Ultrashort echo time (UTE) imaging has shown promise as a technique for imaging tissues with T2 values of a few milliseconds or less. These tissues, such as tendons, menisci, and cortical bone, are normally invisible in conventional magnetic resonance imaging techniques but have signal in UTE imaging. They are difficult to visualize because they are often obscured by tissues with longer T2 values. In this article, new long-T2 suppression RF pulses that improve the contrast of short-T2 species are introduced. These pulses are improvements over previous long-T2 suppression pulses that suffered from poor off-resonance characteristics or T1 sensitivity. Short-T2 tissue contrast can also be improved by suppressing fat in some applications. Dual-band long-T2 suppression pulses that additionally suppress fat are also introduced. Simulations, along with phantom and in vivo experiments using 2D and 3D UTE imaging, demonstrate the feasibility, improved contrast, and improved sensitivity of these new long-T2 suppression pulses. The resulting images show predominantly short-T2 species, while most long-T2 species are suppressed.
ultrashort echo time (UTE) imaging; long-T2 suppression; T2 contrast; short-T2 imaging; RF pulse design
A new method was developed to measure ultrashort T2* relaxation in tissues that contain a focal area of superparamagnetic iron oxide (SPIO) nanoparticle labeled cells in which the T2* decay is too short to be accurately measured using regular multiple gradient echo T2* mapping. The proposed method utilizes the relatively long T2 relaxation of SPIO labeled cells and acquires a series of spin echo images with the readout echo shifted to sample the T2* decay curve. MRI experiments in phantoms and rats with SPIO labeled tumors demonstrated that this method can detect ultrashort T2* down to 1 millisecond or less. Compared to the ultrashort TE technique, the SSE method overestimates the T2* values by about 11%. The longer the TE, the more the measurements deviates from the UTE technique. Combined with the regular multiple gradient echo T2* mapping, the new technique is expected to provide in vivo quantitation of highly concentrated iron labeled cells in tissues from direct cell transplantation.
Superparamagnetic iron oxide nanoparticles; cell labeling; T2* relaxometry
Tendon involvement is common in spondyloarthritis. The MRI signal from the Achilles tendon has been used to quantify mechanical tendinopathy; however, conventional MRI is limited by the short T2 of normal tendon. Short and ultrashort echo time (UTE) MRI have the potential to better measure signal intensity reflecting changes in T2 or gadolinium enhancement. Furthermore, UTE images could be used for normalisation to reduce variability. The aim of this work was to investigate such techniques in patients with spondyloarthritis (SpA).
The Achilles tendons of 14 healthy volunteers and 24 patients with symptomatic spondyloarthritis were studied. Combined UTE (TE=0.07 ms) and gradient echo (TE=4.9 ms) images were acquired before and after intravenous gadolinium together with pre-contrast gradient echo images (TE=2 ms). The signal intensity from a region of interest in the Achilles tendon above the calcaneus was measured. The relative enhancement at echo times of 0.07 ms (RE0.1) and 4.9 ms (RE5) were calculated. The ratios of the signal intensities from both 4.9 ms and 2 ms gradient echo images to the signal intensity from the UTE image were calculated (RTE5 and RTE2 respectively).
Interobserver intraclass correlation coefficients were excellent (≥0.97). The contrast-to-noise ratio was higher for enhancement on UTE images than on gradient echo images. RE0.1, RTE5 and RTE2 were significantly higher in SpA patients than controls.
Signal intensity ratios using UTE images allow quantitative measurements to be made which are sensitive to tendon T2 or contrast enhancement and which are increased in spondyloarthritis. They therefore have the potential for use as measures of tendon disease in spondyloarthritis.
Iron oxide nanoparticles (IONPs) are used in various MRI applications as negative contrast agents. A major challenge is to distinguish regions of signal void due to IONPs from those due to low signal tissues or susceptibility artifacts. To overcome this limitation, several positive contrast strategies have been proposed. Relying on IONP T1 shortening effects to generate positive contrast is a particularly appealing strategy since it should provide additional specificity when associated with the usual negative contrast from T2* effects. In this paper, Ultrashort TE (UTE) imaging is shown to be a powerful technique which can take full advantage of both contrast mechanisms. Methods of comparing T1 and T2* contrast efficiency are described and general rules that allow optimizing IONP detection sensitivity are derived. Contrary to conventional wisdom, optimizing T1 contrast is often a good strategy for imaging IONPs. Under certain conditions, subtraction of a later echo signal from the UTE signal not only improves IONP specificity by providing long T2* background suppression, but also increases detection sensitivity, as it enables a synergistic combination of usually antagonist T1 and T2* contrasts. In vitro experiments support our theory and a molecular imaging application is demonstrated using tumor-targeted IONPs in vivo.
