Labeling Perfusion Magnetic Resonance Imaging Research Articles

Quantification Issues in Arterial Spin Labeling Perfusion Magnetic Resonance Imaging

Arterial spin labeling (ASL) perfusion magnetic resonance imaging has gained wide acceptance for its value in clinical and neuroscience applications during recent years. Its capability for noninvasive and absolute perfusion quantification is a key characteristic that makes ASL attractive for many clinical applications. In the present review, we discuss the main parameters or factors that affect the reliability and accuracy of ASL perfusion measurements. Our secondary goal was to outline potential solutions that may improve the reliability and accuracy of ASL in clinical settings. It was found that, through theoretical analyses, flow quantification is most sensitive to tagging efficiency and estimation of the equilibrium magnetization of blood signal (M(0b)). Variations of blood T1 have a greater effect on perfusion quantification than variations of tissue T1. Arterial transit time becomes an influential factor when it is longer than the postlabeling delay time. The T2's of blood and tissue impose minimal effects on perfusion calculation at field strengths equal to or lower than 3.0 T. Subsequently, we proposed various approaches for in vivo estimation or calibration of the above parameters, such as the use of phase-contrast magnetic resonance imaging for calibration of the labeling efficiency as well as the use of inversion recovery TrueFISP (true fast imaging with steady-state precession) sequence for blood T1 mapping. We also list representative clinical cases in which implicit assumptions for ASL perfusion quantification may be violated, such as the venous outflow effect in children with sickle cell disease. Finally, an optimal imaging protocol including in vivo measurements of several critical parameters was recommended for clinical ASL studies.

Parkinson's Disease Spatial Covariance Pattern: Noninvasive Quantification with Perfusion MRI

Parkinson's disease (PD) is associated with elevated expression of a specific disease-related spatial covariance pattern (PDRP) in radiotracer scans of cerebral blood flow and metabolism. In this study, we scanned nine early-stage patients with PD and nine healthy controls using continuous arterial spin labeling (CASL) perfusion magnetic resonance imaging (pMRI). Parkinson's disease-related metabolic pattern expression in CASL pMRI scans was compared with the corresponding (18)F-fluorodeoxyglucose positron emission tomography values. The PDRP expression was abnormally elevated (P<0.01) in patients scanned with either modality, and the two values were highly intercorrelated (P<0.0001). Perfusion MRI methods can be used for accurate quantification of disease-related covariance patterns.

Arterial spin labeling perfusion MRI at multiple delay times: a correlative study with H(2)(15)O positron emission tomography in patients with symptomatic carotid artery occlusion.

Arterial spin labeling (ASL) perfusion magnetic resonance imaging (MRI) with image acquisition at multiple inversion times is a noninvasive ASL technique able to compensate for spatial heterogeneities in transit times caused by collateral blood flow in patients with severe stenosis of the cerebropetal blood vessels. Our aim was to compare ASL-MRI and H(2)(15)O positron emission tomography (PET), the gold standard for cerebral blood flow (CBF) assessment, in patients with a symptomatic internal carotid artery (ICA) occlusion. Fourteen patients (63+/-14 years) with a symptomatic ICA occlusion underwent both ASL-MRI and H(2)(15)O PET. The ASL-MRI was performed using a pulsed STAR labeling technique at multiple inversion times within 7 days of the PET. The CBF was measured in the gray-matter of the anterior, middle and posterior cerebral artery, and white-matter. Both PET and ASL-MRI showed a significantly decreased CBF in the gray-matter of the middle cerebral artery in the hemisphere ipsilateral to the ICA occlusion. The average gray-matter CBF measured with ASL-MRI (71.8+/-4.3 mL/min/100 g) was higher (P<0.01) than measured with H(2)(15)O PET (43.1+/-1.0 mL/min/100 g). In conclusion, ASL-MRI at multiple TIs is capable of depicting areas of regions with low CBF in patients with an occlusion of the ICA, although a systematic overestimation of CBF relative to H(2)(15)O PET was noted.

Physiological Modulations in Arterial Spin Labeling Perfusion Magnetic Resonance Imaging

The purpose of this study is to evaluate cardiac and respiratory modulations in the signals of arterial spin labeling (ASL) perfusion magnetic resonance imaging (MRI) using RETROICOR, an image domain based retrospective correction method. Systematic comparisons were conducted for tagging schemes, pulsed (PASL) versus frequency-modulated continuous (CASL) methods, and the use of background suppression (BGS). Results showed that cardiac pulsation accounted for more signal fluctuation in PASL than in CASL (two-tailed paired Student's t-test, p < 10(-3)), whereas no significant difference was found in the effect of respiratory motion ( p=0.55) on the two tagging schemes studied. For PASL, significantly more improvement was achieved by the inclusion of cardiac pulsation than respiratory motion in RETROICOR ( p < 0.01). On the contrary, the inclusion of respiratory motion offers more improvement for CASL ( p < 0.02). BGS effectively improved the temporal signal-to-noise ratio (tSNR) as previous studies reported, but no significant difference was measured in the spectral power of physiological modulations relative to the entire spectrum of PASL signals before and after the superimposition of BGS ( p=0.63 for cardiac component, p=0.67 for respiratory component). Thus, we conclude that BGS reduces noise without spectral selectivity, and the improvements of tSNR from RETROICOR and BGS are additive. CASL with a labeling duration at a multiple of an R-R interval can be used to minimize signal fluctuation originating from cardiac pulsation.

