Multiple spin-echo MR imaging of the body: image contrast and motion-induced artifact

For a given TR and TE, image quality changes when the number of spin echoes obtained is varied. To investigate the importance of this in clinical imaging, a total of 4 patients and 9 volunteers had MRI examinations of the abdomen (n = 7) and/or pelvis (n = 8) which included at least 2 sequences with identical TR (2000 or 2500 ms), TE (80 ms) and other parameters, but with a different series of refocusing pulses. Sequences included single-echo (S), asymmetric and symmetric double-echo (AD and SD) and quadruple-echo (Q) techniques. Image contrast and severity of motion-induced artifact was measured via blind examination by 3 independent MRI radiologists and calculation of signal-difference, signal-difference-to-noise ratios and intensity of motion-induced "ghost artifact." The order of decreasing signal differences was S, SD, AD and Q, and all of three liver lesions were better seen with S than with SD techniques. These observations are consistent with signal loss from cumulative inaccuracies from multiple 180 degrees RF pulses. The order of increasing intensity of ghost artifact was Q, SD, AD and S, consistent with the beneficial motion artifact-reducing effects of even-echo rephasing. Knowledge of these effects of multi-echo imaging allows one to make informed decisions about imaging protocols rather than to simply obtain multiple echoes "because they are free."     

Feasibility study of the application of micro-Raman imaging as complement to micro-XRF imaging

X-ray fluorescence (XRF) spectroscopy and Raman spectroscopy are preferential analytical techniques to study cultural heritage objects, since both techniques may provide complementary information in a non-destructive manner. Moreover, the application of microscopic beams allows the investigation of heterogeneous samples on the microscopic level and the study of the heterogeneity of particular samples. The micro-XRF method became already a routine analytical imaging method also because of the well-established spectrum evaluation methodology enabling specific data handling procedures. These include multivariate statistical analysis procedures such as principal components analysis (PCA) in order to explore and describe the acquired data, and clustering techniques in order to find similar pixels (or areas) in the obtained images. In the case of the micro-Raman technique, however, the usual approach is to perform a single spot analysis of only a few selected positions in order to ultimately identify the material on the basis of the comparison with Raman spectra obtained from reference materials. However, when samples are heterogeneous, imaging is still to be preferred in order to deal with the problem of sampling. With the arrival of a new micro-Raman spectrometer at the UGent laboratories, there was the need to explore the possibilities of Raman imaging. One of the most important aspects of imaging is the time needed for the analysis. Therefore, the influence of different instrumental parameters, such as resolution (low or high) and measuring time per pixel, on the quality of Raman spectra and images was investigated in order to evaluate the possibility of performing fast Raman mappings because of the need to identify the regions of interest on the art object in a more systematic manner.     

MR imaging of coarctation of the aorta and its postoperative complications in adults: assessment with spin-echo and cine-MR imaging

To evaluate the ability of ECG-gated spin-echo (SE)-MR imaging vs. cine-MR imaging to assess coarctation of the aorta, 11 patients aged 15–45 years, with known or suspected coarctation of the aorta, and five patients suspected of re-stenosis or postoperative false-aneurysms after coarctation repair were examined by multisection SE-MR imaging and single-section multiphase cine-MR imaging on a 1.0 Tesla device. Aortography was performed in 15, and surgery in 14 of these 16 patients. Qualitatively, the location, severity, and length of the coarctation were shown in all cases with MR imaging, as well as the relationship with the arteries arising from the aortic arch. The respective sensitivities and specificities in the assessment of severity of stenoses were 86% and 100% for SE MR images, and 100% and 100% for cine-MR images. Cine-MR imaging was superior to SE imaging in stenosis diameter assessment with contrast angiography as reference, as well as to identify the site of leakage in cases of postoperative pseudoaneurysm. Pre-stenotic dilation or post-stenotic aneurysm, collateral channels, and associated malformations were better identified on SE images. Quantitatively, a better stenosis diameter correlation was found between cine-MR images and angiography than between SE-MR images and angiography (r = 0.99 vs. r = 0.78; p = 0.001 vs. p = 0.004), related to overestimation of stenoses with SE-MR imaging. The use of a combination of spin-echo and cine-MR imaging correlates well with conventional angiographic findings in this small series of patients with coarctation of aorta or postoperative pseudoaneurysmal complications. Cine-MRI can provide anatomic information that is equivalent to angiography for surgical planning.     

