Comparison of intracranial 3D-ToF-MRA with and without parallel acquisition techniques at 1.5T and 3.0T: preliminary results

PURPOSE: To evaluate the performance of four 3D-ToF magnetic resonance angiography (MRA) sequences with and without integrated parallel acquisition techniques (iPAT) at 1.5T and 3.0T in imaging intracranial vessels. MATERIAL AND METHODS: Seven volunteers and 5 patients (4 aneurysms, 1 AVM) underwent 3D-ToF-MRA at 1.5T (Magnetom Sonata) and 3.0T (Magnetom Trio) with and without parallel acquisition techniques (iPAT) using similarly designed 8-channel phased-array head coils. Imaging time of the pulse sequences was set to 7.15 and 7.35 min, respectively. Images were analyzed quantitatively by calculating signal-to-noise (SNR) and contrast-to-noise (CNR) ratios of proximal M2 segments and qualitatively by using a 5-point scale. RESULTS: SNR and CNR were significantly higher for both 3D-ToF sequences at 3.0T compared with both pulse sequences at 1.5T. The highest SNR and CNR were obtained at 3.0T without iPAT. However, because of a higher spatial resolution (matrix 512 x 640) visualization of small vessel details was best at 3.0T with iPAT. CONCLUSION: Intracranial 3D-ToF-MRA at 3.0T offers superior image quality compared with 1.5T, particular in the delineation of smaller vessels. In contrast to 1.5T, implementation of iPAT at 3.0T is of additional benefit since the high SNR available at 3.0T allows for higher spatial resolution without prolongation of measurement time.     

Improved multi-station peripheral MR angiography with a dedicated vascular coil

Delineation of small branch vessels can be crucial for assessing the peripheral arterial system of patients requiring surgical grafting. Thus signal-to-noise needs to be maximized. We evaluated the performance of a dedicated peripheral vascular coil in four subjects by comparing it to the body coil using DSA as the standard of reference. SNR and CNR values of the dedicated peripheral coil exceeded those obtained with the body coil by a mean of 398%, thus permitting improved delineation of the infrapopliteal arterial morphology.     

New irradiation geometry for microbeam radiation therapy

Microbeam radiation therapy (MRT) has the potential to treat infantile brain tumours when other kinds of radiotherapy would be excessively toxic to the developing normal brain. MRT uses extraordinarily high doses of x-rays but provides unusual resistance to radioneurotoxicity, presumably from the migration of endothelial cells from 'valleys' into 'peaks', i.e., into directly irradiated microslices of tissues. We present a novel irradiation geometry which results in a tolerable valley dose for the normal tissue and a decreased peak-to-valley dose ratio (PVDR) in the tumour area by applying an innovative cross-firing technique. We propose an MRT technique to orthogonally crossfire two arrays of parallel, nonintersecting, mutually interspersed microbeams that produces tumouricidal doses with small PVDRs where the arrays meet and tolerable radiation doses to normal tissues between the microbeams proximal and distal to the tumour in the paths of the arrays.     

The design and use of a dual-frequency surface coil providing proton images for improved localization in 31P spectroscopy of small lesions

Nuclear magnetic resonance (NMR) spectroscopy of small lesions is restricted by the difficulties of localizing the surface coil with respect to the lesion and the problem of ensuring that signal is only obtained from the lesion and not from surrounding tissue. A double-tuned coil has been developed that permits NMR proton images to be obtained from a region of interest, prior to carrying out 31P spectroscopy of the same region with the same coil, without the need for further adjustment. The coil provides a means of accurately localizing the region from which the 31P signal is obtained, whilst offering a means of accurately applying 31P signal localization methods, and the possibility of making corrections for the nonuniform sensitivity of a given surface coil. The coil makes use of two parallel resonant circuits, with independent rf connections, but sharing a common coil. Simulated shorted and open circuit lambda /4 cables are used, respectively, to open circuit each circuit at the resonant frequency of the other circuit and ensure that the simulated lambda /4 line is short circuited for each circuit at the circuit's resonant frequency. At 63.6 MHz, the Q of the coil was 190 unloaded and 90 loaded, and at 25.7 MHz the Q was 210 unloaded and 140 loaded, for a 4-cm-diam coil. The coil has been used to obtain proton images and 31P spectra. A circuit employing only one input was also developed.     

MR imaging of the normal shoulder

Magnetic resonance (MR) images of the shoulders of a healthy volunteer were obtained in axial, sagittal, and coronal orientations using a 0.5-T imaging system. Multiple high-resolution spin-echo images were generated using an off-center zoom technique and a specially designed surface coil. Several anatomic structures, including the rotator cuff, long biceps tendon, articular capsule, muscles, and bones, were visualized. The coronal and sagittal views were the most useful for demonstrating the rotator cuff. MR imaging has potential as a new non-invasive tool for the evaluation of the shoulder region.     

