Measuring in vivo Regional Myocardial Function Using High-Field MRI

Heart failure (HF) is one of the major causes of mortality in the Western world, recognized by a compromised ability of the heart to supply the body with blood. The poor understanding of the disease mechanisms and lack of adequate therapy strategies are reflected in the grim prognosis of HF. Great research efforts over the last decades have been aimed at revealing the factors responsible for the reduction in function the left ventricle (LV) of the heart. In this, small-animal models of cardiac disease plays an irreplaceable role, enabling isolation and identification of structural and functional alterations on cellular and/or subcellular level. However, to be able to relate findings on the microscopic scale to alterations in cardiac function, there is a great need for methodology, preferably noninvasive, that allows detailed assessment of in vivo regional myocardial function. The hearts of mice and rats are more than two orders of magnitudes smaller than human hearts by weight, and beats up to ten times faster. Measurement of cardiac function in mice and rats thus understandably requires considerably higher resolution than in humans to offer comparable data yield. Phase contrast magnetic resonance imaging (PC-MRI) is a well-established and versatile noninvasive imaging technique allowing measurement of time-resolved 3D motion. We have developed an improved PC-MRI technique able to measure myocardial motion in small animals with improved accuracy and resolution compared to earlier approaches (Paper I). A major consequence of pushing the limits of achievable spatiotemporal resolution in PC-MRI is increased generation of eddy currents in the systems, which introduces severe baseline shifts in the measured motion that may render the data unusable. This required further development of eddy current correction techniques (Paper II). We developed this imaging technique further and introduced and validated a protocol for calculating regional myocardial strain from PCMRI velocity data. We applied this protocol, as a proof-of-concept, in normal and regionally dysfunctional rat hearts (Paper III). Finally, we incorporated a mathematical model allowing calculation of regional myocardial work from PC-MRI data, in combination with identification of the mitral and aortic valve events and a simple measurement of peak blood pressure. This protocol was also demonstrated in normal and dysfunctional rat hearts (Paper IV). The work in this thesis demonstrates that PC-MRI allows noninvasive measurement of regional myocardial motion, strain and work in small-animal models of cardiac disease with high resolution. The results are readily extendable to human applications, ultimately allowing higher sensitivity and/or resolution and extended data yield in functional cardiac MRI

The effect of chronic hypoxia and low dose-rate beta-irradiation on the MCF-7 human cancer cell : by in vitro cellular protein incorporation of 3H-valine in 8% O2

Hypoksiske celler som eksponeres for akutt bestråling vil, som hovedregel, være mer strålingsresistente enn veloksygenerte celler. Tidligere studier har derimot vist at kronisk hypoksiske celler tåler ståling med lav doserate dårligere enn enn veloksygenerte celler. I denne oppgaven ble den humane brystkreftcellen MCF-7 eksponert for varierende oksygenkonsentrasjoner og dermed varierende grad av kronisk hypoksi, og cellenes oksygenforbruk (respirasjon) og formeringsevne (proliferasjon) ble undersøkt. I tillegg har 3H-valin, en radioaktiv versjon av den essensielle aminosyren valin, blitt brukt til å bestråle cellene med lav-doserate $\beta$-stråling. En andel 3H-valin ble tilsatt cellenes vekstmedium, og når cellene inkorporerte valin i sine proteiner bestrålte de radioaaktive atomene cellen innenfra. Mikrodosimetriske beregninger på dette systemet ble utført. Til slutt ble respirasjonen til celler eksponert for både hypoksi og lav-doserate bestråling studert. Det følgende ble observert i eksperimentene og tilhørende analyse: * En spesifikk mediumaktivitet på 1.67 uCi/ml resulterte i en doserate på 0.0270 +/- 0.0030 Gy/t til MCF-7-kjernen. Videre ble en spesifikk mediumaktivitet på 0.735 uCi/ml brukt til å oppnå en doserate på 0.0119 +/- 0.0013 Gy/t, sammenliknbar med doseratene brukt i tidligere studier. * En oksygenkonsentrasjon i atmosfæren på 8% ble funnet til å oppfylle kravene om at cellene ikke skulle dø av oksygenmangel, mens de samtidig opplevde tilstrekkelig lav oksygentilgjengelighet til å utvise hypoksiske effekter. *Den målte maksimale cellerespirasjonen for celler som grodde i en en atmosfære med 8% var - 642.7 fmol/(t*celle) for ubestrålte celler - 812.1 fmol/(t*celle) for celler bestrålt med 0.027 Gy/t - 530.3 fmol/(t*celle) for celler bestrålt med 0.012 Gy/t Imidlertid ble ikke eksperimentet med 0.012 Gy/t kjørt lenge nok til å kunne ekskludere noen eventuell videre økning i respirasjon. * Cellene under bestråling opparbeidet en lavere pericellulær oksygenkonsentrasjon (POC) enn ubestrålte celler. Videre så en doserate på 0.012 Gy/t ut til å resultere i enda lavere POC enn en doserate på 0.027 Gy/t. * Den pericellulære oksygenkonsentrasjonen når cellenes respirasjonsmaksimum ble målt var - 2.55 +/- 0.28 %O2 for ubestrålte celler - 3.31 +/- 0.99 %O2 for celler bestrålt med 0.0270 Gy/t - 0.750 +/- 0.029 %O2 for celler bestrålt med 0.012 Gy/t Disse resultatene indikerer at en doserate på 0.012 Gy/t (dvs en spesifikk mediumaktivitet på 0.735 uCi/ml) resulterte i en betydlig redusering i nedre terskelverdi for maksimal respirasjon, sammenliknet med ikke-bestrålte celler. Dette ble derimot ikke observert i cellene bestrålt med 0.027 Gy/t, noe som kan indikere at den lavere doseraten på 0.012 Gy/h resulterer i en større cellulær effekt enn doseraten på 0.027 Gy/h