Peliosis hepatis and neoplastic/dysplastic lesions in aged male Long-Evans Cinnamon rats: MR imaging with pathologic correlation

The Long-Evans Cinnamon (LEC) rat, an animal model of Wilson's disease, abnormally accumulates copper in the liver. There have been a lot of reports on preneoplastic and neoplastic hepatic tumors in LEC rats, but few studies have been focused on other lesions. The aim of this study was to describe the MR findings of the liver of LEC rats with pathologic correlation to characterize the hepatic lesions developed in them. We measured MR images of the liver of six aged (over the age of 70 weeks old) male LEC rats. Measurements of T(1), T(2)-weighted images, and the dynamic and delayed studies after i.v. gadolinium injection were performed. The rats were sacrificed immediately after the measurements, and the diagnosis was histologically made. We identified seven lesions of peliosis hepatis, three neoplastic/dysplastic lesions, three cysts and one cholangiofibrosis. Peliosis hepatis was characterized as showing a significantly long T(2) relaxation time of 57.9 +/- 13.3 ms (mean +/- standard deviation) compared with 41.3 +/- 1.7 ms in normal liver, and prolonged enhancement after a gadolinium injection. Neoplastic/dysplastic lesions tended to show prolonged T(2), and they showed isointensity on T(1)-weighted images. They were best characterized by early enhancement followed by a rapid wash-out after a gadolinium injection. In conclusions, the frequent occurrence of peliosis hepatis observed in the present study suggests this can be a characteristic lesion in aged LEC rats. The characteristic MR findings enable us to distinguish between peliosis hepatis and neoplastic/dysplastic lesions.     

Magnetization prepared rapid gradient-echo (MP-RAGE) MR imaging of the liver: comparison with spin-echo imaging.

We have implemented an MR technique that employs a rapid gradient echo sequence, preceded by magnetization preparation pulses to provide T1- and T2-weighted tissue contrast. With this technique, which can be identified as a member of a new family of pulse sequences, generically named Magnetization Prepared RApid Gradient Echo (MP-RAGE), very short repetition times are used, allowing acquisition times of less than one second and images virtually free of motion-induced artifacts during quiet respiration. Fifteen patients with known liver lesions (metastases, hemangiomas, and cysts) were examined using T1- and T2-weighted 2-dimensional MP-RAGE sequences, and the images were compared with conventional T1- and multi-echo T2-weighted spin-echo (SE) sequences. Signal difference-to-noise ratios (SD/Ns) of the lesions were calculated for all pulse sequences using corresponding axial images and were normalized for voxel volume. The mean normalized SD/Ns of the MP-RAGE sequences were generally comparable to those for the SE sequences. In addition, there were no noticeable respiratory artifacts on the MP-RAGE images whereas these were clearly present on the T2-weighted SE images and to a lesser degree on the T1-weighted SE images. It is concluded that the MP-RAGE technique could become an important method for evaluating the liver for focal disease.     

Evaluation of a contraceptive device with MR imaging.

The purpose of this investigation is to evaluate the positioning and to confirm the volume concept of the Lea's Shield diaphragm utilizing MR imaging. We evaluated the device in two women, one nulliparous and one multiparous. We were able to comprehensively evaluate the device in both patients and answer all questions regarding anatomical positioning and aspects pertaining to the morphology of the device relevant to its function. MRI may be effectively utilized to evaluate contraceptive devices and their relationship to adjacent anatomical structures. This may enhance the gynecologist's clinical assessment of its correct positioning and efficacy.     

