Fast spin echo sequences for BOLD functional MRI

At higher field strengths, spin echo (SE) functional MRI (fMRI) is an attractive alternative to gradient echo (GE) as the increased weighting towards the microvasculature results in intrinsically better localization of the BOLD signal. Images are free of signal voids but the commonly used echo planar imaging (EPI) sampling scheme causes geometric distortions, and T ₂* effects often contribute considerably to the signal changes measured upon brain activation. Multiply refocused SE sequences such as fast spin echo (FSE) are essentially artifact free but their application to fast fMRI is usually hindered due to high energy deposition, and long sampling times. In the work presented here, a combination of parallel imaging and partial Fourier acquisition is used to shorten FSE acquisition times to near those of conventional SE-EPI, permitting sampling of eight slices (matrix 64 × 64) per second. Signal acquisition is preceded by a preparation experiment that aims at increasing the relative contribution of extravascular dynamic averaging to the BOLD signal. Comparisons are made with conventional SE-EPI using a visual stimulation paradigm. While the observed signal changes are approximately 30% lower, most likely due to the absence of T ₂* contamination, activation size and t-scores are comparable for both methods, suggesting that HASTE fMRI is a viable alternative, particularly if distortion free images are required. Our data also indicate that the BOLD post-stimulus undershoot is most probably attributable to persistent elevated oxygen metabolism rather than to delayed vascular compliance.     

Turbo spin-echo sequences in magnetic resonance imaging of the brain: Physics and applications

Fast SE imaging provides considerable measure time reduction, high signal-to-noise ratios as well as similar contrast behavior compared to conventional SE sequences. Besides TR and TE_eff, echo train length (ETL), interecho time , and-space trajectory determine image contrast and image quality in fast SE sequences. True proton density contrast (CSF hypointense) and not too strong T2 contrast are essential requirements in routine brain MRI. A Turbo SE sequence with very short echo train length (ETL=3), short TE_eff and short interecho time (17 ms), and TR=2000 ms was selected for proton density contrast; a Turbo SE sequence with ETL=7, TE_eff=90 ms, =22 ms, and TR=3250 ms was selected for T₂-weighted images. Using both single-echo Turbo SE sequences yielded 50% measure time reduction compared to the conventional SE technique. Conventional SE and optimized Turbo SE sequences were compared in 150 patients resulting in very similar signal and contrast behavior. Furthermore, reduced flow artifacts in proton density—and especially in T₂-weighted Turbo SE images—and better contrast of high-intensity lesions in proton density-weighted Turbo SE images were found. Slightly reduced edge sharpness—mainly in T₂-weighted Turbo SE images—did not reduce diagnostic reliability. Differences between conventional and Turbo SE images concerning image contrast and quality are explained regarding special features of fast SE technique.Address for correspondence: Institut für Röntgendiagnostik, Klinikum der Universität Regensburg, Franz-Josef-Strauß-Allee 11, 93042 Regensburg, Germany. Additional reprints of this chapter may be obtained from the Reprints Department, Chapman & Hall, One Venn Plaza, New York, NY 10119.     

Breathhold imaging of the upper abdomen using a circular polarized-array coil: comparison with standard body coil imaging

Current studies emphasize the use of array coils to decrease noise and increase the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). We applied Tl-weighted and T2-weighted standard nonbreathhold spin echo (SE) sequences and Tl-weighted FLASH, TurboFLASH, T2-weighted spin-echo time (TSE), and heavily T2-weighted half Fourier acquisition single-shot TSE (HASTE) sequences during breathhold for abdominal imaging in 15 normal volunteers. The breathhold scans were performed using both a standard coil and a circular polarized array coil. We analyzed the signal intensity (SI), SNR, and CNR of abdominal organs in all sequences. SNRs increased in all cases by an overall factor of 3 due to an 8% increase in overall Sis and a 50% decrease in noise when applying the array coil. Although the array-coil FLASH sequence performed at least as well as the respective SE sequence, the SNRs of the array-coil TurboFLASH, TSE breathhold, and HASTE sequences were generally lower. We conclude that array-coil imaging significantly improves fast imaging of the abdomen.     