UTE; iron oxide nanoparticle; sequence optimization; positive contrast
Manganese (Mn) is a positive magnetic resonance imaging (MRI) contrast agent that has been used to obtain physiological, biochemical, and molecular biological information. There is great interest to broaden its applications, but a major challenge is to increase detection sensitivity. Another challenge is distinguishing regions of Mn-related signal enhancement from background tissue with inherently similar contrast. To overcome these limitations, this study investigates the use of ultrashort echo time (UTE) and subtraction UTE (SubUTE) imaging for more sensitive and specific determination of Mn accumulation.
Materials and Methods
Simulations were performed to investigate the feasibility of UTE and SubUTE for Mn-enhanced MRI and to optimize imaging parameters. Phantoms containing aqueous Mn solutions were imaged on a MRI scanner to validate simulations predictions. Breast cancer cells that are very aggressive (MDA-MB-231 and a more aggressive variant LM2) and a less aggressive cell line (MCF7) were labeled with Mn and imaged on MRI. All imaging was performed on a 3 Tesla scanner and compared UTE and SubUTE against conventional T1-weighted spoiled gradient echo (SPGR) imaging.
Simulations and phantom imaging demonstrated that UTE and SubUTE provided sustained and linearly increasing positive contrast over a wide range of Mn concentrations, whereas conventional SPGR displayed signal plateau and eventual decrease. Higher flip angles are optimal for imaging higher Mn concentrations. Breast cancer cell imaging demonstrated that UTE and SubUTE provided high sensitivity, with SubUTE providing background suppression for improved specificity and eliminating the need for a pre-contrast baseline image. The SubUTE sequence allowed the best distinction of aggressive breast cancer cells.
UTE and SubUTE allow more sensitive and specific positive-contrast detection of Mn enhancement. This imaging capability can potentially open many new doors for Mn-enhanced MRI in vascular, cellular, and molecular imaging.
To demonstrate the feasibility of combining a chemical shift-based water-fat separation method (IDEAL) with a 2D ultrashort echo time (UTE) sequence for imaging and quantification of the short T2 tissues with robust fat suppression.
Materials and Methods
A 2D multislice UTE data acquisition scheme was combined with IDEAL processing, including
T2∗ estimation, chemical shift artifacts correction, and multifrequency modeling of the fat spectrum to image short T2 tissues such as the Achilles tendon and meniscus both in vitro and in vivo. The integration of an advanced field map estimation technique into this combined method, such as region growing (RG), is also investigated.
The combination of IDEAL with UTE imaging is feasible and excellent water-fat separation can be achieved for the Achilles tendon and meniscus with simultaneous
T2∗ estimation and chemical shift artifact correction. Multifrequency modeling of the fat spectrum yields more complete water-fat separation with more accurate correction for chemical shift artifacts. The RG scheme helps to avoid water-fat swapping.
The combination of UTE data acquisition with IDEAL has potential applications in imaging and quantifying short T2 tissues, eliminating the necessity for fat suppression pulses that may directly suppress the short T2 signals.
ultrashort TE (UTE); ultrashort TE spectroscopic imaging (UTESI); IDEAL; water-fat separation; T*2 estimation; artifact correction
The major thrust of modern day fluorescence laser-scanning microscopy have been towards achieving better and better depth resolution embodied by the invention and subsequent development of confocal and multi-photon microscopic techniques. However, each method bears its own limitations: in having sufficient background fluorescence and photo-damage resulting from out-of-focus illumination for the former, while low multi-photon absorption cross-sections of common fluorophores for the latter. Here we show how the intelligent choice of single-photon ultrashort pulsed illumination can circumvent all these shortcomings by exemplifying the tiny spatial stretch of an ultrashort pulse. Besides achieving a novel way of optical sectioning, this new method offers improved signal-to-noise ratio as well as reduced photo-damage which are crucial for live cell imaging under prolonged exposure to light.