Fast flair imaging of the brain using the fast spin-echo and gradient spin-echo technique

The purpose of this study was to compare the gradient spin-echo (GRASE) to the fast spin-echo (FSE) implementation of fast fluid-attenuated inversion recovery (FLAIR) sequences for brain imaging. Thirty patients with high signal intensity lesions on T₂-weighted images were examined on a 1.5 T MR system. Scan time-minimized thin-section FLAIR-FSE and FLAIR-GRASE sequences were obtained and compared side by side. Image assessment criteria were lesion conspicuity, contrast between different types of normal tissue, image quality, and artifacts. In addition, contrast ratios and contrast-to-noise ratios were determined. Compared to FSE, the GRASE technique allowed a 17% reduction in scan time but conspicuity of small lesions in particular was significantly lower on FLAIR-GRASE images because of higher image noise and increased artifacts. Gray-white differentiation was slightly worse on FLAIR-GRASE. Physiological ferritin deposition appeared slightly darker on FLAIR-GRASE images and susceptibility artifacts were stronger. Fatty tissue was less bright with FLAIR-GRASE. With current standard hardware equipment, the GRASE technique is not an adequate alternative to FSE for the implementation of fast FLAIR sequences in routine clinical MR brain imaging.     

MR and CT imaging of ethanol-treated liver tumors in an animal model

To characterize the radiographic appearance of liver lesions over time following ethanol injection, seven New Zealand white rabbits underwent surgical implantation of small fragments of VX-2 carcinoma within the liver. Upon reaching 1 cm in diameter, a tumor nodule was directly injected with absolute ethanol. Another nodule in the same animal was injected with saline as a control. Imaging was performed 6-24 days after the injections by high resolution CT and MRI, and correlation obtained with the pathologic specimens. Long TR spin-echo MR sequences were found to characterize the ethanol-treated regions of liver most accurately. Liver tissue infarcted by alcohol could be differentiated from tumor and necrosis by virtue of its short T2 relaxation value. There were no distinguishing features by other imaging techniques between the ethanol-treated and control tumor nodules. Peripheral contrast enhancement was demonstrated in both, corresponding to fibrous tissue around the ethanol-injected regions, and to viable tumor in the case of controls.     

Formation of multifunctional Fe_3O_4/Au composite nanoparticles for dual-mode MR/CT imaging applications

Recent advances with iron oxide/gold(Fe3O4/Au) composite nanoparticles(CNPs) in dual-modality magnetic resonance(MR) and computed tomography(CT) imaging applications are reviewed. The synthesis and assembly of "dumbbelllike" and "core/shell" Fe3O4/Au CNPs is introduced. Potential applications of some developed Fe3O4/Au CNPs as contrast agents for dual-mode MR/CT imaging applications are described in detail.     

Gd-enhanced MR imaging of brain metastases: Contrast as a function of dose and lesion size

MR imaging contrast of brain metastases after cumulative doses of gadolinium chelate is quantitated and compared in order to assess the clinical utility of high dosage. T1-weighted spin-echo MR images of 39 patients with metastatic brain tumors were made before and after each of three equal doses cumulating to 0.1, 0.2 and 0.3 mmol Gd-complex per kg body weight. Quantitation of MRI contrast was limited to homogeneous brain metastases larger than 3 mm (n = 246). Post-Gd MRI contrast doubled with dose escalation from 0.1 to 0.3 mmol/kg and also increased with lesion size, by a factor of 2.5 between metastases of 3 and 16 mm diameter, that is after correcting for partial volume effect. At 0.2 and 0.3 mmol/kg the respective numbers of visible metastases increased by 15% and 43% compared with 0.1 mmol/kg (p < 0.0001, both). Image contrast figures differed significantly between doses (p = 0.018). Both the number of metastases and the image contrast is significantly higher when dose escalation is performed. It is indicated that the number of detected metastases will increase further at Gd doses beyond 0.3 mmol/kg. Post-Gd MRI contrast increases with lesion size, to an extent that can not be attributed to partial volume attenuation.     

