Stephen Keevil

Cardiac catheterisation guided by MRI in children and adults with congenital heart disease

BACKGROUND Fluoroscopically guided cardiac catheterisation is an essential tool for diagnosis and treatment of congenital heart disease. Drawbacks include poor soft tissue visualisation and exposure to radiation. We describe the first 16 cases of a novel method of cardiac catheterisation guided by MRI with radiographic support. METHODS In our cardiac catheterisation laboratory, we combine magnetic resonance and radiographic imaging facilities. We used MRI to measure flow and morphology, and real-time MRI sequences to visualise balloon angiographic catheters. 12 patients underwent diagnostic cardiac catheterisation, two had interventional cardiac catheterisations, and for two patients, MRI was used to plan radiofrequency ablation for treatment of tachyarrhythmias. FINDINGS In 14 patients, some or all of the cardiac catheterisation was guided by MRI. In two patients undergoing radiofrequency ablation, catheters were manipulated with use of fluoroscopic guidance and outcome was assessed with MRI. All patients received lower amounts of radiation than controls. There was some discrepancy between pulmonary vascular resistance calculated by flow derived from MRI and the traditional Fick method. We were able to superimpose fluoroscopic images of electro physiology electrode catheters on the three dimensional MRI of the cardiac anatomy. INTERPRETATION We have shown that cardiac catheterisation guided by MRI is safe and practical in a clinical setting, allows better soft tissue visualisation, provides more pertinent physiological information, and results in lower radiation exposure than do fluoroscopically guided procedures. MRI guidance could become the method of choice for diagnostic cardiac catheterisation in patients with congenital heart disease, and an important tool in interventional cardiac catheterisation and radiofrequency ablation.

Magnetic resonance imaging screening in women at genetic risk of breast cancer: imaging and analysis protocol for the UK multicentre study

The imaging and analysis protocol of the UK multicentre study of magnetic resonance imaging (MRI) as a method of screening for breast cancer in women at genetic risk is described. The study will compare the sensitivity and specificity of contrast-enhanced MRI with two-view x-ray mammography. Approximately 500 women below the age of 50 at high genetic risk of breast cancer will be recruited per year for three years, with annual MRI and x-ray mammography continuing for up to 5 years. A symptomatic cohort will be measured in the first year to ensure consistent reporting between centres. The MRI examination comprises a high-sensitivity three-dimensional contrast-enhanced assessment, followed by a high-specificity contrast-enhanced study in equivocal cases. Multiparametric analysis will encompass morphological assessment, the kinetics of contrast agent uptake and determination of quantitative pharmacokinetic parameters. Retrospective analysis will identify the most specific indicators of malignancy. Sensitivity and specificity, together with diagnostic performance, diagnostic impact and therapeutic impact will be assessed with reference to pathology, follow-up and changes in diagnostic certainty and therapeutic decisions. Mammography, lesion localisation, pathology and cytology will be performed in accordance with the UK NHS Breast Screening Programme quality assurance standards. Similar standards of quality assurance will be applied for MR measurements and evaluation.

Automatic compensation of motion artifacts in MRI

Patient motion during the acquisition of a magnetic resonance image can cause blurring and ghosting artifacts in the image. This paper presents a new post‐processing strategy that can reduce artifacts due to in‐plane, rigid‐body motion in times comparable to that required to re‐scan a patient. The algorithm iteratively determines unknown patient motion such that corrections for this motion provide the best image quality, as measured by an entropy‐related focus criterion. The new optimization strategy features a multi‐resolution approach in the phase‐encode direction, separate successive one‐dimensional searches for rotations and translations, and a novel method requiring only one re‐gridding calculation for each rotation angle considered. Applicability to general rigid‐body in‐plane rotational and translational motion and to a range of differently weighted images and k‐space trajectories is demonstrated. Motion artifact reduction is observed for data from a phantom, volunteers, and patients. Magn Reson Med 41:163‐170, 1999.