Fluorescence microscopy; optical sectioning; confocal detection; multi-photon excitation; pulsed illumination; ultra-short pulses; second-harmonic generation; threshold detection
To investigate tissue dependence of the MRI-based thermometry in frozen tissue by quantification and comparison of signal intensity and T2* of ex vivo frozen tissue of three different types: heart muscle, kidney and liver.
Materials and Methods
Tissue samples were frozen and imaged on a 0.5T MRI scanner with ultrashort echo time (UTE) sequence. Signal intensity and T2* were determined as the temperature of the tissue samples was decreased from room temperature to approximately −40°C. Statistical analysis was performed for [−20°C, −5°C] temperature interval.
The findings of this study demonstrate that signal intensity and T2* are consistent across three types of tissue for [−20°C, −5°C] temperature interval.
Both parameters can be used to calculate a single temperature calibration curve for all three types of tissue and potentially in the future serve as a foundation for tissue-independent MRI-based thermometry.
cryoablation; T2*; hydration water; frozen tissue
Ultra-short echo time (UTE) imaging is a technique that can visualize tissues with sub-millisecond T2 values that have little or no signal in conventional MRI techniques. The short-T2 tissues, which include tendons, menisci, calcifications, and cortical bone, are often obscured by long-T2 tissues. This paper introduces a new method of long-T2 component suppression based on adiabatic inversion pulses that significantly improves the contrast of short-T2 tissues. Narrow bandwidth inversion pulses are used to selectively invert only long-T2 components. These components are then suppressed by combining images prepared with and without inversion pulses. Fat suppression can be incorporated by combining images with the pulses applied on the fat and water resonances. Scaling factors must be used in the combination to compensate for relaxation during the preparation pulses. The suppression is insensitive to RF inhomogeneities because it uses adiabatic inversion pulses. Simulations and phantom experiments demonstrate the adiabatic pulse contrast and how the scaling factors are chosen. In vivo 2D UTE images in the ankle and lower leg show excellent, robust long-T2 suppression for visualization of cortical bone and tendons.
Ultra-short Echo Time (UTE) Imaging; long-T2 suppression; short-T2 imaging; adiabatic pulses
To validate 23Na twisted projection magnetic resonance imaging (MRI) as a quantitative technique to assess local brain sodium concentration ([Na+]br) during rat focal ischemia every 5.3 minutes.
Materials and Methods
The MRI protocol included an ultrashort echo-time (0.4 msec), a correction of radiofrequency (RF) inhomogeneities by B1 mapping, and the use of 0–154 mM NaCl calibration standards. To compare MRI [Na+]br values with those obtained by emission flame photometry in precision-punched brain samples of about 0.5 mm3 size, MR images were aligned with a histological three-dimensional reconstruction of the punched brain and regions of interest (ROIs) were placed precisely over the punch voids.
The Bland–Altman analysis of [Na+]br in normal and ischemic cortex and caudate putamen of seven rats quantitated by 23Na MRI and flame photometry yielded a mean bias and limits of agreement (at ±1.96 SD) of 2% and 43% of average, respectively. A linear increase in [Na+]br was observed between 1 and 6 hours after middle cerebral artery occlusion.
23Na MRI provides accurate and reliable results within the whole range of [Na+]br in ischemia with a temporal resolution of 5.3 minutes and precisely targeted submicroliter ROIs in selected brain structures.
rat brain; focal ischemia; tissue sodium; 23Na MRI
In the present work it is shown that by combining the double quantum filtered, magnetization transfer (DQF-MT) and the ultra-short TE (UTE) MRI methods it is possible to obtain contrast between tissue compartments based on the following characteristics: (a) the residual dipolar coupling interaction within the biomacromolecules, which depends on their structure, (b) residual dipolar interactions within the water molecules, and (c) the magnetization exchange rate between biomacromolecules and water. The technique is demonstrated in rat tail specimen where the collagenous tissue, such as tendons and the annulus pulposus of the disc are highlighted in these images, and their macromolecular properties along with those of bones and muscles can be characterized. DQF-MT UTE MRI also holds promise because collagenous tissues that typically are invisible in conventional MRI experiments produce significant signal intensities using this approach.
