurate interpretation.

PURPOSE

To quantify effects of principal component analysis (PCA) denoising on tissue signal intensities and fat fraction (FF) and to determine qualitative image quality improvements from both denoising and water-weighting (WW) algorithms to improve nerve conspicuity and fat suppression.

STUDY TYPE

Prospective.

SUBJECTS

Twenty-one subjects undergoing MR neurography evaluation (11/10 male/female, mean age = 46.3±13.7 years) with 60 image volumes. Twelve subjects (23 image volumes) were determined to have muscle denervation based on diffusely elevated T₂ signal intensity.

FIELD STRENGTH/SEQUENCE

3 T, 2D, two-point Dixon FSE.

ASSESSMENT

Qualitative assessment included overall image quality, nerve conspicuity, fat suppression, pulsation and ringing artifacts by 3 radiologists separately on a three-point scale (1 = poor, 2 = average, 3 = excellent). Quantitative measurements for FF and signal intensity relative to normal muscle were made for nerve, abnormal muscle and subcutaneous fat.

STATISTICAL TESTS

Linear and ordinal regression models were used for quantitative and qualitative comparisons, respectively; 95% confidence intervals (CIs) and p-values for pairwise comparisons were adjusted using the Holm-Bonferroni method. Inter-rater agreement was assessed using Gwet's agreement coefficient (AC₂).

RESULTS

Simulations showed PCA-denoising reduced FF error from 2.0% to 1.0%, and from 7.6% to 3.1% at noise levels of 10% and 30%, respectively. In human subjects, PCA-denoising did not change signal levels and FF quantitatively. WW decreased fat signal significantly (-83.6%, p < 0.001). Nerve conspicuity was improved by WW (odds ratio, OR = 5.8, p < 0.001). Fat suppression was improved by both PCA (OR = 3.6, p < 0.001) and WW (OR = 2.2, p < 0.001). Overall image quality was improved by PCA + WW (OR = 1.7, p = 0.04).

CONCLUSIONS

WW and PCA-denoising improved nerve conspicuity and fat suppression in MR neurography. Denoising can potentially provide improved accuracy of FF maps for assessing fat-infiltrated muscle.

Low-level fat fraction quantification at 3 T: comparative study of different tools for water-fat reconstruction and MR spectroscopy

OBJECTIVES

Chemical Shift Encoded Magnetic Resonance Imaging (CSE-MRI)-based quantification of low-level (< 5% of proton density fat fraction-PDFF) fat infiltration requires highly accurate data reconstruction for the assessment of hepatic or pancreatic fat accumulation in diagnostics and biomedical research.

MATERIALS AND METHODS

We compare three software tools available for water/fat image reconstruction and PDFF quantification with MRS as the reference method. Based on the algorithm exploited in the tested software, the accuracy of fat fraction quantification varies. We evaluate them in phantom and in vivo MRS and MRI measurements.

RESULTS

The signal model of Intralipid 20% emulsion used for phantoms was established for 3 T and 9.4 T fields. In all cases, we noticed a high coefficient of determination (R-squared) between MRS and MRI-PDFF measurements: in phantoms <0.9924-0.9990>; and in vivo <0.8069-0.9552>. Bland-Altman analysis was applied to phantom and in vivo measurements.

DISCUSSION

Multi-echo MRI in combination with an advanced algorithm including multi-peak spectrum modeling appears as a valuable and accurate method for low-level PDFF quantification over large FOV in high resolution, and is much faster than MRS methods. The graph-cut algorithm (GC) showed the fewest water/fat swaps in the PDFF maps, and hence stands out as the most robust method of those tested.

The Dixon technique for MRI of the bone marrow

Dixon sequences are established as a reliable MRI technique that can be used for problem-solving in the assessment of bone marrow lesions. Unlike other fat suppression methods, Dixon techniques rely on the difference in resonance frequency between fat and water and in a single acquisition, fat only, water only, in-phase and out-of-phase images are acquired. This gives Dixon techniques the unique ability to quantify the amount of fat within a bone lesion, allowing discrimination between marrow-infiltrating and non-marrow-infiltrating lesions such as focal nodular marrow hyperplasia. Dixon can be used with gradient echo and spin echo techniques, both two-dimensional and three-dimensional imaging. Another advantage is its rapid acquisition time, especially when using traditional two-point Dixon gradient echo sequences. Overall, Dixon is a robust fat suppression method that can also be used with intravenous contrast agents. After reviewing the available literature, we would like to advocate the implementation of additional Dixon sequences as a problem-solving tool during the assessment of bone marrow pathology.

