Takahiro Natsume

Quantitative analysis of first-pass contrast-enhanced myocardial perfusion MRI using a Patlak plot method and blood saturation correction.

The objectives of this study were to develop a method for quantifying myocardial K1 and blood flow (MBF) with minimal operator interaction by using a Patlak plot method and to compare the MBF obtained by perfusion MRI with that from coronary sinus blood flow in the resting state. A method that can correct for the nonlinearity of the blood time–signal intensity curve on perfusion MR images was developed. Myocardial perfusion MR images were acquired with a saturation‐recovery balanced turbo field‐echo sequence in 10 patients. Coronary sinus blood flow was determined by phase‐contrast cine MRI, and the average MBF was calculated as coronary sinus blood flow divided by left ventricular (LV) mass obtained by cine MRI. Patlak plot analysis was performed using the saturation‐corrected blood time–signal intensity curve as an input function and the regional myocardial time–signal intensity curve as an output function. The mean MBF obtained by perfusion MRI was 86 ± 25 ml/min/100 g, showing good agreement with MBF calculated from coronary sinus blood flow (89 ± 30 ml/min/100 g, r = 0.74). The mean coefficient of variation for measuring regional MBF in 16 LV myocardial segments was 0.11. The current method using Patlak plot permits quantification of MBF with operator interaction limited to tracing the LV wall contours, registration, and time delays. Magn Reson Med, 2009.

Robot‐assisted partial nephrectomy: Superiority over laparoscopic partial nephrectomy

Nephron‐sparing surgery has been proven to positively impact the postoperative quality of life for the treatment of small renal tumors, possibly leading to functional improvements. Laparoscopic partial nephrectomy is still one of the most demanding procedures in urological surgery. Laparoscopic partial nephrectomy sometimes results in extended warm ischemic time and severe complications, such as open conversion, postoperative hemorrhage and urine leakage. Robot‐assisted partial nephrectomy exploits the advantages offered by the da Vinci Surgical System to laparoscopic partial nephrectomy, equipped with 3‐D vision and a better degree in the freedom of surgical instruments. The introduction of the da Vinci Surgical System made nephron‐sparing surgery, specifically robot‐assisted partial nephrectomy, safe with promising results, leading to the shortening of warm ischemic time and a reduction in perioperative complications. Even for complex and challenging tumors, robotic assistance is expected to provide the benefit of minimally‐invasive surgery with safe and satisfactory renal function. Warm ischemic time is the modifiable factor during robot‐assisted partial nephrectomy to affect postoperative kidney function. We analyzed the predictive factors for extended warm ischemic time from our robot‐assisted partial nephrectomy series. The surface area of the tumor attached to the kidney parenchyma was shown to significantly affect the extended warm ischemic time during robot‐assisted partial nephrectomy. In cases with tumor‐attached surface area more than 15 cm2, we should consider switching robot‐assisted partial nephrectomy to open partial nephrectomy under cold ischemia if it is imperative. In Japan, a nationwide prospective study has been carried out to show the superiority of robot‐assisted partial nephrectomy to laparoscopic partial nephrectomy in improving warm ischemic time and complications. By facilitating robotic technology, robot‐assisted partial nephrectomy will be more frequently carried out as a safe, effective and minimally‐invasive nephron‐sparing surgery procedure.

Successful serial imaging of the mouse cerebral arteries using conventional 3-T magnetic resonance imaging.

Serial imaging studies can be useful in characterizing the pathologic and physiologic remodeling of cerebral arteries in various mouse models. We tested the feasibility of using a readily available, conventional 3-T magnetic resonance imaging (MRI) to serially image cerebrovascular remodeling in mice. We utilized a mouse model of intracranial aneurysm as a mouse model of the dynamic, pathologic remodeling of cerebral arteries. Aneurysms were induced by hypertension and a single elastase injection into the cerebrospinal fluid. For the mouse cerebrovascular imaging, we used a conventional 3-T MRI system and a 40-mm saddle coil. We used non-enhanced magnetic resonance angiography (MRA) to detect intracranial aneurysm formation and T2-weighted imaging to detect aneurysmal subarachnoid hemorrhage. A serial MRI was conducted every 2 to 3 days. MRI detection of aneurysm formation and subarachnoid hemorrhage was compared against the postmortem inspection of the brain that was perfused with dye. The imaging times for the MRA and T2-weighted imaging were 3.7 ± 0.5 minutes and 4.8 ± 0.0 minutes, respectively. All aneurysms and subarachnoid hemorrhages were correctly identified by two masked observers on MRI. This MRI-based serial imaging technique was useful in detecting intracranial aneurysm formation and subarachnoid hemorrhage in mice.

Quantitative assessment of regional systolic and diastolic functions and temporal heterogeneity of myocardial contraction in patients with myocardial infarction using cine magnetic resonance imaging and Fourier fitting