DQF; MT; UTE; dipolar interaction; rat tail; tendons; annulus pulposus; intervertebral disc; short T2
Triethyl tin(TET)-induced cerebral oedema has been studied in cats by magnetic resonance imaging (MRI), and the findings correlated with the histology and fine structure of the cerebrum following perfusion-fixation. MRI is a sensitive technique for detecting cerebral oedema, and the distribution and severity of the changes correlate closely with the morphological abnormalities. The relaxation times, T1 and T2 increase progressively as the oedema develops, and the proportional increase in T2 is approximately twice that in T1. Analysis of the magnetisation decay curves reveals slowly-relaxing and rapidly-relaxing components which probably correspond to oedema fluid and intracellular water respectively. The image appearances taken in conjunction with relaxation data provide a basis for determining the nature of the oedema in vivo.
Disorganization of collagen fibers is a sign of early-stage cartilage degeneration in osteoarthritis knees. Water molecules trapped within well-organized collagen fibrils would be sensitive to collagen alterations. Multi-component T2* mapping with ultrashort echo time (UTE) acquisitions is here proposed to probe short T2 relaxations in those trapped water molecules. Six human tibial plateau explants were scanned on a 3T MRI scanner using a home-developed UTE sequence with TEs optimized via Monte Carlo simulations. Time constants and component intensities of T2* decays were calculated at individual pixels using the nonnegative least squares algorithm. Four T2*-decay types were found: 99% of cartilage pixels having mono-, bi-, or non-exponential decay, and 1% showing tri-exponential decay. Short T2* was mainly in 1-6ms while long T2* was ~22ms. A map of decay types presented spatial distribution of these T2* decays. These results showed the technical feasibility of multi-component T2* mapping on human knee cartilage explants.
T2* mapping; UTE imaging; Multi-component exponential fitting; cartilage degeneration; Osteoarthritis knee
The Automatic Quantitative Ultrashort Echo Time imaging (AQUTE) protocol for serial MRI allows quantitative in-vivo monitoring of iron labeled pancreatic islets of Langerhans transplanted into the liver, quantifying graft implantation and persistence in a rodent model.
Rats (n=14), transplanted with iron-oxide loaded cells (0–4000 islet equivalents, IEQ), were imaged using a 3D radial ultrashort echo time difference technique (dUTE) on a Siemens MAGNETOM 3T clinical scanner up to 5 months post-surgery. In-vivo 3D dUTE images gave positive contrast from labeled cells, suppressing liver signal and small vessels, allowing automatic quantification.
Position of labeled islet clusters was consistent over time and quantification of hyperintense pixels correlated with the number of injected IEQs (R2= 0.898, p < 0.0001), and showed persistence over time (5 months post-transplantation). Automatic quantification was superior to standard imaging and manual counting methods, due to the uniform suppressed background and high contrast, resulting in significant timesavings, reproducibility and ease of quantification. 3D coverage of the whole liver in the absence of cardiac/respiratory artifact provided further improvement over conventional imaging.
This imaging protocol reliably quantifies transplanted islet mass and has high translational potential to clinical studies of transplanted pancreatic islets.
MRI; ultrashort TE; rat; islet; iron oxide; quantification
Registration based mapping of geometric differences in MRI anatomy allows the detection of subtle and complex changes in brain anatomy over time that provides an important quantitative window on the process of both brain development and degeneration. However, methods developed for this have so far been aimed at using conventional structural MRI data (T1W imaging) and the resulting maps are limited in their ability to localize patterns of change within sub-regions of uniform tissue. Alternative MRI contrast mechanisms, in particular Diffusion Tensor Imaging (DTI) data are now more commonly being used in serial studies and provide valuable complementary microstructural information within white matter. This paper describes a new approach which incorporates information from DTI data into deformation tensor morphometry of conventional MRI. The key problem of using the additional information provided by DTI data is addressed by proposing a novel mutual information (MI) derived criterion termed diffusion paired MI. This combines conventional and diffusion data in a single registration measure. We compare different formulations of this measure when used in a diffeomorphic fluid registration scheme to map local volume changes. Results on synthetic data and example images from clinical studies of neurodegenerative conditions illustrate the improved localization of tissue volume changes provided by the incorporation of DTI data into the morphometric registration.
