Marija Ivanovic

Functional, dynamic, and anatomic MR urography: feasibility and preliminary findings.

RATIONALE AND OBJECTIVES The authors assessed the feasibility of using magnetic resonance (MR) urography to acquire functional, dynamic, and anatomic information in human subjects with normal and hydronephrotic kidneys. MATERIALS AND METHODS In subjects known to have or suspected of having hydronephrosis, split renal filtration fractions were measured with a customized magnetization-prepared, inversion-prepared gradient-recalled echo sequence to determine the T1 of flowing blood in the inferior vena cava and aorta before and after contrast medium administration and in the renal veins and arteries after contrast medium administration. Multiple timed sets of coronal fast spoiled gradient-echo 70 degrees flip-angle images were acquired before and after contrast medium administration to derive MR renograms from changes in the signal intensity of the cortex and medulla. Precontrast T2-weighted images were obtained with a three-dimensional fast spoiled gradient-echo maximum intensity projection pulse sequence, and postcontrast T1 maximum intensity projection images were also obtained to depict the renal anatomy. RESULTS Split filtration fraction differentiated normal from hydronephrotic kidneys. MR renograms depicted vascular, tubular, and ductal phases and differentiated between normal and hydronephrotic kidneys (P < .05, n = 20). Contrast medium dose correlated with the peak of the cortical signal intensity curves on the renogram (r = 0.7, P < .0005; n = 20). The sensitivities for the visual determination of hydronephrosis and unilateral delayed excretion of contrast material were both 100%, and the specificities were 64% and 85%, respectively. CONCLUSION The preliminary findings show promise for the use of MR urography in the comprehensive assessment of renal function, dynamics, and anatomy.

Multi-pinhole collimator optimization for high resolution SPECT imaging

Recent studies show Tl-201 and Tc-99m MIBI to have high sensitivity for detecting and differentiating tumor from benign lesions in the breast. A study was conducted to optimize the design of a multi-pinhole collimator for high spatial resolution SPECT imaging of the breast to detect small tumors (0.5/spl times/0.5 cm). High sensitivity is achieved by using multiple pinholes, and rotating the camera 90-180 degrees around the breast. Monte Carlo simulation studies and analytical methods are applied to determine the number of holes, hole position, hole aperture size, conical field of view, and focal length of the collimator to optimize spatial resolution and sensitivity for breast imaging. Shielding is optimized to accommodate an energy range of 50-511 keV. An image reconstruction algorithm for cone beam imaging geometry for rotating single pinhole SPECT will be modified to accommodate simultaneous imaging from multiple pinholes with limited angular sampling. Special emphasis is placed on evaluation of imaging with small pinhole apertures (1-2 mm diameter), that are required to achieve the resolution of 3-7 mm at the distance of 10-15 cm with radionuclides having energies /spl les/140 keV. Complete 360/spl deg/ acquisition of the breast tumor model was simulated to study the artifacts in reconstructed images due to limited angle acquisition and to select an optimal reconstruction algorithm.

Spinal Sonography and Magnetic Resonance Imaging in Patients with Repaired Myelomeningocele: Comparison of Modalities

The goals of this study were to evaluate the feasibility of using ultrasonography of the spine in the follow‐up evaluation of patients with repaired myelomeningocele at birth and to compare sonography with the accepted modality of magnetic resonance imaging. Over a period of 4 years we performed 165 sonographic studies in 101 patients; 107 sonographic studies had MR imaging results for comparison. We collected our data prospectively. The quality of the sonograms was good in 110 of 129 studies, acceptable in 17 of 129, and poor in two of 129. The sonographic examinations failed in 33 of 165 studies (20%). Concordant information was obtained between ultrasonography and magnetic resonance imaging in the following percentage of studies: level of the distal end of the cord in 82%, position of the cord in the canal in 59%, presence of hydromyelia in 63%, cord duplication in 96%, adhesions in 16%, intradural mass in 37%, cord measurements in 85%, and dural sac measurements in 83%. At the lumbosacral level, we saw no cord pulsation in 57% of the studies in patients with cord adhesions and in 20% of those without adhesions. At the lower thoracic level, we saw no pulsation in 35% of the studies in patients with cord adhesions and in 7% of those without adhesions. Postoperative studies of cord release surgery in eight patients showed varied findings. We conclude that in those patients who have a spinal defect or interlaminar space allowing proper visualization of the lumbosacral spinal canal, ultrasound can provide fairly similar information to that obtained with magnetic resonance imaging of that area with no need for sedation and at a reduced cost. Ultrasonography seems more sensitive than magnetic resonance imaging in the detection of cord adhesions, which is particularly relevant in the diagnosis of tethering.

Gadolinium-enhanced T1-weighted renal and abdominal MR imaging: quantitative discrepancy between clinical and in vitro findings.