Mapping resting-state functional connectivity using perfusion MRI

Resting-state, low-frequency (<0.08 Hz) fluctuations of blood oxygenation level-dependent (BOLD) magnetic resonance signal have been shown to exhibit high correlation among functionally connected regions. However, correlations of cerebral blood flow (CBF) fluctuations during the resting state have not been extensively studied. The main challenges of using arterial spin labeling perfusion magnetic resonance imaging to detect CBF fluctuations are low sensitivity, low temporal resolution, and contamination from BOLD. This work demonstrates CBF-based quantitative functional connectivity mapping by combining continuous arterial spin labeling (CASL) with a neck labeling coil and a multi-channel receiver coil to achieve high perfusion sensitivity. In order to reduce BOLD contamination, the CBF signal was extracted from the CASL signal time course by high frequency filtering. This processing strategy is compatible with sinc interpolation for reducing the timing mismatch between control and label images and has the flexibility of choosing an optimal filter cutoff frequency to minimize BOLD fluctuations. Most subjects studied showed high CBF correlation in bilateral sensorimotor areas with good suppression of BOLD contamination. Root-mean-square CBF fluctuation contributing to bilateral correlation was estimated to be 29±19% (N=13) of the baseline perfusion, while BOLD fluctuation was 0.26±0.14% of the mean intensity (at 3 T and 12.5 ms echo time).

Integrity of the Cerebral Blood-Flow Response to Hyperoxia After Cardiopulmonary Bypass

In this study, the hypothesis that cardiopulmonary bypass (CPB) alters the cerebral blood flow (CBF) vasoconstrictive response to hyperoxia was tested. A prospective, observational study was conducted. The study was conducted at a single university hospital. Subjects were patients who presented for cardiac surgery with CPB. CBF was measured before and after CPB in 12 subjects while breathing 21% O(2) and 100% O(2). CBF was measured by using continuous arterial spin labeling (CASL) perfusion magnetic resonance imaging. Arterial pO(2) (mmHg), pCO(2) (mmHg), hemoglobin (Hgb), and oxygen content (CaO(2)) were also measured. Mean age of the 12 subjects was 63 +/- 16 years. Hgb decreased from 12.0 (+/-2.4) g/dL to 9.2 (+/-2.9) g/dL postoperatively (p = 0.008). CBF increased by 39%, from 37.2 (+/-10.8) mL/100 g/min to 49.2 (+/-14.3)mL/100 g/min postoperatively (p = 0.01). In response to the hyperoxic challenge CBF decreased by 8.0 (+/-7.1) mL/100 g/min (21%) preoperatively and by 9.4 (+/-6.4) mL/100 g/min (19%) postoperatively (p = 0.58). By using multiple regression, the contribution of CPB to the hyperoxic CBF response (DeltaCBF) was evaluated, while controlling for other potentially important covariates known to influence CBF, including age, baseline CBF on 21% O(2), and changes in arterial pO(2), pCO(2), and CaO(2). CPB state was not found to be a significant covariate in controlling the CBF response to hyperoxia. CPB does not impair the CBF response to hyperoxia.

Recurrence of Stroke After Cardiac Surgery: Insight Into Pathogenesis via Diffusion-Weighted and Continuous Arterial Spin Labeling Perfusion Magnetic Resonance Imaging