PURPOSE The objective of this study is to determine regional left ventricle (LV) function and temporal heterogeneity of LV wall contraction by analyzing regional time-volume curve (TVC) after Fourier fitting and to assess altered systolic and diastolic functions and temporal indices of myocardial contraction in infarcted segments in comparison with noninfarcted myocardium in patients with myocardial infarction (MI). METHODS Steady-state cine magnetic resonance (MR) and late gadolinium-enhanced (LGE) MR images were acquired using a 1.5-T MR system in 60 patients with MI. Regional LV function was determined by analyzing regional TVC in 16 segments. The fitted regional TVC was generated by Fourier curve fitting with five harmonics. Regional LV ejection fraction (EF), peak ejection rate (PER), peak filling rate (PFR), time to end-systole and time to peak filling (TPF) were determined from TVC and the first derivative curve. RESULTS On LGE MR imaging (MRI), MI was observed in 307 of 960 segments (32.0%). Regional EF and PER averaged in LGE segments were 49.3+/-14.5% and 2.83+/-0.65 end-diastolic volume (EDV)/s, significantly lower than those in normal segments (66.7+/-11.9% and 3.63+/-0.60 EDV/s, P<.001 and P<.01, respectively). In addition, regional PFR, an index of diastolic function, was significantly reduced in LGE segments (1.94+/-0.54 vs. 2.86+/-0.68 EDV/s, P<.01). Time to end-systole and TPF were significantly greater in LGE segments (380.2+/-57.6 and 169.3+/-45.4 ms) than in normal segments (300.9+/-55.1 and 132.3+/-43.0 ms, P<.01 and P<.01, respectively). CONCLUSIONS Analysis of regional TVC on cine MRI after Fourier fitting allows quantitative assessment of regional systolic and diastolic LV functions and temporal heterogeneity of LV wall contraction in patients with MI.

Theoretical considerations in measurement of time discrepancies between input and myocardial time-signal intensity curves in estimates of regional myocardial perfusion with first-pass contrast-enhanced MRI.

The purpose of this study was to develop a method to determine time discrepancies between input and myocardial time-signal intensity (TSI) curves for accurate estimation of myocardial perfusion with first-pass contrast-enhanced MRI. Estimation of myocardial perfusion with contrast-enhanced MRI using kinetic models requires faithful recording of contrast content in the blood and myocardium. Typically, the arterial input function (AIF) is obtained by setting a region of interest in the left ventricular cavity. However, there is a small delay between the AIF and the myocardial curves, and such time discrepancies can lead to errors in flow estimation using Patlak plot analysis. In this study, the time discrepancies between the arterial TSI curve and the myocardial tissue TSI curve were estimated based on the compartment model. In the early phase after the arrival of the contrast agent in the myocardium, the relationship between rate constant K1 and the concentrations of Gd-DTPA contrast agent in the myocardium and arterial blood (LV blood) can be described by the equation K1={dCmyo(tpeak)/dt}/Ca(tpeak), where Cmyo(t) and Ca(t) are the relative concentrations of Gd-DTPA contrast agent in the myocardium and in the LV blood, respectively, and tpeak is the time corresponding to the peak of Ca(t). In the ideal case, the time corresponding to the maximum upslope of Cmyo(t), tmax, is equal to tpeak. In practice, however, there is a small difference in the arrival times of the contrast agent into the LV and into the myocardium. This difference was estimated to correspond to the difference between tpeak and tmax. The magnitudes of such time discrepancies and the effectiveness of the correction for these time discrepancies were measured in 18 subjects who underwent myocardial perfusion MRI under rest and stress conditions. The effects of the time discrepancies could be corrected effectively in the myocardial perfusion estimates.

Development of calibration phantoms for generating quantitative perfusion images using 2D angiography data

Blood flow quantification by perfusion imaging or fractional flow reserve estimation using 2D x-ray angiography data requires scatter correction and beam hardening correction to ensure a linear relationship between the concentration of contrast agent in the human body and the x-ray image signal intensity. These types of correction usually require complicated calibration and numerous correction tables. To simplify correction, we developed a set of spherical epoxy resin phantoms containing different concentrations of barium sulfate. Prior to use, the concentration of iodinated contrast agent that results in the same x-ray absorption as each spherical phantom was determined. These values are hereinafter referred to as the equivalent concentrations of the spherical phantoms. To confirm that the spherical phantoms were suitable for estimating the contrast agent concentration, the curve of the image signal intensity vs. equivalent concentration was compared with the corresponding curve for the contrast agent. The mean difference between the two curves was 18 mgI/mL for 6 sets of experimental conditions. The mean of the p-values obtained in the paired t-tests was greater than 0.05. To validate the accuracy of our method, the spherical phantoms and test samples were placed on the surface of a chest phantom and an x-ray image including all the spherical phantoms and test samples was acquired. Using the relationship between the equivalent concentrations of the spherical phantoms and the image signal intensities, the concentrations of the test samples were estimated from their measured image signal intensities. The estimated and true concentrations of the test samples were then compared. Their cross correlation was R>0.99 for each of the 8 sets of experimental conditions used. The mean of the p-values obtained in the paired t-tests was greater than 0.05. Based on the promising results of these initial tests, we plan to proceed to clinical studies on these phantoms.

Fermi function constrained deconvolution underestimates myocardial blood flow and myocardial perfusion reserve regardless of saturation correction of arterial input curve

Background Myocardial blood flow (MBF) can be quantified from arterial input function (AIF) and myocardial output function in perfusion MRI by using tracer kinetic modeling. Fermi function constrained deconvolution has been widely used to determine MBF and myocardial perfusion reserve (MPR). However, stress MBF and MPR in healthy subjects quantified by the Fermi deconvolution approach were reported to be 2 to 3 mL/min/g and 2 to 3, respectively, which are substantially smaller than those quantified by O-water PET (3 to 5 mL/min/g and 3 to 5, respectively). Tissue compartment model analysis with Patlak plot is an alternative approach to estimate MBF and MPR. In a recent study employing corrections for AIF saturation and extraction fraction of gadolinium contrast medium, MBF and MPR by model-based Patlak plot method showed an excellent agreement with those quantified by O-water PET. In the current study, we compare MBF and MPR calculated by Fermi function constrained deconvolution with those by model-based Patlak plot method.