Absolute metabolite quantification by in vivo NMR spectroscopy: II. A multicentre trial of protocols for in vivo localised proton studies of human brain

We have performed a multicentre trial to assess the performance of three techniques for absolute quantification of cerebral metabolites using in vivo proton nuclear magnetic resonance (NMR). The techniques included were 1) an internal water standard method, 2) an external standard method based on phantom replacement, and 3) a more sophisticated method incorporating elements of both the internal and external standard approaches, together with compartmental analysis of brain water. Only the internal water standard technique could be readily implemented at all participating sites and gave acceptable precision and interlaboratory reproducibility. This method was insensitive to many of the experimental factors affecting the performance of the alternative techniques, including effects related to loading, standing waves and B1 inhomogeneities; and practical issues of phantom positioning, user expertise and examination duration. However, the internal water standard method assumes a value for the concentration of NMR-visible water within the spectroscopic volume of interest. In general, it is necessary to modify this assumed concentration on the basis of the grey matter, white matter and cerebrospinal fluid (CSF) content of the volume, and the NMR-visible water content of the grey and white matter fractions. Combining data from 11 sites, the concentrations of the principal NMR-visible metabolites in the brains of healthy subjects (age range 20-35 years) determined using the internal water standard method were (mean+/-SD): [NAA]=10.0+/-3.4 mM (n=53), [tCho]=1.9+/-1.0 mM (n=51), [Cr + PCr]=6.5+/-3.7 mM (n=51). Evidence of system instability and other sources of error at some participating sites reinforces the need for rigorous quality assurance in quantitative spectroscopy.

Fourier transform Raman spectroscopy of polyacrylamide gels (PAGs) for radiation dosimetry

Polyacrylamide gels (PAGs) are used for magnetic resonance imaging radiation dosimetry. Fourier transform (FT) Raman spectroscopy studies were undertaken to investigate cross-linking changes during the copolymerization of polyacrylamide gels in the spectral range of 200-3500 cm(-1). Vibrational bands of 1285 cm(-1) and 1256 cm(-1) were assigned to acrylamide and bis-acrylamide single CH2 deltaCH2 binding modes. Bands were found to decrease in amplitude with increasing absorbed radiation dose as a result of copolymerization. Principal component regression was performed on FT-Raman spectra of PAG samples irradiated to 50 Gy. Two components were found to be sufficient to account for 98.7% of the variance in the data. Cross validation was used to establish the absorbed radiation dose of an unknown PAG sample from the FT-Raman spectra. The calculated correlation coefficient between measured and predictive samples was 0.997 with a standard error of estimate of 0.976 and a standard error of prediction of 1.140. Results demonstrate the potential of FT-Raman spectroscopy for ionizing radiation dosimetry using polyacrylamide gels.

MAGNETIC RESONANCE RENOGRAPHY: OPTIMISATION OF PULSE SEQUENCE PARAMETERS AND Gd-DTPA DOSE, AND COMPARISON WITH RADIONUCLIDE RENOGRAPHY

The aim of this study was to assess the feasibility of magnetic resonance renography (MRR) using gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA) in comparison with conventional radionuclide renography (RR) using technetium-99m-DTPA (99mTc-DTPA). MRR has many advantages over RR, including lack of ionising radiation, increased spatial resolution, and visible background anatomy. By optimising the pulse sequence, we developed an MRR protocol in which signal intensity is linear with Gd-DTPA concentration over a clinically relevant range. Twenty-nine patients and a volunteer were studied using this protocol. Magnetic resonance renography was performed using three different doses of Gd-DTPA: 0.1 mmol kg-1 (n = 13), 0.05 mmol kg-1 (n = 7), and 0.025 mmol kg-1 (n = 9). Each patient was also assessed using radionuclide renography. The resulting renograms were assessed in terms of time to peak signal intensity, signal decrease after peak, and kidney function ratios calculated from both the areas underneath and the slopes of the uptake curves. We have shown that the MR renograms obtained using low dose Gd-DTPA correlate best with the radionuclide renograms. Remaining discrepancies may be explained by variations in the injection procedures (hence in arterial input functions) and the limited coverage of the three MRR slices compared to the whole body projection of RR. Furthermore, at high local concentrations, signal becomes independent of T1 and is dominated by T2.

