Reduce Imaging Time Research Articles

Potential of three-dimensional ultrasound in neonatal and paediatric neurosonography.

The aim of this study was to describe the potential of three-dimensional ultrasound (3D US) in paediatric and neonatal neurosonography. The potential applications are illustrated based on our experience in 150 patients using three different 3D US techniques at two different sites. Various disease entities throughout the paediatric age have been evaluated. The potential of 3D US, including 3D US of the cerebral vessels based on colour Doppler data, is discussed based on comparison with conventional 2D US or other imaging (as available), and with regard to the literature. In our experience, 3D US is feasible in neonatal and paediatric neurosonography. It reduces imaging time, improves demonstration of complex anatomy and vasculature, and allows for evaluation of anatomy/pathology in any plane. The 3D US furthermore improves volume assessment (e.g. in hydrocephalus), and comparison with CT, MRI and during follow-up, with a potentially improved standardisation and documentation. The 3D US additionally offers an ideal modality for training and education, as the brain and the neonatal spine can be virtually rescanned at the workstation. Yet, limitations such as areas inaccessible to 2D US, limited resolution and motion artefacts have to be acknowledged. Three-dimensional US has the potential to become a valuable additional imaging tool in paediatric neurosonography.

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

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. 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. 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. 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.

Prospective navigator gating with a dual acceptance window technique to reduce respiratory motion artifacts in 3D MR coronary angiography.

A prospective navigator algorithm with a dual acceptance window (DAW) technique was developed to reduce image artifacts induced by respiratory motion without increasing imaging time. A phantom study shows that the ghost level measured by ghost-to-image ratio was reduced by 27.1 % (p < 0.005) using the DAW technique compared to the conventional single acceptance window technique. This DAW technique can also be used to reduce imaging time while maintaining comparable ghosting level.

Stress cine MRI for detection of coronary artery disease

Stress testing is the cornerstone in the diagnosis of patients with suspected coronary artery disease (CAD). Stress echocardiography has become a well-established modality for the detection of ischemia-induced wall motion abnormalities. However, display and reliable interpretation of stress echocardiography studies are user-dependent, the test reproducibility is low, and 10 to 15 % of patients yield suboptimal or non-diagnostic images. Due to its high spatial and contrast resolution, MRI is known to permit an accurate determination of left ventricular function and wall thickness at rest. Early stress MRI studies provided promising results with respect to the detection of CAD. However, the clinical impact was limited due to long imaging time and problematic patient monitoring in the MRI environment. Recent technical improvements - namely ultrafast MR image acquisition - led to a significant reduction of imaging time and improved patient safety. Stress can be induced by physical exercise or pharmacologically by administration of a beta1-agonist (dobutamine) or vasodilatator (dipyridamole and adenosine). The best developed and most promising stress MRI technique is a high-dose dobutamine/atropine stress protocol (10, 20, 30, 40 microgram/kg/min; optionally 0.25-mg fractions of atropine up to maximal dose 1 mg). Severe complications (myocardial infarction, ventricular fibrillation and sustained tachycardia, cardiogenic shock) may be expected in 0.25 % of patients. Currently, data of three high-dose dobutamine stress MRI studies are available, revealing a good sensitivity (83 - 87 %) and specificity (83 - 86 %) in the assessment of CAD. The direct comparison between echocardiography and MRI for the detection of stress-induced wall motion abnormalities yielded better results for dobutamine-MRI in terms of sensitivity (86.2 % vs. 74.3 %; p < 0.05) and specificity (85.7 % vs. 69.8 % p < 0.05) as compared to dobutamine stress echocardiography. The superior results of MRI can mainly be explained by the better image quality with sharp delineation of the endocardial and epicardial borders. Currently, stress MRI is already a realistic clinical alternative for the non-invasive assessment of CAD in patients with impaired image quality in echocardiography.

Abdominal imaging with a modular combination of spin and gradient echoes.

MR-CAT (combined acquisition technique), a modular hybrid imaging concept, was introduced recently. In this article it is demonstrated that the CAT principles can be applied to form a versatile combination of spin and gradient echoes for abdominal imaging. This CAT approach, which essentially integrates RARE and EPI modules in a sequential fashion, was used to implement a set of segmented and single-shot RARE/EPI-CAT imaging techniques. CAT was used in in vivo studies to perform high-resolution abdominal imaging in five healthy subjects. The results demonstrate the feasibility of abdominal imaging using the proposed CAT approach and the potential of this technique to reduce imaging time while preserving image quality.