Diffeomorphic fluid registration; Deformation morphometry; Mutual information; Multi channel mutual information; Diffusion tensor imaging; Dense feature morphometry
Ultrashort echo-time enhanced T2* (UTE-T2*) mapping of articular cartilage is a novel quantitative MRI technique with the potential to visualize deep cartilage characteristics better than standard T2 mapping. The feasibility and intersession repeatability of UTE-T2* mapping of cartilage in vivo has not previously been evaluated.
Eleven asymptomatic subjects underwent repeat UTE-T2* imaging on a whole-body 3T MRI scanner on three consecutive days. Full-thickness, superficial and deep regions of interest (ROIs) were evaluated in the central weight-bearing medial femoral condyle (cMFC) and tibial plateau (cMTP). Intersession precision error across subjects was evaluated by the root-mean-square average coefficients of variation (RMSA-CV) and by the median of intra-subject standard deviations (SD) of UTE-T2* values in each ROIs.
UTE-T2* values in vivo were found to be repeatable with relative (RMSA-CV) intersession precision errors of 8%, 6%, 16% for full-thickness, superficial and deep cMFC ROIs, corresponding to absolute errors (SD) of 1.2, 1.5, 1.5ms, respectively. In cMTP tissue, UTE-T2* relative repeatability was 8%, 8%, 13%, corresponding to absolute repeatability of 1.0, 1.5, 2.1ms (full-thickness, superficial, deep). UTE-T2* values were higher in superficial cartilage compared to deep in both cMFC (p<<0.001) and cMTP (p=0.0004) regions.
In vivo 3-D UTE-T2* mapping at 3T is feasible and can be implemented using a standard clinical MRI scanner and knee coil. Intersession precision error of UTE-T2* values in full-thickness ROIs in the weight-bearing regions of asymptomatic subjects is under 1.2ms or 8% (RMSA-CV). Significant zonal and regional variations of UTE-T2* were seen.
Ultrashort echo time; T2* Mapping; Cartilage
Recently developed MRI techniques have enabled clinical imaging of short-lived 1H NMR signals with T2 < 1 ms. Using these techniques, novel signal enhancement has been observed in myelinated tissues, although the source of this enhancement has not been identified. Herein, we report studies of the nature and origins of ultra-short T2 (uT2) signals (50 μs < T2 < 1 ms) from amphibian and mammalian myelinated nerves. NMR measurements and comparisons with myelin phantoms and expected myelin components indicate that these uT2 signals arise predominantly from methylene 1H on/in the myelin membranes, which suggests that direct measurement of uT2 signals can be used as a new means for quantitative myelin mapping.
MRI; myelin; T2; ultra short TE; magnetization transfer
Examinations with a visualisation of the anatomy and pathology of the gastrointestinal (GI) tract are often necessary for the diagnosis of GI diseases. Traditional radiology played a crucial role for many years. Endoscopy, despite some limitations, remains the main technique in the differential diagnosis and treatment of GI diseases. In the last decades, the introduction of, and advances in, non-invasive cross-sectional imaging modalities, including ultrasound (US), computed tomography (CT), positron-emission tomography (PET), and magnetic resonance imaging, as well as improvements in the resolution of imaging data, the acquisition of 3D images, and the introduction of contrast-enhancement, have modified the approach to the examination of the GI tract. Moreover, additional co-registration techniques, such as PET-CT and PET-MRI, allow multimodal data acquisition with better sensitivity and specificity in the study of tissue pathology. US has had a growing role in the development and application of the techniques for diagnosis and management of GI diseases because it is inexpensive, non-invasive, and more comfortable for the patient, and it has sufficient diagnostic accuracy to provide the clinician with image data of high temporal and spatial resolution. Moreover, Doppler and contrast-enhanced ultrasound (CEUS) add important information about blood flow. This article provides a general review of the current literature regarding imaging modalities used for the evaluation of bowel diseases, highlighting the role of US and recent developments in CEUS.