Design and development of an MR-compatible PET scanner for imaging small animals

An MR compatible PET system has been designed and is currently under construction. It will consist of four concentric rings of LSO crystals, each coupled to one of eight multi-channel photomultiplier tubes via 3.5 m optical fibres. The photomultiplier tubes may be located outside the main magnetic field of the MR scanner. A highly reproducible method has been established to optimise the amount of scintillation light that reaches the PM tubes, as this factor will heavily influence the scanner performance. Two small sections of the scanner, each containing 4 by 4 crystal arrays, demonstrated good flood position histograms with all sixteen channels clearly identifiable. The light loss through a fibre of length of 3.25 m was approximately 70%. The spatial resolution of the two arrays in coincidence was measured at 1.6 mm (FWHM). The temporal resolution of one array in coincidence with a single LSO crystal was measured to be 10.9 ns. A technique for improving sampling at the centre of the field of view within the scanner has also been investigated, whereby the concentric rings are offset with respect to one another. An offset of one quarter of the crystal width between layers results in significantly improved sampling. These results indicate that the scanner will be capable of carrying out the studies for which it has been designed.

Visualization and tracking of an inflatable balloon catheter using SSFP in a flow phantom and in the heart and great vessels of patients

Passive catheter tracking involves direct interaction between the device and its surroundings, creating a local signal loss or enhancement of the image. Using only standard balloon catheters filled with CO2 and imaged with a steady‐state free precession sequence, it was possible to visualize and passively track catheters in a flow phantom and in the heart and great vessels of 20 patients without any additional image processing. The phantom work demonstrated that it was advantageous to sacrifice spatial resolution in order to increase temporal resolution. Frame rates greater than 10/sec were necessary for ease of catheter manipulation. Although only the tip of the catheter was visualized, this technique proved to be effective in patients undergoing cardiac catheterization. Magn Reson Med 51:988–995, 2004.

Spatial localization in nuclear magnetic resonance spectroscopy.

The ability to select a discrete region within the body for signal acquisition is a fundamental requirement of in vivo NMR spectroscopy. Ideally, it should be possible to tailor the selected volume to coincide exactly with the lesion or tissue of interest, without loss of signal from within this volume or contamination with extraneous signals. Many techniques have been developed over the past 25 years employing a combination of RF coil properties, static magnetic field gradients and pulse sequence design in an attempt to meet these goals. This review presents a comprehensive survey of these techniques, their various advantages and disadvantages, and implications for clinical applications. Particular emphasis is placed on the reliability of the techniques in terms of signal loss, contamination and the effect of nuclear relaxation and J-coupling. The survey includes techniques based on RF coil and pulse design alone, those using static magnetic field gradients, and magnetic resonance spectroscopic imaging. Although there is an emphasis on techniques currently in widespread use (PRESS, STEAM, ISIS and MRSI), the review also includes earlier techniques, in order to provide historical context, and techniques that are promising for future use in clinical and biomedical applications.

Lower limb muscle volumes in bilateral spastic cerebral palsy.

AIM Muscle weakness is a feature of individuals with spastic cerebral palsy (SCP) but there are few reports in the literature of muscle volume in this group. This study compares muscle volumes in adolescents and young adults with SCP with those of their typically developing (TD) peers. DESIGN Measurements of the volumes of nine major lower limb muscles in 19 independently ambulant subjects with SCP (mean age 14.2 years (sd 2.7), 11 male, GMFCS I (n=5); GMFCS II (n=14)), 19 TD subjects (mean age 16.5 years (sd 3.0), 11 male) were made using magnetic resonance imaging. RESULTS Lower limb muscles were smaller in the SCP group (p≤0.023 in all muscles) than the TD group with the exception of the vastii (lateralis+intermedius; p=0.868) and gluteus maximus (p=0.056). Average muscle volume deficit was 27.9%. Muscle volume deficits were significantly greater for distal muscles than proximal muscles (p<0.001). CONCLUSIONS Reduced muscle size in adolescence and the natural history of sarcopenia in adulthood may contribute to the early loss of mobility of adults with SCP.