Clinical experience with two-point mDixon turbo spin echo as an alternative to conventional turbo spin echo for magnetic resonance imaging of the pediatric knee

BACKGROUND

Two-point modified Dixon (mDixon) turbo spin-echo (TSE) sequence provides an efficient, robust method of fat suppression. In one mDixon acquisition, four image types can be generated: water-only, fat-only, in-phase and opposed-phase images.

OBJECTIVE

To determine whether PD mDixon TSE water-only and, by proxy, PD in-phase images generated by one acquisition can replace two conventional PD TSE sequences with and without fat suppression in routine clinical MR examination of the knee.

MATERIALS AND METHODS

This is a retrospective study of 50 consecutive pediatric knee MR examinations. PD mDixon TSE water-only and PD fat-saturated TSE sequences (acquired in the sagittal plane with identical spatial resolution) were reviewed independently by two pediatric radiologists for homogeneity of fat suppression and detection of intra-articular pathology. Thirteen of the 50 patients underwent arthroscopy, and we used the arthroscopic results as a reference standard for the proton-density fat-saturated and proton-density mDixon results. We used the Kruskal-Wallis rank test to assess difference in fat suppression between the proton-density mDixon and proton-density fat-saturated techniques. We used kappa statistics to compare the agreement of detection of intra-articular pathology between readers and techniques. We also calculated sensitivity, specificity and accuracy between arthroscopy and MR interpretations.

RESULTS

Proton-density mDixon water-only imaging showed significant improvement with the fat suppression compared with proton-density fat-saturated sequence (P=0.02). Each observer demonstrated near-perfect agreement between both techniques for detecting meniscal and ligamentous pathology and fair to substantial agreement for bone contusions, and chondral and osteochondral lesions.

CONCLUSION

Two-point mDixon water-only imaging can replace conventional proton-density fat-saturated sequence. When same-plane proton-density fat-saturated and non-fat-saturated sequences are required, proton-density water-only and proton-density in-phase image types acquired in the same acquisition shorten the overall examination time while maintaining excellent intra-articular lesion conspicuity.

Review of key concepts in magnetic resonance physics

MR physics can be a challenging subject for practicing pediatric radiologists. Although many excellent texts provide very comprehensive reviews of the field of MR physics at various levels of understanding, the authors of this paper explain several key concepts in MR physics that are germane to clinical practice in a non-rigorous but practical fashion. With the basic understanding of these key concepts, practicing pediatric radiologists can build on their knowledge of current clinical MR techniques and future advances in MR applications. Given the challenges of both the increased need for rapid imaging in non-sedated children and the rapid physiological cardiovascular and respiratory motion in pediatric patients, many advances in complex MR techniques are being applied to imaging these children. The key concepts are as follows: (1) structure of a pulse sequence, (2) k-space, (3) "trade-off triangle" and (4) fat suppression. This review is the first of five manuscripts in a minisymposium on pediatric MR. The authors' goal for this review is to aid in understanding the MR techniques described in the subsequent manuscripts on brain imaging and body imaging in this minisymposium.

Fat-containing soft-tissue masses in children

The diagnosis of soft-tissue masses in children can be difficult because of the frequently nonspecific clinical and imaging characteristics of these lesions. However key findings on imaging can aid in diagnosis. The identification of macroscopic fat within a soft-tissue mass narrows the differential diagnosis considerably and suggests a high likelihood of a benign etiology in children. Fat can be difficult to detect with sonography because of the variable appearance of fat using this modality. Fat is easier to recognize using MRI, particularly with the aid of fat-suppression techniques. Although a large portion of fat-containing masses in children are adipocytic tumors, a variety of other tumors and mass-like conditions that contain fat should be considered by the radiologist confronted with a fat-containing mass in a child. In this article we review the sonographic and MRI findings in the most relevant fat-containing soft-tissue masses in the pediatric age group, including adipocytic tumors (lipoma, angiolipoma, lipomatosis, lipoblastoma, lipomatosis of nerve, and liposarcoma); fibroblastic/myofibroblastic tumors (fibrous hamartoma of infancy and lipofibromatosis); vascular anomalies (involuting hemangioma, intramuscular capillary hemangioma, phosphate and tensin homologue (PTEN) hamartoma of soft tissue, fibro-adipose vascular anomaly), and other miscellaneous entities, such as fat necrosis and epigastric hernia.