Magnetic resonance imaging differentiation of adrenal masses at 1.5 t: t2-weighted images,chemical shift imaging,and gd-dtpa dynamic studies

The purpose of our study was to assess the potential role of spin-echo (SE), chemical shift, and gadolinium-enhanced magnetic resonance imaging (MRI) in the differentiation of adrenal masses.Seventy-two adrenal masses (26 nonhyperfunctioning adenomas, 16 aldosterone-secreting adenomas and 6 other different benign cortical masses, 18 pheochromocytomas, and 6 malignant masses) in 63 patients were evaluated with spin-echo sequences, chemical shift imaging (CSI) and gadolinium diethylenetriamine penta-acetic acid (Gd-DTPA) dynamic studies. Ratios and indices of signal intensity for all examined MRI methods were calculated and examined for significance of difference between different types of adrenal masses.Quantitative magnetic resonance evaluation of adrenal masses showed significant differences (at least<0.01) between nonhyperfunctioning adenomasvs. pheochromocytomas orvs. malignant lesions orvs. aldosterone-secreting adenomas and between pheochromocytomasvs. malignant lesions. The most specific indicators of adrenal mass character proved to be the CSI ratio based on opposed-phase and in-phase two-dimensional fast low-angle shot (FLASH) images, reflecting lipid content in the lesion, and Gd-DTPA dynamic studies ratios reflecting contrast agent inflow and washout in the lesion: Wo_max/last and Dyn_1.2–3.2. There was no overlap of CSI ratio between adenomas and pheochromocytomas. The overlap of ranges of CSI ratio between nonhyperfunctioning adenomas and aldosterone-secreting adenomas was only 18.5%. There was no overlap of Wo_max/last ratio between adenomas and pheochromocytomas, or adenomas and malignant lesions. The overlap of ranges of Dyn_{1.2– 3.2} ratio between pheochromocytomas and malignant lesions was only 17.6%.MRI enables good visualization and specific characterization of adrenal masses. The optimal MRI protocol for the adrenal region is presented.Address for correspondence: Imaging Department, Warsatv Province Hospital, ul. Kondratowicza 8, 03-285 Warsaw, Poland.     

Single-shot t1-and t2-weighted magnetic resonance imaging of the heart with black blood: preliminary experience

Purpose: To implement and evaluate two robust methods for T1-and T2-weighted snapshot imaging of the heart with data acquisition within a single heart beat and suppression of blood signal. Methods: Both Tl-and T2-weighted diastolic images of the heart can be obtained with half Fourier single-shot turbo spin echo (HASTE) and turbo fast low-angle shot (turboFLASH) sequences, respectively, in less than 350 ms. Signal from flowing blood in the ventricles and large vessels can be suppressed by a preceding inversion recovery preparing pulse pair (PRESTO). Fifteen volunteers and five patients have been evaluated quantitatively for signal-to-noise ratio (SNR) contrast-to-noise ratio (CNR) and flow void and qualitatively for image quality, artifacts, and black-blood effect. Results: Both PRESTO-HASTE and PRESTO-turboFLASH achieved consistently good image quality and blood signal suppression. In contrast to gradient-echo (GRE) echo-planar imaging techniques, (EPI) HASTE and turboFLASH are much less sensitive to local susceptibility differences in the thorax, resulting in a more robust imaging technique without the need for time-consuming system tuning. Compared to standard spin-echo sequences with cardiac triggering, HASTE and turboFLASH have significantly shorter image acquisition times and are not vulnerable to respiratory motion artifacts. Conclusion: PRESTO-HASTE and PRESTO-turboFLASH constitute suitable methods for fast and high-quality cardiac magnetic resonance imaging (MRI).     

Contrast-modified gradient echo imaging using rotary echo preparatory pulses

The use of on-resonance binomial composite pulses in two- or three-dimensional magnetization-prepared gradient-recalled echo magnetic resonance imaging experiments generates rotary echoes, leading to an increase in contrast range that is, in part, determined by the ratio ofT ₂ toT ₁. In comparison with other fast gradient-recalled echo imaging techniques designed for enhancedT ₂ contrast, this method is more robust with respect to radiofrequency field inhomogeneity and less sensitive with respect to motion artifacts. Three-dimensional parametric images may be calculated using least-squares fitting based on a simple model for steady-state longitudinal magnetization during the imaging sequences.     