Coded Aperture Nuclear Scintigraphy: A Novel Small Animal Imaging Technique

We introduce and demonstrate the utility of coded aperture (CA) nuclear scintigraphy for imaging small animals. CA imaging uses multiple pinholes in a carefully designed mask pattern, mounted on a conventional gamma camera. System performance was assessed using point sources and phantoms, while several animal experiments were performed to test the usefulness of the imaging system in vivo, with commonly used radiopharmaceuticals. The sensitivity of the CA system for 99mTc was 4.2 x 10(3) cps/Bq (9400 cpm/microCi), compared to 4.4 x 10(4) cps/Bq (990 cpm/microCi) for a conventional collimator system. The system resolution was 1.7 mm, as compared to 4-6 mm for the conventional imaging system (using a high-sensitivity low-energy collimator). Animal imaging demonstrated artifact-free imaging with superior resolution and image quality compared to conventional collimator images in several mouse and rat models. We conclude that: (a) CA imaging is a useful nuclear imaging technique for small animal imaging. The advantage in signal-to-noise can be traded to achieve higher resolution, decreased dose or reduced imaging time. (b) CA imaging works best for images where activity is concentrated in small volumes; a low count outline may be better demonstrated using conventional collimator imaging. Thus, CA imaging should be viewed as a technique to complement rather than replace traditional nuclear imaging methods. (c) CA hardware and software can be readily adapted to existing gamma cameras, making their implementation a relatively inexpensive retrofit to most systems.

What’s up in nuclear medicine?

Well, a lot of it. SPET/CT and indeed PET/CT have arrived, and we are witnessing the first applications. Instantaneous image fusion, accurate co-registration, and often reduced imaging times are real benefits. The world of radiotherapy planning may change forever—who knows how intensity modulated radiation therapy will change and benefit from routinely available PET/CT? Of course all of this presents us with new opportunities, challenges and training development. But the buzz is here, and here to stay for a number of years. Oh yes, I’ve heard it all before, in the 70s, 80s, and 90s, that nuclear medicine is about to fall off the plane—a little like MRI pushing CT off balance—and look what has really happened … In the meantime the reader would be well advised to not just read but to carefully consider the article written in Scientific American in its October 2001 issue. Simply entitled ‘Magic bullets fly again’, it describes the real progress achieved with labelled monoclonal antibodies now available to treat lymphoma. A large number of these have or are about to obtain FDA approval for large-scale application in a variety of cancers. Rituximab may exceed the one billion dollar market—the first such antibody to pass this milestone. In the meantime many of us are still busy with the sentinel lymph node approach in the management of the surgical and oncological patient. The next world conference in Yokohama will attract over 1000 attendees, all engaged in this application of the radioactive tracer. Melanoma, ca breast, head and neck, gynaecological ca, oesophageal ca, will all be subject to intense debate. And yes, nuclear medicine has raised the level of debate: on what is the standard of care in respect to axillary lymph node dissections, or extensive basin dissections when the sentinel node is clearly negative? Nuclear medicine has offered new opportunities for pathologists to focus their efforts on a few preselected nodes rather than on a large number of randomly sampled ones… And the discussion rages between H&E,IHC,PCR, and all because a tiny amount of labelled colloid allows you to take out the villain in question… And so that I do not forget, many of us are also engaged in the study of movement disorders and making use of the new ligand for the dopamine transporter, an I-123 labelled compound recently introduced in the UK market. Indeed the buzz is here, and here to stay…

Coil-by-coil image reconstruction with SMASH.

The SiMultaneous Acquisition of Spatial Harmonics (SMASH) technique uses linear combinations of undersampled datasets from the component coils of an RF coil array to reconstruct fully sampled composite datasets in reduced imaging times. In previously reported implementations, SMASH reconstructions were designed to reproduce the images that would otherwise be obtained by simple sums of fully gradient encoded component coil images. This strategy has left SMASH images vulnerable to phase cancellation artifacts when the sensitivities of RF coil array elements are not suitably phase-aligned. In fully gradient encoded imaging schemes these artifacts can be eliminated using a variety of methods for combining the individual coil images, including matched filter combinations as well as sum of squares combinations. Until now, these reconstruction schemes have been unavailable to SMASH reconstructions as SMASH produced a final composite image directly from the raw component coil k-space datasets. This article demonstrates a modification to SMASH that allows reconstruction of a full set of accelerated individual component coil images by fitting component coil sensitivity functions to a complete set of spatial harmonics tailored for each coil in the array. Standard component coil combinations applied to the individual reconstructed images produce final composite images free of phase cancellation artifacts.