Usefulness of the fast spin-echo three-point Dixon (mDixon) image of the knee joint on 3.0-T MRI: comparison with conventional fast spin-echo T2 weighted image

OBJECTIVE

To compare the quality of two different imaging methods, three-point Dixon (mDixon) and fast spin-echo (FSE) T2 weighted image (T2WI) [and fat suppression (FS) T2WI], and to assess the utility of mDixon for the imaging of knee joint pathology.

METHODS

This retrospective study included 66 patients who underwent both mDixon and FSE T2WI (and FS T2WI) of the knee joint. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of the two sequences at the articular cartilage and ligament were measured. Two radiologists independently evaluated the anatomic identification score and diagnostic performances of the two sequences.

RESULTS

The mean SNRs and CNRs of the patellar cartilage, femoral cartilage and anterior cruciate ligament (ACL) were significantly higher on T2WI and FS T2WI than on mDixon imaging, with the exception of the mean SNR of ACL on in-phase images. Most of the anatomic identification scores did not show significant differences, except for inferiorities of the in-phase mDixon in the evaluation of the cruciate ligament. There were no significant differences in sensitivity, specificity and accuracy between mDixon and T2WI regarding diagnostic performance.

CONCLUSION

mDixon images have equivalent anatomic identification ability with the exception of cruciate ligament delineation on in-phase images and have a diagnostic performance comparable with that of FSE T2WI for meniscal, cartilage and ligament injuries of the knee joint. There would be a net saving in time, if mDixon was the only sequence used.

ADVANCES IN KNOWLEDGE

The mDixon images have equivalent anatomic identification abilities, with the exception of cruciate ligament delineation on in-phase images and have a diagnostic performance comparable with that of FSE T2WI for meniscal, cartilage and ligament injuries of the knee joint.

Fat-suppression techniques for 3-T MR imaging of the musculoskeletal system

Fat suppression is an important technique in musculoskeletal imaging to improve the visibility of bone-marrow lesions; evaluate fat in soft-tissue masses; optimize the contrast-to-noise ratio in magnetic resonance (MR) arthrography; better define lesions after administration of contrast material; and avoid chemical shift artifacts, primarily at 3-T MR imaging. High-field-strength (eg, 3-T) MR imaging has specific technical characteristics compared with lower-field-strength MR imaging that influence the use and outcome of various fat-suppression techniques. The most commonly used fat-suppression techniques for musculoskeletal 3-T MR imaging include chemical shift (spectral) selective (CHESS) fat saturation, inversion recovery pulse sequences (eg, short inversion time inversion recovery [STIR]), hybrid pulse sequences with spectral and inversion-recovery (eg, spectral adiabatic inversion recovery and spectral attenuated inversion recovery [SPAIR]), spatial-spectral pulse sequences (ie, water excitation), and the Dixon techniques. Understanding the different fat-suppression options allows radiologists to adopt the most appropriate technique for their clinical practice.

Diffusion tensor imaging and T2 mapping in early denervated skeletal muscle in rats

BACKGROUND

To evaluate the temporal changes of diffusion tensor imaging (DTI) indices, T2 values, and visual signal intensity on various fat suppression techniques in the early state of denervated skeletal muscle in a rat model.

METHODS

Institutional Animal Care and Use Committee approval was obtained. Sciatic nerves of eight rats were transected for irreversible neurotmesis model. We examined normal lower leg and denervated muscles at 3 days, 1 week, and 2 weeks on a 3 Tesla MR. fractional anisotropy (FA), mean apparent diffusion coefficient (mADC), and T2 values were measured by using DTI and T2 mapping scan. We subjectively classified the signal intensity change on various fat suppression images into the following three grades: negative, suspicious, and definite change. Wilcoxon-sign rank test and Kruskal-Wallis test were used for the comparison of FA, mADC, T2 values. McNemar's test was used for comparing signal intensity change among fat suppression techniques.