Dynamic snapshot gradient-echo imaging of head and neck malignancies: time dependency and quality of contrast-to-noise ratio

The purpose of this study was to evaluate the time dependency of the contrast-to-noise ratio (CNR) of head and neck malignancies during contrast-enhanced MR imaging. Then we would compare the CNR of dynamic snapshot gradient-echo (SGE) images with conventional spin-echo (SE) and fast spin-echo (FSE) sequences. Fifteen patients with squamous cell carcinomas were examined with T1W-SE, T2W-FSE, contrast-enhanced Gd-TlW-SE, and T1W-SGE sequences, the latter statically and contrast-enhanced dynamically. The CNR for all sequences and adjacent tissues was computed and the time to reach maximal CNR (T_max) was determined for dynamic studies. The CNR was time dependent with two distinct T_max at 6–18 and 60–160 s which corresponded to two different tumor enhancement patterns. Neither enhancement pattern correlated with distinct histologic findings or tumor grading. The CNR improved for the Gd-TlW-SGE images. The improvement was statistically significant in relation to T1W-SE and Gd-TlW-SE images at the floor of the mouth and at the tongue base. The good CNR of the dynamic Gd-TlW-SGE measurements justifies further investigations of this method in order to improve tumor delineation.     

Adapted techniques for clinical MR imaging of tendons

To determine whether the echo time of magnetic resonance gradient-echo and spin-echo imaging sequences may be important for the occurrence of high signal strength from tendon with pathological alterations, imaging sequences with sufficient spatial resolution and very short echo times were developed for whole-body imagers with standard gradient system. The sequences were applied on the Achilles tendons of five healthy volunteers and seven patients with achillodynia. Some affected regions inside tendon, probably corresponding with tissue with subtle edema in the collagen bundles were only revealed in images recorded with very short echo times TE<5 ms, whereas stronger affections and protons in liquids between the fiber bundles were also shown in images with longer echo times TE>10 ms. Gradient-echo methods allow shorter echo times than spin-echo techniques for a given gradient system of the imager and given spatial resolution. So minimum echo time gradient-echo sequences should be used for sensitive imaging of tendon alterations, because no considerable signal dephasing due to susceptibility effects were found in tendon.     

MR imaging of focal liver lesions: comparison between turbo spin-echo and conventional spin-echo pulse sequences

The aim of this study was to compare conventional spin-echo (CSE)T ₂-weighted (T2W) images with turbo spin-echo (TSE) T2W pulse sequences in their ability to detect focal liver lesions. Seventy-eight consecutive patients with focal liver lesions were entered into this study. All patients were imaged using the gradient-echo (GE) sequence with the breath-hold technique forT ₁-weighted (T1W) images, and CSE and TSE sequences for T2W images. Qualitative evaluation included lesion detection (number of lesions detected) and conspicuity (extent of visualization of lesional borders); quantitative evaluation included the signal-to-noise (S/N) ratio and the contrast-to-noise (C/N) ratio. TSE showed the best performance in terms of lesion detection; however, the difference between TSE and CSE was significant only in the case of benign cysts (p<0.01). Conspicuity was higher with TSE and CSE, and lower with GE. The S/N and C/N ratios of the two T2W sequences were also comparable, and better than those of GE. However, the combined use of GE and TSE resulted in improved lesion detection. The results show that, because the acquisition time is greatly reduced with TSE sequences, these should be considered as first-line approach to magnetic resonance imaging of the liver for the study of focal lesions.     