MR imaging of hand and wrist with a dedicated 0.1-T low-field imaging system

We describe the first results of a new magnetic resonance imaging (MRI) system specially developed for hand and wrist imaging. The system uses a small resistive water-cooled magnet with a vertical magnetic field of 0.1 T in an air gap of 15 cm. The console is based on a microcomputer with a vector signal processor and an image-processing board. There is actually no Faraday cage. For the whole hand, the in-plane spatial resolution is less than 1 mm in the 128 × 128-pixels format for typical slice thicknesses of 3 to 5 mm. Solenoidal volume coils for fingers were developed, giving, in the same matrix format, an in-plane high spatial resolution of 0.22 mm for a typical slice thickness of 3 mm.     

Encoding to the longitudinal magnetization for MR imaging and flow velocity mapping

Phase-encoding to the longitudinal magnetization is implemented by adding encoding gradient pulses in the evolution period tau of the NMR pulse sequence 90 degrees+x-tau-90 degrees-x. This work focuses on the effect of the spin-lattice relaxation and its removal and on the constraint that the 90 degrees-x pulse can only transform the phase of the transverse magnetization partially to the longitudinal magnetization. Theoretical analysis shows that the encoded phase information and the spin-lattice relaxation effect are separable and the latter is identical in each repetition in collecting phase-encoding data. Thus the relaxation effect can be eliminated by subtracting a second data set whose phase information is inverted or by alternating the polarity of the relaxation contribution. From data with partial phase information, Fourier-transform image reconstruction results in mirror aliasing in which the two halves of the Fourier spectrum of positive and negative coordinates overlap. Removal of mirror aliasing requires imaging data of the orthogonal component. Nevertheless mirror aliasing is not necessarily a problem, depending on the subject of study. Phase-encoding to the longitudinal magnetization for spatial MRI and flow velocity mapping are demonstrated using the rotating ultra-fast imaging sequence (RUFIS).     

Improving background suppression in diffusion-weighted imaging of the abdomen and pelvis using STIR with single-axis diffusion encoding

Bowel contents and peripheral nerves often show high signal intensity at diffusion-weighted imaging (DWI) of the abdomen and pelvis, obscuring or mimicking pathology. This study introduced and compared short tau inversion recovery (STIR) with single-axis DWI to the usual combination of spectral fat suppression (SFS) with trace DWI in the suppression of bowel contents and peripheral nerves. Five volunteers underwent both STIR with single-axis DWI and SFS with trace DWI of the abdomen and pelvis. Images were evaluated by two observers with respect to the suppression of signal of bowel contents and peripheral nerves using four-point grading scales (4=poor suppression; 3=moderate suppression; 2=good suppression; 1=excellent suppression). Mean scores (±S.D.) regarding the suppression of bowel contents were 1.60±0.55 and 1.40±0.89 for STIR with single-axis DWI and 3.40±0.55 and 3.00±1.00 for SFS with trace DWI for observers 1 and 2, respectively. Mean scores regarding the suppression of peripheral nerves were 1.20±0.45 and 1.20±0.45 for STIR with single-axis DWI and 2.40±0.89 and 2.80±0.84 for SFS with trace DWI for observers 1 and 2, respectively. In conclusion, STIR with single-axis DWI is superior to SFS with trace DWI in the suppression of bowel signal and peripheral nerves.     