RESULTS

FA values of denervated muscles at 3 days (0.35 ± 0.06), 1 week (0.29 ± 0.04), and 2 weeks (0.34 ± 0.05) were significantly (P < 0.05) lower than that in the control group (0.54 ± 0.17). mADC of denervated muscles decreased without statistically significant (P > 0.05) change. T2 values were significantly increased at 1 week (38.11 ± 6.42 ms, P = 0.017) and markedly increased at 2 weeks (46.53 ± 5.17 ms, P = 0.012). The grade of visual signal intensity change on chemical shift selective fat saturation, STIR and IDEAL images were identical in all cases (P = 1.000).

CONCLUSION

FA and T2 values can demonstrate the early temporal changes in denervated rat skeletal muscle.

Gradient-echo in-phase and opposed-phase chemical shift imaging: role in evaluating bone marrow

Chemical shift imaging (CSI) provides valuable information for assessing the bone marrow, while adding little to total examination time. In this article, we review the uses of CSI for evaluating bone marrow abnormalities. CSI can be used for differentiating marrow-replacing lesions from a range of non-marrow-replacing processes, although the sequence is associated with technical limitations and pitfalls. Particularly at 3 T, susceptibility artefacts are prevalent, and optimal technical parameters must be implemented with appropriate choices for echo times.

High-resolution, large dynamic range field map estimation

PURPOSE

We present a theory and a corresponding method to compute high-resolution field maps over a large dynamic range.

THEORY AND METHODS

We derive a closed-form expression for the error in the field map value when computed from two echoes. We formulate an optimization problem to choose three echo times which result in a pair of maximally distinct error distributions. We use standard field mapping sequences at the prescribed echo times. We then design a corresponding estimation algorithm which takes advantage of the optimized echo times to disambiguate the field offset value.

RESULTS

We validate our method using high-resolution images of a phantom at 7T. The resulting field maps demonstrate robust mapping over both a large dynamic range, and in low SNR regions. We also present high-resolution offset maps in vivo using both, GRE and multiecho gradient echo sequences. Even though the proposed echo time spacings are larger than the well known phase aliasing cutoff, the resulting field maps exhibit a large dynamic range without the use of phase unwrapping or spatial regularization techniques.

CONCLUSION

We demonstrate a novel three-echo field map estimation method which overcomes the traditional noise-dynamic range trade-off.

Quantification of fat infiltration in oculopharyngeal muscular dystrophy: comparison of three MR imaging methods

PURPOSE

To analyze and compare three quantitative MRI methods to determine the degree of muscle involvement in oculopharyngeal muscular dystrophy (OPMD).

MATERIALS AND METHODS

Muscle fat content (MFC) was determined based on water-fat quantification using a 2-point Dixon (2PD) method and on a histogram analysis of the free induction decay (FID) signal of a gradient-spoiled steady-state free precession (SSFP) sequence. In addition, transverse relaxation times (T₂) of muscle tissue were calculated using a monoexponential decay model.

RESULTS

We observed an increased mean MFC in OPMD patients as compared to healthy controls with the adductor magnus and soleus muscles being the most involved muscles in the thigh and calf, respectively. Furthermore, strong correlations (0.78 < R² < 0.94) between different quantitative MR methods were observed. Fewer outliers, however, were obtained by the 2PD method and T₂ measurements, suggesting these methods being superior to the SSFP-FID method.

CONCLUSION

Quantitative MR techniques, such as fast multiecho Dixon methods and T₂ imaging, can reliably differentiate between healthy and dystrophic muscles in OPMD, even if muscles are only marginally affected. Quantitative methods thus represent a promising tool that may be able to monitor more objectively the individual disease progression and treatment response in future clinical trials in muscular dystrophies.

Fast spin-echo triple-echo Dixon: initial clinical experience with a novel pulse sequence for fat-suppressed T2-weighted abdominal MR imaging

PURPOSE

To evaluate a prototype fast spin echo (FSE) triple-echo-Dixon (fTED) technique for breath-hold, fat-suppressed, T2-weighted abdominal imaging.