Principles and applications of echo-planar imaging: a review for the general radiologist.

Echo-planar imaging is a very fast magnetic resonance (MR) imaging technique capable of acquiring an entire MR image in only a fraction of a second. In single-shot echo-planar imaging, all the spatial-encoding data of an image can be obtained after a single radio-frequency excitation. Multishot echo-planar imaging results in high-quality images comparable to conventional MR images. However, echo-planar imaging offers major advantages over conventional MR imaging, including reduced imaging time, decreased motion artifact, and the ability to image rapid physiologic processes of the human body. The use of echo-planar imaging has already resulted in significant advances in clinical diagnosis and scientific investigation, such as in evaluation of stroke and functional imaging of the human brain, respectively. The clinical indications for echo-planar imaging are expanding rapidly, and it can now be applied to many parts of the body, including the brain, abdomen, and heart. Today, with the availability of echo-planar imaging-capable MR imagers at many sites, the general radiologist can benefit from echo-planar imaging and its clinical applications.

Magnetic Resonance Imaging of the Fetal and Neonatal Central Nervous System

After completing this article, readers should be able to: 1. Compare and contrast magnetic resonance technology with computed tomography and ultrasonography. 2. Delineate the primary clinical uses of magnetic resonance spectroscopy, perfusion magnetic resonance, functional magnetic resonance imaging, and diffusion imaging. 3. Describe the primary uses of magnetic resonance imaging in congenital and developmental abnormalities, neurovascular disease, trauma, infections, inflammatory processes, and metabolic and neurodegenerative disorders. Magnetic resonance (MR) is one of several neuroimaging technologies used to diagnose fetal and neonatal central nervous system (CNS) abnormalities. It can aid in differentiating static from progressive encephalopathies and in diagnosing such conditions as congenital and developmental abnormalities, trauma, neurovascular disease, infections and inflammatory processes, and metabolic and neurodegenerative disorders. Although it also is helpful in diagnosing neoplastic processes, which are rare but important causes of progressive encephalopathy, these presentations are not considered in this article. MR, which employs magnetic fields and radiowaves, is a less invasive imaging technology than ultrasonography (US), computed tomography (CT), and nuclear medicine (NM), but it is more expensive. In contrast to the other modalities, which derive their signals from single parameters, the MR signal is derived exponentially from multiple parameters (eg, T1, T2, proton density, T2*, proton flow, proton relaxation enhancement, chemical shift, and molecular diffusion). MR also employs many more basic imaging techniques than other modalities, including spin echo, inversion recovery, gradient echo, and chemical shift imaging methods. Advancing MR capabilities have improved its sensitivity, specificity, and efficiency, particularly compared with US, CT, and NM. Among these advanced capabilities are the flow-attenuated inversion recovery technique (FLAIR), fat suppression short inversion time-inversion recovery imaging, and magnetization transfer imaging for increased structural resolution. Fast and ultrafast MR imaging techniques (fast spin echo, fast gradient echo, echo planar imaging) have reduced imaging times, improved structural resolution, and provided functional imaging. Specific applications …

300 MHz continuous wave electron paramagnetic resonance spectrometer for small animal in vivo imaging

Design and construction of an electron paramagnetic resonance (EPR) spectrometer, operating in the continuous wave mode in the radio frequency (rf) region, and capable of performing spectroscopy and in vivo imaging of paramagnetic spin probes is described. A resonant frequency of 300 MHz was chosen to provide the required sensitivity at nontoxic levels of commonly used spin probes and penetration of the rf in small animals. Three major components, the magnet, the radio frequency signal detection bridge, and the data acquisition module are described in this article. Integration of a rapid scan capability to reduce imaging time is also described. Two- and three-dimensional EPR images of the spin probe distribution in phantom objects as well as from in vivo experiments are reported. From the EPR images, morphology of some internal organs could be recognized. EPR images of the spin probe distribution in mice suggest differences in perfusion of the spin probe between normal and tumor regions. Addition of a spectral dimension to spatial images should enable differentiation of oxygen status in normal and pathological conditions.

Simultaneous noninvasive determination of regional myocardial perfusion and oxygen content in rabbits: toward direct measurement of myocardial oxygen consumption at MR imaging.