Time-resolved contrast-enhanced three-dimensional magnetic resonance angiography of the chest: combination of parallel imaging with view sharing (TREAT)

RATIONALE AND OBJECTIVES: In view sharing, some parts of k-space are updated more often than others, leading to an effective shortening of the total acquisition time. Undersampling of high-frequency k-space data, however, can result in artifacts at the edges of blood vessels, especially during the rapid signal intensity changes. The objective of this study was to evaluate a new time-resolved echo-shared angiographic technique (TREAT) combining parallel imaging with view sharing. First, the presence of artifacts arising from different temporal interpolation schemes was evaluated in simulations of the point spread function. Second, the image quality and presence of artifacts of time-resolved parallel three-dimensional magnetic resonance angiography (3D MRA) of the chest, acquired with and without view sharing, was assessed in a clinical study of patients with cardiovascular or pulmonary disease. MATERIALS AND METHODS: Using parameters from a time-resolved parallel 3D MRA sequence without view sharing (parallel MRA), giving a 33% increase in spatial resolution, our simulations have revealed that k-space segmentation in 3 regions provides acceptable artifacts. Thirty-six consecutive patients (mean age, 50 +/- 16 years; 15 females, 22 males) were examined in a clinical study with TREAT. The image data were compared with that of a group of 31 consecutive patients (mean age, 46 +/- 19 years; 12 females, 19 males) examined with a conventional time-resolved parallel MRA sequence without view sharing (parallel MRA). The image quality and presence of artifacts was assessed in a blind comparison by 2 radiologists in consensus using MPR and MIP reconstructions. Furthermore, the peak SNR of the pulmonary artery and aorta was compared between both MRA sequences. RESULTS: The image quality of TREAT was rated significantly higher than that of the parallel MRA sequence without view sharing: depending on the orientation of MPR and MIP reconstructions, an excellent image quality was found in 69-89% with TREAT and in 45-71% with the parallel MRA protocol without view sharing, respectively. The presence of artifacts was equal with both sequences. CONCLUSION: View sharing can be successfully combined with other acceleration techniques, such as parallel imaging. TREAT allows the assessment of the thoracic vasculature with a high temporal and spatial resolution.     

Contrast-enhanced MR angiography of the run-off vasculature: intraindividual comparison of gadobenate dimeglumine with gadopentetate dimeglumine

PURPOSE: To compare intraindividually gadobenate dimeglumine (Gd-BOPTA) with gadopentetate dimeglumine (Gd-DTPA) for multi-station MR Angiography of the run-off vessels. MATERIALS AND METHODS: Twenty-one randomized healthy volunteers received either Gd-BOPTA or Gd-DTPA as a first injection and then the other agent as a second injection after a minimum interval of 6 days. Each agent was administered at a dose of 0.1 mmol/kg bodyweight followed by a 25-mL saline flush at a single constant flow rate of 0.8 mL/second. Images were acquired sequentially at the level of the pelvis, thigh, and calf using a fast three-dimensional (3D) gradient echo sequence. Source, subtracted source, maximum intensity projection (MIP), and subtracted MIP image sets from each examination were evaluated quantitatively and qualitatively on a segmental basis involving nine vascular segments. RESULTS: Significantly (P < 0.05) higher signal-to-noise and contrast-to-noise ratios were noted for Gd-BOPTA compared to Gd-DTPA, with the more pronounced differences evident in the more distal vessels. Qualitative assessmentrevealed no differences in the abdominal vasculature, a preference for Gd-BOPTA in the pelvic vasculature, and markedly better performance for Gd-BOPTA in the femoral and tibial vasculature. Summation of individual diagnostic quality scores for each segment revealed a significantly (P = 0.0001) better performance for Gd-BOPTA compared to Gd-DTPA. CONCLUSION: Greater vascular enhancement of the run-off vasculature is obtained after Gd-BOPTA, particularly in the smaller more distal vessels. Enhancement differences are not merely dose dependent, but may be due to different vascular enhancement characteristics of the agents.     

Switched array coils

Switched array coils (SACs) are a useful tool in local coil imaging since they allow in a user-friendly manner one to make the best choice in the trade-off between field of view and signal-to-noise ratio. This is done by selecting the current path within a conductor array equipped with suitable switches. Since the switching can be controlled by the system, this allows changing of the coil dimensions within multislice sequences. Thus image quality can be improved due to smaller coil dimensions for a given slice and a larger area can be covered by electronically shifting this sensitive area. The principle can be applied to surface--as well as volume--coil designs.     

Effects of injection rate and dose on image quality in time-resolved magnetic resonance angiography (MRA) by using 1.0M contrast agents

In time-resolved MRA (TR MRA), injection parameters and contrast agent (CA) dose are important factors influencing image quality. In this study, three different injection schemes with different CA volumes were evaluated in 12 healthy volunteers. Injection rates between 0.2 and 0.8 ml/s were evaluated with CA volumes of 10 and 20 ml. To measure circulatory parameters, cine cardiac MRI was performed before each exam. Spatial resolution could be reduced to 2×1.4×2 mm³, temporal resolution was 2.25 s/frame. To exclude signal saturation at high CA concentrations, a phantom with fixed CA concentrations was placed in the field of view. SNR was measured, and the area under the curve of the arterial signal of the different injection schemes was calculated. Results showed the largest diagnostic window at a relatively slow injection rate of 0.4 ml/s and a CA volume of 10 ml. Circulatory parameters have an important impact on CA arrival, so delay times have to be set depending on these parameters.