A generalized method of converting CT image to PET linear attenuation coefficient distribution in PET/CT imaging

The accuracy of attenuation correction in positron emission tomography scanners depends mainly on deriving the reliable 511-keV linear attenuation coefficient distribution in the scanned objects. In the PET/CT system, the linear attenu- ation distribution is usually obtained from the intensities of the CT image. However, the intensities of the CT image relate to the attenuation of photons in an energy range of 40 keV-140 keV. Before implementing PET attenuation correction, the intensities of CT images must be transformed into the PET 511-keV linear attenuation coefficients. However, the CT scan parameters can affect the effective energy of CT X-ray photons and thus affect the intensities of the CT image. Therefore, for PET/CT attenuation correction, it is crucial to determine the conversion curve with a given set of CT scan parameters and convert the CT image into a PET linear attenuation coefficient distribution. A generalized method is proposed for con- verting a CT image into a PET linear attenuation coefficient distribution. Instead of some parameter-dependent phantom calibration experiments, the conversion curve is calculated directly by employing the consistency conditions to yield the most consistent attenuation map with the measured PET data. The method is evaluated with phantom experiments and small animal experiments. In phantom studies, the estimated conversion curve fits the true attenuation coefficients accurately, and accurate PET attenuation maps are obtained by the estimated conversion curves and provide nearly the same correction results as the true attenuation map. In small animal studies, a more complicated attenuation distribution of the mouse is obtained successfully to remove the attenuation artifact and improve the PET image contrast efficiently.     

Single-shot fast spin-echo diffusion tensor imaging of the lumbar spine at 1.5 and 3 T

Diffusion tensor imaging (DTI) of the lumbar spine could improve diagnostic specificity. The purpose of this work was to determine the feasibility of and to validate DTI with single-shot fast spin-echo (SSFSE) for lumbar intervertebral discs at 1.5 and 3 T. Six normal volunteers were scanned with DTI-SSFSE using an eight- and a three-b-value protocol at 1.5 and 3 T, respectively. Apparent diffusion coefficient (ADC) values were computed and validated based on those obtained at 1.5 T from corresponding diffusion tensor scans using line scan diffusion imaging (LSDI), a technique that has been previously validated for use in the spine. Pearson correlation coefficients for LSDI and DTI-SSFSE ADC values were .88 and .89 for 1.5 and 3 T, respectively, with good quantitative agreement according to the Bland-Altman method. Results indicate that DTI-SSFSE is a candidate as a clinical sequence for obtaining diffusion tensor images of the lumbar intervertebral discs with scan times shorter than 4 min.     

Evaluation of a PbTe detector for infrared imaging purposes

Summary An infra-red detector produced by the Elettronica SPA Company for the European Space Agency to be used in the Earth surface mapping from aircraft has been tested in our laboratory. We present the characteristics of the detector working in the spectral range (1÷2.5) μm. The acquisition system and the electronics are discussed and a detailed study of all the noise sources is presented; the measured NEP is 3.2·10⁻¹⁴W Hz^−1/2 at a temperature of 108 K. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Quantitative mapping of transverse relaxivity (1/T(2)) in hepatic iron overload: a single spin-echo imaging methodology

Recent research into the non-invasive assessment of hepatic iron concentrations using magnetic resonance imaging has shown that the proton transverse relaxivity (1/T(2)) varies linearly with liver iron concentration. However, the development of an image-based system for the assessment of hepatic iron distribution has been confounded by the presence of motion induced artifacts in the T(2)-weighted images. We report on the development of a single spin-echo imaging methodology that enables the generation of transverse relaxivity maps over the liver. A simple smoothing technique is used to accommodate the image intensity perturbations caused by abdominal motion. The relaxivity maps are consistent with the variation of iron concentration throughout the liver. A Parzen density estimate and histogram of the relaxivity distribution are generated to assist in the visual assessment of the degree and variability of T(2) shortening with liver iron loading. It was found that one or two Gaussian functions could be used to characterize the relaxivity distributions with a small number of parameters. We propose that this methodology may be used in the clinical setting to monitor hepatic iron concentrations in the advent of an accurate transverse relaxivity calibration curve.