Gastrointestinal tract; Bowel; Imaging; Ultrasound; Colour-Doppler; Contrast-enhancement; Time-intensity curve
The loss of proteoglycans in the articular cartilage is an early signature of osteoarthritis. The ensuing changes in the fixed charge density in the cartilage can be directly linked to sodium concentration via charge balance. Sodium ions in the knee joint appear in two pools: in the synovial fluids or joint effusion where the ions are in free motion and bound within the cartilage tissue where the Na+ ions have a restricted motion. The ions in these two compartments have therefore different T1 and T2 relaxation times. The purpose of this study is to demonstrate the feasibility of a fluid-suppressed 3D ultrashort TE radial sodium sequence by implementing an inversion recovery (IR) preparation of the magnetization at 7T. This method could allow a more accurate and more sensitive quantification of loss of PG in patients with OA. It is shown that adiabatic pulses offer significantly improved performance in terms of robustness to B1 and B0 inhomogeneities when compared to the hard pulse sequence. Power deposition considerations further pose a limit to the RF inversion power, and we demonstrate in simulations and experiments how a practical compromise can be struck between clean suppression of fluid signals and power deposition levels. Two IR sequences with different types of inversion pulses (a rectangular pulse and an adiabatic pulse) were tested on a liquid phantom, ex vivo on a human knee cadaver and then in vivo on 5 healthy volunteers, with a (Nyquist) resolution of ~3.6 mm and a signal-to-noise ratio of ~30 in cartilage without IR and ~20 with IR. Due to specific absorption rate limitations, the total acquisition time was ~17 min for the 3D radial sequence without inversion or with the rectangular IR, and 24:30 min for the adiabatic IR sequence. It is shown that the adiabatic IR sequence generates a more uniform fluid suppression over the whole sample than the rectangular IR sequence.
osteoarthritis; cartilage; sodium; magnetic resonance imaging; inversion recovery; adiabatic inversion
Extrapolation of pharmacokinetic data between species has been simplified by the advent of more sensitive methods of analysis of chemicals in body tissues and by the capability of inexpensive computers to perform complex calculations. These new methods enable investigators to observe the rates at which target tissues reach equilibrium in different species and to develop mathematical models of these processes. The evaluation of physiological pharmacokinetics from classical or compartmental kinetics is improving the ability to project the long-term behavior of chemicals in body fluids and organs based on independently derived physical, chemical, and physiological constants obtained from simple chemical reactions, tissue culture experiments, or short-term animal studies. Accurate prediction of chemical behavior by such models gives support to hypothetical mechanisms of distribution and accumulation, while significant deviations from predicted behavior signal the existence of previously unsuspected pathways. These techniques permit the simulation of the impact of linear, nonlinear, and saturation kinetics on chemical behavior; the prediction of integrated tissue exposure; and the mapping of the sequence of alternate metabolic pathways that lead to toxicity or detoxification. The discussion will identify the research needs for improving extrapolations between species.
We report the unique depiction of brown adipose tissue (BAT) by MRI and computed tomography (CT) in a human three month-old infant. Based on cellular differences between BAT and more lipid-rich white adipose tissue (WAT), chemical-shift MRI and CT were both capable of generating distinct signal contrasts between the two tissues and against surrounding anatomy, utilizing fat-signal fraction metrics in the former and X-ray attenuation values in the latter. While numerous BAT imaging experiments have been performed previously in rodents, the identification of BAT in humans has only recently been described with fusion positron emission and computed tomography in adults. The imaging of BAT in children has not been widely reported and furthermore, MRI of human BAT in general has not been demonstrated. In the present work, large bilateral supraclavicular BAT depots were clearly visualized with MRI and CT. Tissue identity was subsequently confirmed by histology. BAT has important implications in regulating energy metabolism and non-shivering thermogenesis and has the potential to combat the onset of weight gain and the development of obesity. Current findings suggest that BAT is present in significant amounts in children and that MRI and CT can differentiate BAT from WAT based on intrinsic tissue properties.
brown adipose tissue; white adipose tissue; histology; human; fat-signal fraction; X-ray attenuation
Magnetic resonance imaging (MRI) has been used quantitatively to define the characteristics of two different models of experimental cerebral oedema in cats: vasogenic oedema produced by cortical freezing and cytotoxic oedema induced by triethyl tin. The MRI results have been correlated with the ultrastructural changes. The images accurately delineated the anatomical extent of the oedema in the two lesions, but did not otherwise discriminate between them. The patterns of measured increase in T1' and T2' were, however, characteristic for each type of oedema, and reflected the protein content. The magnetisation decay characteristics of both normal and oedematous white matter were monoexponential for T1 but biexponential for T2 decay. The relative sizes of the two component exponentials of the latter corresponded with the physical sizes of the major tissue water compartments. Quantitative MRI data can provide reliable information about the physico-chemical environment of tissue water in normal and oedematous cerebral tissue, and are useful for distinguishing between acute and chronic lesions in multiple sclerosis.