MATERIALS AND METHODS

Forty patients underwent breath-hold T2-weighted abdominal imaging with fTED and conventional fast recovery (FR) FSE with chemical shift-selective saturation (CHESS). FRFSE and fTED images were compared for overall image quality, homogeneity of fat suppression, image sharpness, anatomic detail, and phase artifact. Depiction of disease was recorded separately for FRFSE and fTED images.

RESULTS

FTED successfully reconstructed water-only and fat-only images from source images in all 40 cases. Water and fat separation was perfect in 36 (0.90) patients. Homogeneity of fat suppression was superior on the fTED images in 38 (0.95) of 40 cases. FTED images showed better anatomic detail in 27 (0.68), and less susceptibility artifact in 20 (0.50). FRFSE images showed less vascular pulsation artifact in 30 (0.75) cases, and less phase artifact in 21 (0.53) cases. There was no difference in depiction of disease for FRFSE and fTED images.

CONCLUSION

FTED is a robust sequence providing breath-hold T2-weighted images with superior fat suppression, excellent image quality, and at least equal depiction of disease compared to conventional breath-hold T2-weighted FRFSE imaging.

Non-invasive assessment and quantification of liver steatosis by ultrasound, computed tomography and magnetic resonance

Hepatic steatosis is the most prevalent liver disorder in the developed world. It is closely associated with features of metabolic syndrome, especially insulin resistance and obesity. The two most common conditions associated with fatty liver are alcoholic liver disease (ALD) and non-alcoholic fatty liver disease (NAFLD). Liver biopsy is considered the gold standard for the assessment of liver fat, but there is a need for less invasive diagnostic techniques. New imaging modalities are emerging, which could provide more detailed information about hepatic tissue or even replace biopsy. In the present review, available imaging modalities (ultrasound, computed tomography, magnetic resonance imaging and proton magnetic resonance spectroscopy) are presented which are employed to detect or even quantify the fat content of the liver. The advantages and disadvantages of the above-mentioned imaging modalities are discussed. Although none of these techniques is able to differentiate between microvesicular and macrovesicular steatosis and to reveal all features visible using histology, the proposed diagnostic modalities offer a wide range of additional information such as anatomical and morphological information non-invasively. In particular, magnetic resonance imaging and proton magnetic resonance spectroscopy are able to quantify the hepatic fat content hence avoiding exposure to radiation. Except for proton magnetic resonance spectroscopy, all modalities offer additional information about regional fat distribution within the liver. MR elastography, which can estimate the amount of fibrosis, also appears promising in the differentiation between simple steatosis and steatohepatitis.

Pelvic imaging using a T1W fat-suppressed three-dimensional dual echo Dixon technique at 3T

PURPOSE

To compare two T1-weighted (T1W) fat-suppressed sequences for 3D breath-hold pre- and postcontrast fat-suppressed T1W imaging of the female pelvis at 3T.

MATERIALS AND METHODS

Pelvic MRI scans of 16 female patients were retrospectively identified who were scanned with two 3D breath-hold sequences: 1) a fast spoiled gradient echo sequence with spectral inversion at lipids (SPECIAL) (called 3D FSPGR), and 2) a dual-echo two-point Dixon (DE Dixon) sequence. Contrast between soft tissue and fat, soft tissue and fluid, and fat and fluid was measured on pre- and postcontrast images. Additionally, two readers subjectively scored the images for degree and homogeneity of fat suppression plus presence and severity of artifacts.

RESULTS

Contrast between muscle and myometrium to fat was improved with the Dixon technique (0.61 vs. 0.09 and 0.7 vs. 0.3, respectively, P < 0.001). Both readers agreed that fat suppression was stronger with the Dixon sequence (P < 0.001 and P = 0.06). Artifacts were equivalent (P = 0.53 and 0.65).

CONCLUSION

The 3D DE Dixon sequence achieved stronger fat suppression in the female pelvis when compared to a 3D FSPGR sequence with SPECIAL.

A single-point Dixon technique for fat-suppressed fast 3D gradient-echo imaging with a flexible echo time

PURPOSE

To develop a single-point Dixon (SPD) technique that requires only data of a single echo with a flexible echo time, and to demonstrate its use for fat-suppressed, T1-weighted contrast agent enhancement studies.