A 4D continuous representation of myocardial velocity fields from tissue phase mapping magnetic resonance imaging.

Myocardial velocities carry important diagnostic information in a range of cardiac diseases, and play an important role in diagnosing and grading left ventricular diastolic dysfunction. Tissue Phase Mapping (TPM) Magnetic Resonance Imaging (MRI) enables discrete sampling of the myocardium's underlying smooth and continuous velocity field. This paper presents a post-processing framework for constructing a spatially and temporally smooth and continuous representation of the myocardium's velocity field from TPM data. In the proposed scheme, the velocity field is represented through either linear or cubic B-spline basis functions. The framework facilitates both interpolation and noise reducing approximation. As a proof-of-concept, the framework was evaluated using artificially noisy (i.e., synthetic) velocity fields created by adding different levels of noise to an original TPM data. The framework's ability to restore the original velocity field was investigated using Bland-Altman statistics. Moreover, we calculated myocardial material point trajectories through temporal integration of the original and synthetic fields. The effect of noise reduction on the calculated trajectories was investigated by assessing the distance between the start and end position of material points after one complete cardiac cycle (end point error). We found that the Bland-Altman limits of agreement between the original and the synthetic velocity fields were reduced after application of the framework. Furthermore, the integrated trajectories exhibited consistently lower end point error. These results suggest that the proposed method generates a realistic continuous representation of myocardial velocity fields from noisy and discrete TPM data. Linear B-splines resulted in narrower limits of agreement between the original and synthetic fields, compared to Cubic B-splines. The end point errors were also consistently lower for Linear B-splines than for cubic. Linear B-splines therefore appear to be more suitable for TPM data

Three-directional evaluation of mitral flow in the rat heart by phase-contrast cardiovascular magnetic resonance

