Modelling carbon responses of tundra ecosystems to historical and projected climate: sensitivity of pan-Arctic carbon storage to temporal and spatial variation in climate.

Historical and projected climate trends for high latitudes show substantial temporal and spatial variability. To identify uncertainties in simulating carbon (C) dynamics for pan-Arctic tundra, we compare the historical and projected responses of tundra C storage from 1921 to 2100 between simulations by the Terrestrial Ecosystem Model (TEM) for the pan-Arctic and the Kuparuk River Basin, which was the focus of an integrated study of C dynamics from 1994 to 1996. In the historical period from 1921 to 1994, the responses of net primary production (NPP) and heterotrophic respiration (RH ) simulated for the Kuparuk River Basin and the pan-Arctic are correlated with the same factors; NPP is positively correlated with net nitrogen mineralization (NMIN) and RH is negatively correlated with mean annual soil moisture. In comparison to the historical period, the spatially aggregated responses of NPP and RH for the Kuparuk River Basin and the pan-Arctic in our simulations for the projected period have different sensitivities to temperature, soil moisture and NMIN. In addition to being sensitive to soil moisture during the projected period, RH is also sensitive to temperature and there is a significant correlation between RH and NMIN. We interpret the increases in NPP during the projected period as being driven primarily by increases in NMIN, and that the correlation between NPP and temperature in the projected period is a result primarily of the causal linkage between temperature, RH , and NMIN. Although similar factors appear to be controlling simulated regional-and biome-scale C dynamics, simulated C dynamics at the two scales differ in magnitude with higher increases in C storage simulated for the Kuparuk River Basin than for the pan-Arctic at the end of the historical period and throughout the projected period. Also, the results of the simulations indicate that responses of C storage show different climate sensitivities at regional and pan-Arctic spatial scales and that these sensitivities change across the temporal scope of the simulations. The results of the TEM simulations indicate that the scaling of C dynamics to a region of arctic tundra may not represent C dynamics of pan-Arctic tundra because of the limited spatial variation in climate and vegetation within a region relative to the pan-Arctic. For reducing uncertainties, our analyses highlight the importance of incorporating the understanding gained from process-level studies of C dynamics in a region of arctic tundra into process-based models that simulate C dynamics in a spatially explicit fashion across the spatial domain of pan-Arctic tundra. Also, efforts to improve gridded datasets of historical climate for the pan-Arctic would advance the ability to assess the responses of C dynamics for pan-Arctic tundra in a more realistic fashion. A major challenge will be to incorporate topographic controls over soil moisture in assessing the response of C storage for pan-Arctic tundra.

Magnetic resonance angiography demonstration of congenital heart disease in adults.

OBJECTIVE: There are limitations when using spin-echo MR imaging to evaluate congenital heart disease (CHD). These include limited ability to detect small shunts, long acquisition times, in-plane or slow flow signal and limited ability to represent complicated three-dimensional (3D) anatomy. However, MR angiography can image flow direction and disturbances, assess function and easily represents 3D anatomy. This may be valuable when evaluating adults with CHD. PATIENTS AND METHODS: Fifty consecutive adult patients were referred for MR evaluation of possible or known CHD using time-of-flight MR angiography. Cine, breathhold and presaturation cine MR angiography were acquired as appropriate, with 3D (MIP) reconstruction of all extracardiac anomalies. RESULTS: Forty-nine patients had a diagnostic examination (one was unsatisfactory because of arrhythmia). Correlation with alternative imaging (TTE = 36, TOE = 13, cardiac catheter = 16) or surgery was available in 39 patients (MR angiography correct in 36 patients). MRA demonstrated or excluded all confirmed congenital valve (12/12), aortic (9/9), and venous (7/7) anomalies. Thirty-five patients were evaluated for shunts. MR angiography detected 31/33 confirmed shunts (shunts present in 26 patients, sensitivity 94%, specificity 95%). Although not used in all cases, spin-echo was unreliable in demonstrating shunts as signal loss in the region of the secundum septum frequently mimicked atrial septal defects (reducing accuracy for excluding intracardiac shunts, specificity 58%). CONCLUSION: MR angiography accurately defines intra- and extra-cardiac anatomy and is superior to spin-echo in detecting or excluding shunts. MR angiography safely and accurately demonstrates many aspects of CHD in adults.

MR angiography of congenital heart disease in adults.

Magnetic resonance (MR) angiographic techniques are useful in evaluation of adult patients with congenital heart disease (CHD). The available techniques include cine, breath-hold ungated, and segmented k-space time-of-flight MR angiography. Three-dimensional (3D) image reconstruction with maximum-intensity projection can be used with all of these techniques to demonstrate great vessel anomalies associated with CHD. Selective presaturation MR angiography can be used to detect intracardiac shunts. MR angiography allows clarification of difficult diagnostic points that are not fully demonstrated with other imaging techniques, such as cardiac catheterization and angiocardiography, echocardiography, nuclear medicine studies, and conventional spin-echo MR imaging. In many patients with CHD in adulthood, the use of appropriate MR angiographic techniques may allow definitive diagnosis and preclude the need for cardiac catheterization. The wide field of view, sensitivity to shunts, and rapid 3D imaging capability of MR angiography make it a valuable method of evaluating CHD in adults.

MRI of right atrial pseudomass: is it really a diagnostic problem?

OBJECTIVE: To determine whether the high proportion of patients reported to have prominence of normal right atrial structures by MRI may lead to inappropriate diagnosis of intracardiac tumors. MATERIALS AND METHODS: One hundred forty-nine subjects were examined by spin-echo MRI: patients with cardiac (no. 40), pericardial (no. 30), or thoracic aortic disease (no. 40) and mediastinal tumor (no. 15), and normal volunteers (no. 24). Imaging was reviewed to determine the frequency of a prominent crista terminalis/Chiari network and the likelihood of misdiagnosis of cardiac tumor. RESULTS: Prominent intraatrial structures were seen in 59% of subjects, a single prominent nodule in 36%, an intraatrial strand in 13%, and both in 10%. In no case were these findings originally or on review thought to represent a pathological mass or was it felt likely that they could reasonably be misinterpreted as such. CONCLUSION: Normal structures within the right atrium, such as the crista terminalis and Chiari network, may be seen more commonly with MRI than with other imaging modalities. An appreciation of the frequency with which these findings are seen should prevent inappropriate misdiagnosis of pathological masses when none is present.

3D image reconstruction of right subclavian artery aneurysms.

OBJECTIVE: Three-dimensional (3D) image reconstruction methods allied to spiral CT (SCT) or MR angiography (MRA) are used in clarifying the anatomy of complex aortic anatomy. MATERIALS AND METHODS: Two patients with superior mediastinal masses suggestive of aneurysms of the anomalous right subclavian artery were examined. Both patients were examined using breath-hold SCT and one by breath-hold MRA. Three-dimensional images were reconstructed using a surface rendering technique for the SCT examinations and by a maximum intensity projection technique for MRA. RESULTS: Two cases of aneurysmal anomalous right subclavian artery were diagnosed by SCT with 3D image reconstruction. One case was also examined by MRA with 3D image reconstruction which confirmed the findings of SCT. The use of the 3D techniques clarified the complicated anatomy and avoided the need for angiography. CONCLUSION: Two cases are illustrated of 3D imaging of complicated aortic branch anatomy due to aneurysms of anomalous right subclavian artery. Image quality was good and no further imaging by more invasive techniques was required.