Contrast-enhanced abdominal echo planar imaging: Dynamic signal-intensity changes following intravenous injection of gadolinium-DOTA

Objectives: After I.V. administration of gadolinium-DOTA, the early contrast enhancement pattern and related signal-intensity (SI) changes in normal abdominal organs (kidney, spleen, liver) are evaluated over the first 4 min by using ultrafast spin-echo echo planar imaging (SE-EPI). Methods: On a 1.5-T magnetic resonance unit ultrafast EPI of the upper abdomen was performed in 12 patients in order to show the contrast enhancement pattern and related measurable SI changes onT ₁ andT ₂-weighted (w) images over the first 4 min after I.V. bolus injection of 0.1 mmol kg^–1 gadolinium (Gd)-DOTA in the spleen, liver, renal cortex, and renal medulla. A TR/TE of 500/44 or 45 ms inT ₁w SE-EPI and a TR/TE of 2000/80 or 100 ms inT ₂-w SE-EPI were used. Results: Typical time-dependent SI changes were noticed onT ₁w images: Subsequent to a SI increase in the renal cortex (starting 7 s after the I.V. injection of Gd-DOTA) SI increased first in the outer renal medulla (6 s later) and then in the inner renal medulla (21 s later). A SI increase was observed in the spleen (starting after 15 s) and in the liver (starting 7 s later). OnT ₂-w images, a SI decrease in the renal cortex (starting after 14 s) was followed by migration of a dark band from the outer (after 46 s) to the inner medulla (after 70 s). Only minimal changes were noticed in the spleen and liver. Conclusions: Ultrafast SE-EPI following I.V. bolus injection of Gd-DOTA enables the observation of the very early contrast agent kinetics in various abdominal organs. The associated SI changes onT ₁- andT ₂- SE EPI are related to organ perfusion and contrast agent tissue concentration and biodistribution.Additional reprints of this chapter may be obtained from the Reprints Department, Chapman & Hall, One Penn Plaza, New York, NY 10119.     

T 2 relaxometry measurements in low spatial frequency brain regions differ between fast spin-echo and multiple-echo spin-echo sequences

Object

Dual-echo fast spin-echo (FSE) sequences are used in T ₂ relaxometry studies of neurological disorders because of shorter clinical scanning times and protocol simplicity. However, FSE sequences have possible spatial frequency-dependent effects, and derived T ₂ values may include errors that depend on the spatial frequency characteristics of the brain region of interest.

Materials and methods

Dual-echo FSE and multi-echo spin-echo (MESE) sequences were acquired in nine subjects. The T ₂ decay curves for FSE and MESE sequences were estimated and percent error maps were generated. T ₂ error values were obtained along each patient’s corticospinal tract (CST). Whole-brain white matter (WM) and gray matter (GM) T ₂ error values were also obtained. The paired t test was performed to evaluate differences in T ₂ values in the CST between FSE and MESE sequences.

Results

Histograms of error values in CST and in whole-brain WM and GM structures revealed systematic errors in FSE sequences. Significant differences (P < 0.001) in CST T ₂ values were also observed between FSE and MESE sequences.

Conclusion

Our findings indicate that T ₂ values derived from FSE sequences are prone to large errors, even in low spatial frequency regions such as the CST, when compared to MESE sequences. Future studies should be aware of this limitation of FSE sequences.     

Dynamic contrast-enhanced MR imaging of the entire breast with spectral-selective inversion fast three dimensional sequence

Dynamic contrast-enhanced images with high spatial and temporal resolutions were acquired with a fast 3D spoiled gradient echo (SPGR) sequence using spectral selective inversion recovery (IR) pulse. Five healthy volunteers and 12 patients with 14 pathologically proven breast lesions were studied. Fat suppressed volume image data covering the entire breast were obtained with a sufficient spatial resolution (0.9×1.5×3.0 mm³) and an imaging time of 57 s. By using the criteria including peripheral enhancement and presence of spiculation, sensitivity, specificity, and accuracy in detecting malignant lesions were 88.9, 80.0 and 85.%, respectively. Although the C/N and S/N ratios were approximately 30% less than those of the conventional fat suppressed 3D technique, fast 3D SPGR imaging with spectral IR method demonstrated sufficient image quality for both time intensity analysis and morphological evaluation of the breast lesions with a data acquisition time less than half of the conventional method. This technique can substantially improve spatial and temporal resolutions of dynamic MR images of the breast and will be useful in evaluating malignant and benign breast lesions.     