MATERIALS AND METHODS

Raw data were collected using a product fast 3D gradient-echo pulse sequence. Phase-error removal and fat-suppression (FS) were achieved using a fully-automated region-growing algorithm. A water and fat phantom and the abdomen and breast of cancer patients before and after injection of gadolinium contrast agent were imaged at varying echo times. Scan time efficiency and overall FS quality were compared to those by the product fast 3D gradient-echo technique with conventional FS.

RESULTS

In phantom, the SPD technique achieved uniform FS for a wide range of echo times corresponding to the water and fat relative phase angles between 100 degrees and 160 degrees. In patients, the technique was able to achieve approximately 30% scan time reduction and more uniform FS when compared to using the conventional FS technique but otherwise identical scan parameters.

CONCLUSION

The SPD technique compares favorably in scan time efficiency and FS uniformity and can be useful for fast T1-weighted and fat-suppressed imaging with contrast agent administration.

3.0 Tesla imaging of the musculoskeletal system

High-field MRI at 3.0T is rapidly gaining clinical acceptance and experiencing more widespread use. The superiority of high-field imaging has clearly been demonstrated for neurological imaging. The impact of 3.0T imaging of the musculoskeletal system has been less dramatic due to complex optimization issues. Areas under consideration include coil technology, protocol modification, artifact reduction, and patient safety. In this article we review these issues and describe our experience with 3.0T musculoskeletal MRI. Fundamentally, an increased signal-to-noise ratio (SNR) is responsible for improved imaging at higher field strength. Increased SNR allows more headroom to adjust parameters that affect image resolution and examination time. It has been established that T1 relaxation time increases at 3.0T, while T2 time decreases. Consequently, scanner parameters require adjustment for optimization of images. Chemical shift and magnetic susceptibility artifacts are more pronounced and require special techniques to minimize the effect on image quality. Spectral fat saturation techniques can take advantage of the increased chemical shift. The specific absorption rate (SAR) and acoustic noise thresholds must be kept in mind at these higher fields. We additionally present some of the clinical issues we have experienced at 3.0T. A decision must be made as to whether to trade higher resolution for reduced scanning time. In general, we believe that routine imaging at 3.0T increases diagnostic confidence, especially for evaluations of cartilaginous and ligamentous structures.

Fast spin-echo triple-echo dixon (fTED) technique for efficientT2-weighted water and fat imaging

Previously published fast spin-echo (FSE) implementations of a Dixon method for water and fat separation all require multiple scans and thus a relatively long scan time. Further, the minimum echo spacing (esp), a time critical for FSE image quality and scan efficiency, often needs to be increased in order to bring about the required phase shift between the water and fat signals. This work proposes and implements a novel FSE triple-echo Dixon (fTED) technique that can address these limitations. In the new technique, three raw images are acquired in a single FSE scan by replacing each frequency-encoding gradient in a conventional FSE with three consecutive gradients of alternating polarity. The timing of the three gradients is adjusted by selecting an appropriate receiver bandwidth (RBW) so that the water and fat signals for the three corresponding echoes have a relative phase shift of -180 degrees , 0 degrees , and 180 degrees , respectively. A fully automated postprocessing algorithm is then used to generate separate water-only and fat-only images for each slice. The technique was implemented with and without parallel imaging. We demonstrate that the new fTED technique enables both uniform water/fat separation and fast scanning with uncompromised scan parameters, including applications such as T(2)-weighted separate water and fat imaging of the abdomen during breath-holding.

Head and neck imaging: The role of CT and MRI

High-resolution computed tomography (CT) and magnetic resonance imaging (MRI) have become indispensable tools for the evaluation of conditions involving the head and neck. Complex anatomic structures and regions, such as the orbit, skull base, paranasal sinuses, deep spaces of the neck, larynx, and lymph nodes, require that the radiologist be familiar with the imaging modalities available and their appropriate applications. The purpose of this article is to review the techniques of CT and MRI and the roles they play in clinical practice, including head and neck disorders.

T1- and T2-weighted fast spin-echo imaging of the brachial plexus and cervical spine with IDEAL water–fat separation

PURPOSE

To compare the iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) method with fat-saturated T1-weighted (T1W) and T2W fast spin-echo (FSE) and short-TI inversion recovery (STIR) imaging of the brachial plexus and cervical spine.