To determine whether myocardial arterial perfusion and oxygen concentration can be quantified simultaneously from the same images by using spin labeling and the blood oxygenation level-dependent (BOLD) effect with fast spin-echo (SE) imaging. A T2-weighted fast SE pulse sequence was written to image isolated, arrested, blood-perfused rabbit hearts (n = 6) at 4.7 T. Perfusion images with intensity in units of milliliters per minute per gram that covered the entire left ventricle with 0.39 x 0.39 x 3.00-mm resolution were obtained in less than 15 minutes with a 32-fold reduction in imaging time from that of a previous study. Estimates of oxygen concentration were made from the same images acquired for calculation of perfusion images. Estimates of regional myocardial oxygen content could be made from the perfusion images; this demonstrated the feasibility of three-dimensional calculation of regional oxygen consumption, which requires concomitant measurement of both oxygen content and flow. Fast SE imaging was shown to be as sensitive to hemoglobin desaturation as standard SE imaging. Perfusion abnormalities and oxygen deficits were easily identified and verified qualitatively with gadopentetate dimeglumine on both perfusion and BOLD images obtained after coronary arterial ligation. T2-weighted fast SE imaging combined with perfusion-sensitive spin labeling can be used to measure myocardial arterial perfusion and oxygen concentration. This provides the groundwork for calculation of regional myocardial oxygen consumption.

Motion-selective encoding for fast cine imaging

In magnetic resonance (MR) cine imaging applications, it is common that only a fraction of the field of view is used for delineating dynamic features whereas surrounding static structures are merely served as landmarks for three-dimensional orientation. This paper presents a method of using motion-selective encoding for fast cine imaging. By the use of a simple RF saturation pulse, the imaging field of view was divided into two areas, one with motion and the other without. This allowed the static material to be imaged by fast ungated acquisition and dynamic structures by reduced phase encoding. Issues related to phase error correction were addressed and the proposed algorithm was validated using images acquired from five normal subjects. The saving in imaging time was dependent on the size of the moving structures and a 35% reduction in imaging time did not cause visually noticeable changes in image quality. The application of the technique is limited to cine images with large surrounding static structures and a theoretical analysis of maximum savings in imaging time in relation to the size of the moving structures within the image field of view is provided. Magn Reson Med 42:430-435, 1999.

Fast magnetic resonance coronary angiography with a three-dimensional stack of spirals trajectory.

In this work, three-dimensional (3D) spiral imaging has been utilized for magnetic resonance coronary angiography. Spiral-based 3D techniques can dramatically reduce imaging time requirements compared with 3D Fourier Transform imaging. The method developed here utilized a "stack of spirals" trajectory, to traverse 3D k-space rapidly. Both thick-slab volumes encompassing the entire coronary tree with isotropic resolution and thin-slab volumes targeted to a particular vessel of interest were acquired. Respiratory compensation was achieved using the diminishing variance algorithm. T2-prepared contrast was also applied in some cases to improve contrast between vessel and myocardium, while off-resonance blurring was minimized by applying a linear correction to the acquired data. Images from healthy volunteers were displayed using a curved reformatting technique to view long segments of vessel in a single projection. The results demonstrate that this 3D spiral technique is capable of producing high-quality coronary magnetic resonance angiograms.

Few-view tomography using roughness-penalized nonparametric regression and periodic spline interpolation

The ability to reconstruct high-quality tomographic images from a smaller number of projections than is usually used could reduce imaging time for many nuclear-medicine studies. This would particularly benefit studies such as cardiac SPECT where patient motion during long acquisitions can lead to motion artifacts in the reconstructed images. To this end, the authors have investigated sinogram pre-processing techniques designed to enable filtered backprojection (FBP) to produce high-quality reconstructions from a small number of views. Each projection is first smoothed by performing roughness-penalized nonparametric regression using a generalized linear model that explicitly accounts for the Poisson statistics of the data. The resulting fit curves are natural cubic splines. After smoothing, additional angular views are generated using periodic spline interpolation, and images are reconstructed using FBP. The algorithm was tested on data from SPECT studies of a cardiac phantom placed at various radial offsets to enable examination of the algorithm's dependence on the radial extent of the object being imaged.

Velocity encoding using both phase and magnitude.

Imaging time constitutes a major limitation of phase-contrast (PC) angiography. It is possibly the main disadvantage of PC methods over the time-of-flight (TOF) methods that actually are used clinically. This relatively long imaging time comes from the fact that conventional PC methods require the acquisition of at least four images with different velocity sensitization to reconstruct a single angiogram (1, 2). However, more than one-half of the information gathered through the acquisition of these four images is either redundant or simply discarded. We propose a faster approach to making PC angiograms in which the quantity of data acquired is diminished by as much as a factor 2. This is made possible by encoding velocity information in both the phase and magnitude of the image. Due to the use of extra radiofrequency (RF) and gradient waveforms, decreases in data requirements do not translate in a direct manner into decreases in imaging time. Nevertheless, significant reductions in imaging time are achieved with the present approach.