Multivariate imaging techniques such as dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) have been shown to provide valuable information for medical diagnosis. Even though these techniques provide new information, integrating and evaluating the much wider range of information is a challenging task for the human observer. This task may be assisted with the use of image fusion algorithms.
In this paper, image fusion based on Kernel Principal Component Analysis (KPCA) is proposed for the first time. It is demonstrated that a priori knowledge about the data domain can be easily incorporated into the parametrisation of the KPCA, leading to task-oriented visualisations of the multivariate data. The results of the fusion process are compared with those of the well-known and established standard linear Principal Component Analysis (PCA) by means of temporal sequences of 3D MRI volumes from six patients who took part in a breast cancer screening study.
The PCA and KPCA algorithms are able to integrate information from a sequence of MRI volumes into informative gray value or colour images. By incorporating a priori knowledge, the fusion process can be automated and optimised in order to visualise suspicious lesions with high contrast to normal tissue.
Our machine learning based image fusion approach maps the full signal space of a temporal DCE-MRI sequence to a single meaningful visualisation with good tissue/lesion contrast and thus supports the radiologist during manual image evaluation.
Conventional MRI sequences do not permit the distinction between the different pathological characteristics (oedema, demyelination, gliosis, axonal loss) of the multiple sclerosis plaque. Magnetisation transfer imaging and transverse magnetisation decay curve (tMDC) analysis may be more specific. These techniques have been applied to the optic nerves in 20 patients with optic neuritis and the results correlated with clinical and visual evoked potential (VEP) findings. tMDC analysis failed to identify separate intracellular and extracellular water compartments within the optic nerve but gave a measure of transverse relaxation time (T2) without the confounding effects of CSF in the nerve sheath. Both T2 and magnetisation transfer ratio (MTR) were abnormal after an episode of optic neuritis. T2 did not correlate with visual function or with VEP latency or amplitude. There was a significant correlation between MTR reduction and prolongation of VEP latency: this increased latency may reflect an effect of myelin loss on MTR. Longer lesions were associated with worse visual outcome, implying that the overall extent of pathological involvement is likely to influence the degree of functional deficit.
Dynamic contrast-enhanced magnetic resonance imaging (DCE MRI) of the breast is a routinely used imaging method which is highly sensitive for detecting breast malignancy. Specificity, though, remains suboptimal. Dynamic susceptibility contrast magnetic resonance imaging (DSC MRI), an alternative dynamic contrast imaging technique, evaluates perfusion-related parameters unique from DCE MRI. Previous work has shown that the combination of DSC MRI with DCE MRI can improve diagnostic specificity, though an additional administration of intravenous contrast is required. Dual-echo MRI can measure both T1W DCE MRI and T2*W DSC MRI parameters with a single contrast bolus, but has not been previously implemented in breast imaging. We have developed a dual-echo gradient-echo sequence to perform such simultaneous measurements in the breast, and use it to calculate the semi-quantitative T1W and T2*W related parameters such as peak enhancement ratio, time of maximal enhancement, regional blood flow, and regional blood volume in 20 malignant lesions and 10 benign fibroadenomas in 38 patients. Imaging parameters were compared to surgical or biopsy obtained tissue samples. Receiver operating characteristic (ROC) curves and area under the ROC curves were calculated for each parameter and combination of parameters. The time of maximal enhancement derived from DCE MRI had a 90% sensitivity and 69% specificity for predicting malignancy. When combined with DSC MRI derived regional blood flow and volume parameters, sensitivity remained unchanged at 90% but specificity increased to 80%. In conclusion, we show that dual-echo MRI with a single administration of contrast agent can simultaneously measure both T1W and T2*W related perfusion and kinetic parameters in the breast and the combination of DCE MRI and DSC MRI parameters improves the diagnostic performance of breast MRI to differentiate breast cancer from benign fibroadenomas.