MATERIALS AND METHODS

Images acquired at 1.5T in five volunteers using fat-saturated T1W and T2W FSE imaging and STIR were compared with T1W and T2W IDEAL-FSE images. Examples of T1W and T2W IDEAL-FSE images acquired in patients are also shown.

RESULTS

T1W and T2W IDEAL-FSE demonstrated superior fat suppression (P<0.05) and image quality (P<0.05), compared to T1W and T2W fat-saturated FSE, respectively. SNR performance of T1W-IDEAL-FSE was similar to T1W FSE in the spinal cord (P=0.250) and paraspinous muscles (P=0.78), while T2W IDEAL-FSE had superior SNR in muscle (P=0.02) and CSF (P=0.02), and marginally higher cord SNR (P=0.09). Compared to STIR, T2W IDEAL-FSE demonstrated superior image quality (P<0.05), comparable fat suppression (excellent, P=1.0), and higher SNR performance (P<0.001).

CONCLUSION

IDEAL-FSE is a promising method for T1W and T2W imaging of the brachial plexus and cervical spine.

Iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL): application with fast spin-echo imaging

Chemical shift based methods are often used to achieve uniform water-fat separation that is insensitive to Bo inhomogeneities. Many spin-echo (SE) or fast SE (FSE) approaches acquire three echoes shifted symmetrically about the SE, creating time-dependent phase shifts caused by water-fat chemical shift. This work demonstrates that symmetrically acquired echoes cause artifacts that degrade image quality. According to theory, the noise performance of any water-fat separation method is dependent on the proportion of water and fat within a voxel, and the position of echoes relative to the SE. To address this problem, we propose a method termed "iterative decomposition of water and fat with echo asymmetric and least-squares estimation" (IDEAL). This technique combines asymmetrically acquired echoes with an iterative least-squares decomposition algorithm to maximize noise performance. Theoretical calculations predict that the optimal echo combination occurs when the relative phase of the echoes is separated by 2pi/3, with the middle echo centered at pi/2+pik (k=any integer), i.e., (-pi/6+pik, pi/2+pik, 7pi/6+pik). Only with these echo combinations can noise performance reach the maximum possible and be independent of the proportion of water and fat. Close agreement between theoretical and experimental results obtained from an oil-water phantom was observed, demonstrating that the iterative least-squares decomposition method is an efficient estimator.

T2-weighted spine imaging with a fast three-point dixon technique: Comparison with chemical shift selective fat suppression

PURPOSE

To develop a phased-array coil-compatible, fast three-point Dixon (TPD) technique, and compare its performance in T2-weighted spine imaging with that of the standard chemical shift selective (CHESS) fat suppression technique.

MATERIALS AND METHODS

We acquired T2-weighted spine images of 27 patients using essentially identical scanning parameters with the fast TPD technique and standard fast spin echo (FSE) with CHESS fat suppression. A phased-array coil-compatible image reconstruction algorithm was developed to generate separate water and fat images from the data acquired with the fast TPD technique. Three neuroradiologists independently scored the images from the two different techniques for uniformity of fat suppression and lesion conspicuity using a four-point system (1 = poor, 2 = fair, 3 = good, 4 = best).

RESULTS

The reviewers' mean scores were 3.2 and 2.1 for the uniformity of fat suppression, and 3.0 and 2.0 for the lesion conspicuity for the fast TPD and the CHESS fat suppression techniques, respectively. The fast TPD technique was statistically superior to the CHESS technique at P < 0.0005.

CONCLUSION

The fast TPD technique provides superior fat suppression and lesion conspicuity, and potentially can be used as an alternative to T2-weighted imaging of the spine.

Using a phantom to compare MR techniques for determining the ratio of intraabdominal to subcutaneous adipose tissue

OBJECTIVE

Patients who have a greater distribution of intraabdominal adipose tissue as compared with subcutaneous adipose tissue and an increased ratio of intraabdominal adipose tissue to subcutaneous adipose tissue are at greater risk for developing cardiovascular disease and type 2 diabetes mellitus. In previous MR investigations, researchers have used conventional T1-weighted spin-echo images to determine the ratio of intraabdominal adipose tissue to subcutaneous adipose tissue. However, no investigation, to our knowledge, has been performed to determine the accuracy of using different MR sequences to estimate adipose distribution. The purpose of our investigation was to compare MR imaging and segmentation techniques in calculating the ratio of intraabdominal to subcutaneous adipose tissue using an adiposity phantom.