Purpose Determination of mitral flow is an important aspect in assessment of cardiac function. Traditionally, mitral flow is measured by Doppler echocardiography which suffers from several challenges, particularly related to the direction and the spatial inhomogeneity of flow. These challenges are especially prominent in rodents. The purpose of this study was to establish a cardiovascular magnetic resonance (CMR) protocol for evaluation of three-directional mitral flow in a rodent model of cardiac disease. Materials and Methods Three-directional mitral flow were evaluated by phase contrast CMR (PC-CMR) in rats with aortic banding (AB) (N = 7) and sham-operated controls (N = 7). Peak mitral flow and deceleration rate from PC-CMR was compared to conventional Doppler echocardiography. The accuracy of PC-CMR was investigated by comparison of spatiotemporally integrated mitral flow with left ventricular stroke volume assessed by cine CMR. Results PC-CMR portrayed the spatial distribution of mitral flow and flow direction in the atrioventricular plane throughout diastole. Both PC-CMR and echocardiography demonstrated increased peak mitral flow velocity and higher deceleration rate in AB compared to sham. Comparison with cine CMR revealed that PC-CMR measured mitral flow with excellent accuracy. Echocardiography presented significantly lower values of flow compared to PC-CMR. Conclusions For the first time, we show that PC-CMR offers accurate evaluation of three-directional mitral blood flow in rodents. The method successfully detects alterations in the mitral flow pattern in response to cardiac disease and provides novel insight into the characteristics of mitral flow

Quantifying left ventricular function in heart failure: What makes a clinically valuable parameter?

In heart failure (HF) management, noninvasive quantification of left ventricular (LV) function is rapidly evolving. Deformation parameters, such as strain, continue to challenge the central role of ejection fraction (EF) in diagnosis and prognostication of LV dysfunction in HF. The increasing recognition and use of deformation parameters motivates a conceptual discussion about what makes a parameter clinically valuable. To do this, we introduce a framework for parameter evaluation. The framework considers three aspects that are important for parameter value; 1) how these parameters couple with underlying myocardial function; 2) the evidence base of the parameters; and 3) the technical feasibility of their measurement. In particular, we emphasize that the coupling of each parameter to the underlying myocardial function (aspect 1) is crucial for parameter value. While EF offers information about cardiac dysfunction trough measuring changes in LV volume, deformation parameters more closely reflect underlying myocardial processes that contribute to cardiac pumping function. This is a fundamental advantage of deformation parameters that could explain why a growing number of studies supports their use. A close coupling to underlying function is, however, not sufficient for high clinical value by itself. A parameter also needs a strong evidence base (aspect 2) and a high degree of technical feasibility (aspect 3). By considering these three aspects, this review discusses the present and potential clinical value of EF and deformation parameters in HF management

Regional right ventricular function in rats: a novel magnetic resonance imaging method for measurement of right ventricular strain

The function of the right ventricle (RV) is linked to clinical outcome in many cardiovascular diseases, but its role in experimental heart failure remains largely unexplored due to difficulties in measuring RV function in vivo. We aimed to advance RV imaging by establishing phase-contrast MRI (PC-MRI) as a robust method for measuring RV function in rodents. A total of 46 Wistar-Hannover rats with left ventricular (LV) myocardial infarction and 10 control rats (sham) were examined 6 wk after surgery. Using a 9.4-T preclinical MRI system, we utilized PC-MRI to measure strain/strain rate in the RV free wall under isoflurane anesthesia. Cine MRI was used to measure RV volumes. LV end-diastolic pressure (LVEDP) was measured and used to identify pulmonary congestion. The infarct rats were divided into two groups: those with signs of pulmonary congestion (PC), with LVEDP ≥ 15 mmHg ( n = 26) and those without signs of pulmonary congestion (NPC), with LVEDP < 15 mmHg ( n = 20). The NPC rats exhibited preserved RV strains/strain rates, whereas the PC rats exhibited reduced strains/strain rates (26–48% lower than sham). Of the strain parameters, longitudinal strain and strain rate exhibited the highest correlations to LVEDP and lung weight (rho = 0.65–0.72, P < 0.001). Basal longitudinal strain was most closely associated with signs of pulmonary congestion and indexes of RV remodeling. Longitudinal RV strain had higher area under the curve than ejection fraction for detecting subtle RV dysfunction (area under the curve = 0.85 vs. 0.67). In conclusion, we show for the first time that global and regional RV myocardial strain can be measured robustly in rodents. Reduced RV strain was closely associated with indexes of pulmonary congestion and molecular markers of RV remodeling. NEW & NOTEWORTHY Global and regional right ventricular myocardial strain can be measured with high reproducibility and low interobserver variability in rodents using tissue phase mapping MRI. Reduced right ventricular strain was associated with indexes of pulmonary congestion and molecular markers of right ventricular remodeling. Regional strain in the basal myocardium was considerably higher than in the apical myocardium