Endorectal coil MR imaging findings in hemospermia

The purpose of this study was to illustrate the spectrum of abnormalities found at endorectal coil magnetic resonance (MR) imaging in patients with hemospermia. Ninety patients aged between 23 and 71 years (mean age 41 years) presenting with hemospermia underwent evaluation with endorectal coil MR imaging at 1.5 T. Duration of hemospermia ranged between 5 days and 4 years (mean 15 months). MR examination protocol included T1-weighted spin-echo (SE) and T2-weighted fast SE MR images obtained in the sagittal, coronal, and axial planes. Abnormalities were observed on endorectal-coil MR images in 49 of 90 patients (54%). Blood within the seminal vesicle or the ejaculatory duct was recognized in 23 of 90 patients (25%). Dilatation of the seminal vesicles or the ejaculatory duct was observed in 31 of 90 patients (34%). Cystic lesions were identified in 14 cases, eight of which involved the utriculus and six the ejaculatory duct. Calculi within the seminal vesicles were depicted in seven patients. No malignant disease was demonstrated. In conclusion, despite the evidence that hemospermia is most often a benign and self-limiting condition, we found that endorectal coil MR imaging can detect abnormal findings in more than half of the patients, and may be helpful in assessing the level at which hemorrhage occurred and in defining the cause of the disease. Presented as Category I Scientific Exhibit at the 1996 Annual Meeting of the Radiological Society of North America.     

Interaction between grounding pads used for RF ablation therapy and magnetic resonance imaging

Objectives: To characterize artifacts and imaging problems in the presence of conductive grounding pads for RF ablation therapy as well as potential heating problems due to induction of eddy currents in the pads. Strategies for avoidance of those problems are developed. Materials and methods: Underlying principles of interactions between grounding pads and MR imaging are reported. Influential parameters, e.g., orientation in relation to the magnetic field, shape of the grounding pad, sequence type (spin-echo versus gradient echo) and magnetic field strength (0.2 T, 1.5 T, 3 T) were varied in systematic phantom studies. Heating effects due to induced eddy currents were estimated theoretically and measured by infrared imaging in an adapted set-up. Results: MR imaging artifacts are markedly dependent on the orientation and geometrical shape of the grounding pads. Visible signal extinction artifacts were more pronounced using spin-echo techniques than in gradient echo images and increased for higher field strengths. Suitable incisions in the grounding pad reduced eddy currents markedly and minimized image artifacts. Heating problems due to induced eddy currents by the RF transmitted for MR imaging were excluded by phantom measurements. Conclusions: Suitable positioning of the grounding pads and adaptation of their geometry provide clearly reduced artifacts in MR imaging.     

Biexponential and diffusional kurtosis imaging, and generalised diffusion-tensor imaging (GDTI) with rank-4 tensors: a study in a group of healthy subjects

Object Clinical diffusion imaging is based on two assumptions of limited validity: that the radial projections of the diffusion propagator are Gaussian, and that a single directional diffusivity maximum exists in each voxel. The former can be removed using the biexponential and diffusional kurtosis models, the latter using generalised diffusion-tensor imaging. This study provides normative data for these three models. Materials and methods Eighteen healthy subjects were imaged. Maps of the biexponential parameters D _fast, D _slow and f _slow, of D and K from the diffusional kurtosis model, and of diffusivity D′ were obtained. Maps of generalised anisotropy (GA) and scaled entropy(SE) were also generated, for second and fourth rank tensors. Normative values were obtained for 26 regions. Results In grey versus white matter, D _slow and D′ were higher and D _fast, f _slow and K were lower. With respect to maps of D′, anatomical contrast was stronger in maps of D _slow and K. Elevating tensor rank increased SE, generally more significantly than GA, in: anterior limb of internal capsule, corpus callosum, deep frontal and subcortical white matter, along superior longitudinal fasciculus and cingulum. Conclusion The values reported herein can be used for reference in future studies and in clinical settings.     

3D diffusion tensor imaging with 2D navigated turbo spin echo

A Cartesian two-dimensional navigator with variable orientation for online motion correction is introduced. It corrects for all possible zeroth- and first-order phase errors due to rigid-body motion of a subject during the diffusion-weighting preparation. The technique is developed for the application of three-dimensional (3D) imaging sequences, which offer the opportunity of high-resolution diffusion-weighted imaging, or diffusion tensor imaging (DTI) with isotropic voxel resolution. The navigator was applied to a displaced 3D turbo spin-echo sequence with an ECG-gated diffusion preparation to avoid phase errors due to gross brain pulsation. Online and offline corrected in vivo images acquired with this sequence are compared to investigate the advantages of online correction. Also eigenvector maps of the diffusion tensor are presented with an isotropic resolution of 1 mm³, which indicate that this new navigator technique is a promising approach for high-resolution DTI.     