MATERIALS AND METHODS

A phantom was created to simulate the distribution of subcutaneous and intraabdominal fat (with known volumes). Axial MR images were obtained twice through the phantom using a 5-mm slice thickness and zero gap for the following T1-weighted sequences: spin-echo, fast Dixon, and three-dimensional (3D) spoiled gradient-echo. An in-house computer software program was then used to segment the volumes of fat and calculate the volume of intraabdominal adipose tissue and subcutaneous adipose tissue and the ratio of intraabdominal to subcutaneous adipose tissue. Each imaging data set was segmented three times, so six sets of data were yielded for each imaging technique. The percentage predicted of the true volume was calculated for each MR imaging technique for each fat variable. The mean percentages for each variable were then compared using one-factor analysis of variance to determine whether differences exist among the three MR techniques.

RESULTS

The three MR imaging techniques had statistically significant different means for the predicted true volume of two variables: volume of subcutaneous adipose tissue (p < 0.001) and volume of intraabdominal adipose tissue (p = 0.0426). Estimates based on fast Dixon images were closest to the true volumes for all the variables. All MR imaging techniques performed similarly in estimating the ratio of intraabdominal adipose tissue to subcutaneous adipose tissue (p = 0.9117). The acquisition time for the 3D spoiled gradient-echo images was 10-22 times faster than for the other sequences.

CONCLUSION

Conventional T1-weighted spin-echo MR imaging, the current sequence used in practice for measuring visceral adiposity, may not be the optimal MR sequence for this purpose. We found that the T1-weighted fast Dixon sequence was the most accurate at estimating all fat volumes. The T1-weighted 3D spoiled gradient-echo sequence generated similar ratios of intraabdominal to subcutaneous adipose tissue in a fraction of the acquisition time.

Rapid MR imaging of articular cartilage with steady-state free precession and multipoint fat-water separation

OBJECTIVE

To obtain high-quality high-resolution images of articular cartilage with reduced imaging time, we combined a novel technique of generalized multipoint fat-water separation with three-dimensional (3D) steady-state free precession (SSFP) imaging.

SUBJECTS AND METHODS

The cartilage of 10 knees in five healthy volunteers was imaged with 3D SSFP imaging and a multipoint fat-water separation method capable of separating fat and water with short TE increments. Fat-saturated 3D spoiled gradient-echo (SPGR) images were obtained for comparison.

RESULTS

High-quality images of the knee with excellent fat-water separation were obtained with 3D SSFP imaging. Total imaging time required was 58% less than that required for 3D SPGR imaging with a comparable cartilage signal-to-noise ratio and spatial resolution. Unlike 3D SPGR images, 3D SSFP images exhibited bright synovial fluid, providing a potential arthrographic effect.

CONCLUSION

High-quality high-resolution images of articular cartilage with improved fat-water separation, bright synovial fluid, and markedly reduced acquisition times can be obtained with 3D SSFP imaging combined with a fat-water separation technique.

Multicoil Dixon chemical species separation with an iterative least-squares estimation method

This work describes a new approach to multipoint Dixon fat-water separation that is amenable to pulse sequences that require short echo time (TE) increments, such as steady-state free precession (SSFP) and fast spin-echo (FSE) imaging. Using an iterative linear least-squares method that decomposes water and fat images from source images acquired at short TE increments, images with a high signal-to-noise ratio (SNR) and uniform separation of water and fat are obtained. This algorithm extends to multicoil reconstruction with minimal additional complexity. Examples of single- and multicoil fat-water decompositions are shown from source images acquired at both 1.5T and 3.0T. Examples in the knee, ankle, pelvis, abdomen, and heart are shown, using FSE, SSFP, and spoiled gradient-echo (SPGR) pulse sequences. The algorithm was applied to systems with multiple chemical species, and an example of water-fat-silicone separation is shown. An analysis of the noise performance of this method is described, and methods to improve noise performance through multicoil acquisition and field map smoothing are discussed.