RATIONALE AND OBJECTIVES This study was conducted to compare the magnetic resonance (MR) contrast medium enhancement of abdominal organs in vivo with the signal intensity (SI) values of known in vitro gadolinium solutions. MATERIALS AND METHODS A phantom was imaged with the MR contrast medium gadodiamide (Omniscan; Nycomed, Princeton, NJ) of solutions at full-strength (0.5 mmol/mL), one-third, 1/10, and 1/100 concentrations. A fat-suppressed fast spoiled gradient-echo pulse sequence with flip angles ranging from 10 degrees to 170 degrees (at 20 degrees increments) was performed with a 1.5-T magnet. In 12 subjects, the SIs of abdominal organs were determined with identical imaging parameters, before and after administration of gadodiamide injection at 0.1 mmol/kg. RESULTS As anticipated, the plot of SI in relation to gadodiamide concentration is nonlinear, with a decrease in SI due to T2 effects at concentrations above 0.05 mmol/mL. The kidney showed the highest SI after gadodiamide enhancement (125.2 +/- 11.6 [standard error] at 2.5 minutes), followed by the liver (76.5 +/- 11.5 at 1 minute) and spleen (57.26 +/- 9.35 at 30 seconds). The SI of the renal medulla (114.2 +/- 9.8 at 4.5 minutes) was approximately one-third that in phantom observations. CONCLUSION The authors observed a marked discrepancy between empirical contrast medium performance in abdominal organs and SI values for comparable gadodiamide concentrations in vitro. One possible reason is the intracellular compartmentalization of water molecules in vivo. These results suggest a need for a better understanding of MR contrast medium performance in vivo.

Detection of motion in hybrid PET/SPECT imaging based on the correlation of partial sinograms

Describes a motion detection method specific to hybrid positron emission tomography/single photon emission computed tomography systems. The method relies on temporal fractionation of the acquisition into three data sets followed by an algorithm based on the cross correlation (CC) of partial sinograms from successive sets at different rotations of the camera. Spatial inconsistencies due to motion are detected by decreases in the CC between two sets. This permits to separate data into premotion and postmotion sets of consistent data that are reconstructed independently then registered and summed. Rigid motions greater than 1-cm translation or 10/spl deg/ rotation were detected with this method from experimental data obtained by manually moving phantoms made of radioactive spheres as well as from a patient lung study corrupted by artificial motion. The different motion studies showed that the image contrast does not seem to be a limiting factor and that the motion is best detected when the gantry is parallel to the direction of motion. The registration and fusion of the reconstructed premotion and postmotion sets lead in all cases to a reduction of the motion artifacts and an increase in signal-to-noise ratio.

Cortical, Medullary, and Pelvocaliceal MR Renography with and without Diuretic Modification

RATIONALE AND OBJECTIVES This study was conducted to define and characterize magnetic resonance (MR) contrast medium enhancement of the renal cortex, medulla, and pelvocaliceal system in normal and hydronephrotic kidneys with and without furosemide administration. MATERIALS AND METHODS In 30 subjects known or suspected to have unilateral hydronephrosis and normal serum creatinine levels, multiple timed sets of coronal fast spoiled gradient-echo images were acquired before and after contrast medium administration. MR renograms were derived from changes in the signal intensity (SI) of the cortex, medulla, and pelvocaliceal system. Ten subjects received 40 mg of intravenous furosemide approximately 10 minutes before contrast medium administration. RESULTS The following values were significantly different between subjects who were given furosemide and those who were not: the peak cortical, medullary, and pelvocaliceal system SIs measured in the normal kidneys during the 4 1/2 minutes following contrast medium administration (163.2 +/- 17.7 [mean arbitrary units plus or minus standard error] vs 120.5 +/- 10.2 [P = .033], 155.5 +/- 18.8 vs 111.5 +/- 9.4 [P = .025], and 332.5 +/- 27.2 vs 229.3 +/- 31.9 [P = .026], respectively); the crossover time between the SI curves of the pelvocaliceal system and the medulla in the normal kidney (2.45 minutes +/- 0.2 vs 3.27 minutes +/- 0.25 [P = .02]); and the peak SIs of the cortex, medulla, and pelvocaliceal system in the unilateral obstructive hydronephrotic kidneys throughout the first 4 1/2 minutes after contrast medium administration (174.6 +/- 16.4 vs 90.6 +/- 13.7 [P = .003], 117.6 +/- 14.1 vs 86.7 +/- 11.8 [P = .015], and 337.2 +/- 41.4 vs 143.1 +/- 74.4 [P = .034], respectively). CONCLUSION MR renography can be used to depict three separate components of renal enhancement: cortical, medullary, and pelvocaliceal. Furosemide-induced diuresis increases renal parenchymal and pelvocaliceal SI and urinary flow rates.