Dual-contrast single breath-hold 3D abdominal MR imaging

Object: Multiple contrasts are often helpful for a comprehensive diagnosis. In 3D abdominal MRI, breath-hold techniques are preferred for single contrast acquisitions to avoid respiratory artifacts. In this paper, highly accelerated parallel MRI is used to acquire large 3D abdominal volumes with two different contrasts within a single breath-hold. Material and methods: In vivo studies have been performed on six healthy volunteers, combining T ₁- and T ₂-weighted, gradient- or spin-echo based scans, as well as water/fat resolved imaging in a single breath-hold. These 3D scans were acquired with an acceleration factor of six, using a prototype 32-element receive array. Results: The presented approach was tested successfully on all volunteers. The whole liver area was covered by a FOV of 350 × 250 × 200 mm³ for all scans with reasonable spatial resolution. Arbitrary scan protocols generating different contrasts have been shown to be combinable in this single breath-hold approach. Good spatial correspondence with negligible spatial offset was achieved for all different scan combinations acquired in overall breath-hold times between 15 and 25 s. Conclusion: Enabled by highly parallel imaging technology, this study demonstrates the technical feasibility and the promising image quality of single breath-hold dual contrast MRI.     

Feasibility study of non Carr Purcell Meiboom Gill single shot fast spin echo in spinal cord diffusion imaging

The fast spin echo (FPE) sequence is sensitive to the phase of the magnetization, hindering its use in procedures such as diffusion imaging. The current solutions to this problem reduce the available signal by one half. We present the first volunteer study of a sequence which does not suffer from this loss of signal while measuring diffusion coefficients. This work was presented in part during the Eighth Scientific Meeting of International Society for Magnetic Resonance in Medicine. Denver. 2000 (number 1501).     

In vivo mouse spinal cord imaging using echo-planar imaging at 11.75 T

Object To evaluate the feasibility of mouse spinal cord MR imaging using echo-planar imaging (EPI). Materials and methods Optimized multi-shot spin-echo-EPI sequences were compared to conventional spin-echo (c-SE) at 11.75 T and used for high-spatially resolved acquisitions and relaxation-time measurements. Results Good quality images were obtained, with clear delineation of gray and white matter. Acquisition-time gain factor was up to 6 (vs. c-SE) and resolution up to 74 × 94 μm² was achieved. T ₁ and T ₂ relaxation times were reliably measured. Conclusion High-temporally and spatially resolved mouse spinal cord EPI imaging is feasible. This technique should greatly benefit to long acquisition-time experiments (diffusion imaging) and imaging of rapidly-evolving pathologies. V. Callot and G. Duhamel equally contributed to this work.     

Breath-hold spin-echo MR imaging for evaluation of dynamic enhancement of native and treated hepatocellular carcinoma after intravenous Gd-DTPA administration

Using a simple modification of a standard spin-echo sequence which enables acquisition of three breath-hold images in 15 s, dynamic enhancement of 30 histologically proven hepatocellular carcinomas (17 native tumors, 6 completely necrotic tumors after nonsurgical treatments, and 7 tumors with viable and necrotic portions) after intravenous injection of gadolinium-DTPA was evaluated. Native hepatocellular carcinomas and viable portions in treated nodules showed elective enhancement in images obtained 40 s after contrast injection. Contrast between these lesions and the normal liver decreased thereafter. No contrast uptake was seen in entirely necrotic nodules and necrotic portions of treated nodules. Because of the capability to demonstrate the elective arterial blood supply typical of hepatocellular carcinoma, breath-holdT ₁-weighted spin-echo sequence should replace conventionalT ₁-weighted images for the evaluation of intravenously administered gadolinium-DTPA enhancement of this tumor before and after nonsurgical treatments.