High Resolution Magnetic Resonance Imaging of Neurovascular Orbital Anatomy

This study describes the magnetic resonance imaging (MRI) anatomy of the blood vessels and nerves of the orbit to provide a morphological basis for the interpretation of clinical findings. Seven volunteers aged 29 to 54 years underwent high-resolution MRI of the orbit on a 1 Tesla unit (Impact, Siemens, Germany). T1-weighted oblique-sagittal, coronal, and axial images were obtained using a surface coil. Anatomic structures on the magnetic resonance images were identified by comparison with corresponding histologic sections of the orbit. The ophthalmic artery and most of its branches (central retinal artery, posterior ciliary arteries, lacrimal artery, anterior and posterior ethmoidal arteries, supratrochlear artery, supraorbital artery, dorsal nasal artery) are visualized. The superior ophthalmic vein, the lacrimal vein, the medial ophthalmic vein, the inferior ophthalmic vein, the medial and lateral collateral veins, and the vorticose veins are also delineated. Furthermore, branches of the oculomotor nerve, the abducens nerve, the frontal nerve, the nasociliary nerve, the lacrimal nerve and the infraorbital nerve are identified in the magnetic resonance images. High-resolution MRI is capable of delineating the orbital arteries, veins, and nerves. This is mainly based on two principles. First, blood vessels appear mostly dark on magnetic resonance images because of the signal void of flowing blood. Second, the bright background of the orbital fat on T1-weighted magnetic resonance images accounts for the good soft-tissue contrast in the orbit. With improved MRI technology and reduced imaging time, high-resolution-MRI may be applied routinely for diagnostic purposes.

Functional MRI of the patellofemoral joint: comparison of ultrafast MRI, motion-triggered cine MRI and static MRI

In recent years ultrafast gradient echo sequences have been used in MRI that allow functional MRI studies of the joints during active motion in an acquisition time of seconds. To evaluate the feasibility and usefulness of ultrafast MRI (u), patellar tracking from 30° of flexion to knee extension (0°) was analysed and compared with motion-triggered cine MRI (m) and a static MRI technique (s). The different imaging methods were compared in respect of the patellofemoral relationship, the examination time and image quality. Eight healthy subjects and four patients (in total 18 joints) with patellar subluxation or luxation were examined. Significant differences between the static MRI series without quadriceps contraction and the functional MRI studies (motion-triggered cine MRI and ultrafast MRI) were found for the patellar tilt angle (PTA: P m/s = 0.01; P u/s < 0.01). In the dynamic joint studies there was no statistical difference of the regression coefficients between the motion-triggered cine MRI studies and the ultrafast MRI studies (P m/u = 0.8). The findings of the functional MRI studies compared with the static MRI images were significantly different for the lateralisation of the patella, expressed by the lateral patellar displacement (LPD: P m/s < 0.01; P u/s = 0.01) and bisect offset (BSO: P m/s = 0.01; P u/s < 0.05). No significant differences in patellar lateralisation were found between motion-triggered cine MRI and ultrafast MRI (LPD: P m/u = 0.89; BSO: P m/u = 0.33). Ultrafast MRI was superior to motion-triggered cine MRI in terms of the reduction in imaging time and improvement of the image quality.

Deconvolution of planar scintigrams by maximum entropy

Planar scintigrams are deconvolved with a point spread function using the maximum entropy method with the aim of improving image quality. The technique requires the specification of several parameters. These are related to the level of noise present in the data and the one's a priori knowledge of the object imaged. The performance of the technique is tested for a wide range of these parameters using images of a Williams phantom in scattering material and a figure of merit, derived from the detectability of the smallest cold spot, is calculated. For close to optimal values of the parameters a factor of two improvement in the figure is found. A processed bone image shows improved contrast and resolution. Maximum entropy processing could be used to increase image quality or allow comparable image quality with reduced imaging time or patient dose.

Reduction of field of view for dynamic imaging

This paper describes a simple technique that improves the temporal resolution for certain dynamic imaging applications. The technique is based on the assumption that the image to image intensity changes sought in dynamic imaging studies are sometimes localized, and a smaller field of view can be used to reduce imaging time. Technical details and experimental results are presented. Experimental results show that this technique works reasonably well for in vivo applications.