Initial Results of the Effects of Diuresis on Gadolinium Enhancement in MR Imaging of the Abdomen

RATIONALE AND OBJECTIVES The authors performed this study to assess the effects of furosemide-induced diuresis on paramagnetic contrast material enhancement at magnetic resonance (MR) imaging of the kidney, liver, spleen, and psoas muscle. MATERIALS AND METHODS Twenty-five patients (average age, 44.9 years; age range, 23-74 years; 13 men, 12 women) who were suspected of having unilateral renal hydronephrosis received 0.1 mmol/kg contrast material with a standardized injection and imaging protocol to assess organ signal intensity at 0-5 minutes after injection. All patients had a normal serum creatinine level. Imaging was performed with a 1.5-T magnet by using a fat-suppressed fast spoiled gradient-echo pulse sequence and a 70 degrees flip angle. Eight patients received 40 mg of furosemide 10 minutes before contrast material injection. RESULTS The areas of the renal cortical and medullary signal intensity curves minus baseline in the unilateral normal kidneys were significantly greater in the group who received furosemide (P = .026 and P = .037, respectively). The areas of the renal cortical and medullary signal intensity minus baseline in the unilateral hydronephrotic kidneys were also significantly greater in the group that received furosemide (P = .036 and P = .026, respectively). There was a statistically significant increase in splenic enhancement (P = .02) and a tendency for increased liver (P = .09) and psoas muscle (P = .08) enhancement. CONCLUSION Furosemide-induced diuresis appears to potentiate the cortical and medullary MR renogram, as well as the MR splenogram. A rapid shift in water compartmentalization from the intracellular to the extracellular space and increased renal water content with diuresis are possible explanations.

Effects of patient motion in coincidence studies on hybrid PET/SPECT system

Patient motion frequently occurs during coincidence imaging studies performed on hybrid PET/SPECT systems due to long acquisition times (30-50 min). The influence of patient motion on image quality and a motion correction method with hybrid PET/SPECT systems were investigated on a Picker AXIS dual head system. The evaluation was done using phantoms made of point, line and spherical sources filled with F-18 solution and inserted in a cylinder. Translations of 0.5 to 4.0 cm in the axial direction, and rotations from 5/spl deg/ to 30/spl deg/ at various times during the acquisition were applied. SNR, object contrast, FWHM and FWTM were calculated. Object deformation caused by motion was expressed as the ratio of the moments of inertia corresponding to the principal axes. The motion correction method applied is based on acquiring multiple fast scans and comparing them for motion. Scans prior to and after rotation and translation of the phantom were reconstructed separately, and combined using image registration. Changes in position of the phantom caused a decrease in contrast and resolution, as well as mispositioning and splitting of the sources. Motion in the axial direction splits the source in two parts that can appear several slices apart depending on the size of the motion. FWHM increased from 8.4 mm to 11.3 mm, and FWTM increased from 14.6 mm to 21.2 mm when 30/spl deg/ rotation was applied. The deformation of the spherical sources changed from 7% (no motion) up to 91% over the of motions applied in this study. Detection and correction of motion from coincidence data is a complex problem. Since motion at a given angular position of the camera can affect 20-40 projections, recording pre and post motion events in the same image. The proposed correction method successfully corrects for arbitrary motion, however further work is required to evaluate performance and quantitative accuracy with clinical data.

Motion detection in hybrid PET/SPECT imaging based on the spatial cross-correlation of temporal sinograms

Patient motion in gamma camera coincidence imaging results in severe reconstruction artifacts. A protocol is proposed to automatically detect and correct motion from SPECT coincidence studies. The method is based on fractionating the acquisition into three full temporal sets of coincidence data. For each set and camera position, partial sinograms are calculated by rebinning events acquired at the same rotation. Partial sinograms from successive angular positions as well as from successive sets are cross- correlated along their common range of projections. Decreases in the cross-correlation values indicate that data from two successive rotations or sets became inconsistent and permit localization of the motion that occurred during the study. Events acquired during motion are eliminated while pre and post motion events are recombined into sets of consistent rebinned data that are reconstructed independently and fused to provide a motion-artifact free reconstructed image. The methods were tested using a wide range of experimental motion data obtained from cylindrical phantoms containing spheres filled with Fluorine-18. Single arbitrary motions that occurred during the study could be detected and further corrected in all phantom studies when the total number of coincident events acquired was greater than 5x106 for lesion-to-background ratios greater than 5.

Detection of motion in SPECT using multi-head data combination

An algorithm to automatically detect and eliminate motion in projection data from triple head SPECT systems is proposed. The approach relies on a specific protocol in which each camera rotates 3 times faster than in conventional acquisitions and performs a full 360-degree orbit, instead of the usual partial 120 degrees orbit. This permits to obtain three full sets of data. The hypothesis is that by adequately combining the sets, it is possible in most eases to reconstitute a motion-free set of projection data. The motion detection consists of searching consistent frames through the study by cross-correlating the projection sets as a function of the angle. Plateaus of high correlation are estimated and matched in order to identify projections corresponding to the same position of the subject throughout the study. Different motion situations were created by manually moving a cardiac phantom at different times. In all cases, it was possible to reconstitute the evolution of the motion during the study. When no motion occurred during an interval equal to at least one third of the total acquisition time, high correlation plateaus could be identified. It was then possible to build an optimal complete set of consistent frames from the longest matching plateaus regardless of the frequency and magnitude of motion. Otherwise, an averaging of the three sets was performed, which still improved the quality of the reconstructed slices compared to conventional imaging protocol.