STROKE AFTER CARDIAC surgery occurs with a higher incidence in patients with a history of prior stroke 1 Redmond J.M. Greene P.S. Goldsborough M.A. et al. Neurologic injury in cardiac surgical patients with a history of stroke. Ann Thorac Surg. 1996; 61: 42-47 Abstract Full Text PDF PubMed Scopus (88) Google Scholar , 2 Hogue Jr, C.W. De Wet C.J. Schechtman K.B. et al. The importance of prior stroke for the adjusted risk of neurologic injury after cardiac surgery for women and men. Anesthesiology. 2003; 98: 823-829 Crossref PubMed Scopus (30) Google Scholar and is often delayed. 3 Peel G.K. Stamou S.C. Dullum M.K. et al. Chronologic distribution of stroke after minimally invasive versus conventional coronary artery bypass. J Am Coll Cardiol. 2004; 43: 752-756 Abstract Full Text Full Text PDF PubMed Scopus (36) Google Scholar , 4 Hogue Jr, C.W. Murphy S.F. Schechtman K.B. et al. Risk factors for early or delayed stroke after cardiac surgery. Circulation. 1999; 100: 642-647 Crossref PubMed Scopus (437) Google Scholar Recurrence of stroke after cardiac surgery, thought to be primarily embolic in nature, 5 Libman R.B. Wirkowski E. Neystat M. et al. Stroke associated with cardiac surgery Determinants, timing, and stroke subtypes. Arch Neurol. 1997; 54: 83-87 Crossref PubMed Scopus (99) Google Scholar , 6 Borger M.A. Ivanov J. Weisel R.D. et al. Stroke during coronary bypass surgery: Principal role of cerebral macroemboli. Eur J Cardiothorac Surg. 2001; 19: 627-632 Crossref PubMed Scopus (94) Google Scholar , 7 Likosky D.S. Marrin C.A. Caplan L.R. et al. Determination of etiologic mechanisms of strokes secondary to coronary artery bypass graft surgery. Stroke. 2003; 34: 2830-2834 Crossref PubMed Scopus (237) Google Scholar has been noted to frequently follow the anatomic and vascular distribution of the previous stroke. 1 Redmond J.M. Greene P.S. Goldsborough M.A. et al. Neurologic injury in cardiac surgical patients with a history of stroke. Ann Thorac Surg. 1996; 61: 42-47 Abstract Full Text PDF PubMed Scopus (88) Google Scholar Stroke is more frequent after aortic valve replacement for aortic stenosis than after coronary artery bypass grafting 8 Wolman R.L. Nussmeier N.A. Aggarwal A. et al. Multicenter Study of Perioperative Ischemia Research Group (McSPI) and the Ischemia Research Education Foundation (IREF) InvestigatorsCerebral injury after cardiac surgery: Identification of a group at extraordinary risk. Stroke. 1999; 30: 514-522 Crossref PubMed Scopus (254) Google Scholar , 9 Stolz E. Gerriets T. Kluge A. et al. Diffusion-weighted magnetic resonance imaging and neurobiochemical markers after aortic valve replacement: Implications for future neuroprotective trials?. Stroke. 2004; 35: 888-892 Crossref PubMed Scopus (91) Google Scholar but may be, in both types of surgery, related to the extent of calcification in the ascending aorta 10 Barbut D. Gold J.P. Aortic atheromatosis and risks of cerebral embolization. J Cardiothorac Vasc Anesth. 1996; 10: 24-30 Abstract Full Text PDF PubMed Scopus (47) Google Scholar , 11 van der Linden J. Hadjinikolaou L. Bergman P. et al. Postoperative stroke in cardiac surgery is related to the location and extent of atherosclerotic disease in the ascending aorta. J Am Coll Cardiol. 2001; 38: 131-135 Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar , 12 Djaiani G. Fedorko L. Borger M. et al. Mild-to-moderate atheromatous disease of the thoracic aorta and new ischemic brain lesions after conventional coronary artery bypass graft surgery. Stroke. 2004; 35: e356-e358 Crossref PubMed Google Scholar or within the valve itself.

Blood Flow Imaging of the Brain: 50 Years Experience

Blood Flow Imaging of the Brain: 50 Years Experience

Altered Flow Territories after Extracranial-Intracranial Bypass Surgery

To prevent stroke after carotid sacrifice and to augment cerebral perfusion in patients with internal carotid artery (ICA) occlusion, high-flow extracranial-intracranial (EC-IC) bypass operations are performed. Although the function and efficacy of the bypass is monitored during surgery, the postoperative flow through the bypass is significantly lower than the flow in the contralateral ICA. Thus far, it is unknown whether decreased bypass flow is caused by a low tissue perfusion or by a relatively small flow territory. Seven patients, four with an atherosclerotic ICA occlusion and three with a giant aneurysm of the ICA, were investigated; each underwent a high-flow EC-IC bypass and permanent occlusion of the ICA. Cerebral blood flow was measured with arterial spin labeling perfusion magnetic resonance imaging. Separate flow territory mapping of the EC-IC bypass, contralateral ICA, and posterior circulation was performed with selective arterial spin labeling magnetic resonance imaging. No significant difference was found in cerebral blood flow between the hemisphere ipsilateral to the EC-IC bypass (70.9 +/- 11.3 ml/min/100 g tissue), contralateral to the EC-IC bypass (71.9 +/- 14.3 ml/min/100 g tissue), and comparable findings in 50 healthy control participants (69.1 +/- 17.5 ml/min/100 g tissue). Paired analysis of the individual flow territories demonstrated a 15% volume reduction (P = 0.018) in flow territory of the EC-IC bypass compared with the contralateral side. In the present study, we demonstrate the feasibility of selective arterial spin labeling magnetic resonance imaging for clinical follow-up of patients after high-flow EC/IC bypass surgery, providing both information on flow territories and the level of regional cerebral blood flow.

Continuous Arterial Spin Labeled Perfusion Magnetic Resonance Imaging in Patients before and after Carotid Endarterectomy

Continuous arterial spin labeling perfusion magnetic resonance imaging (CASL-pMRI) uses magnetically labeled arterial blood water as a tracer to obtain quantifiable measurements of cerebral blood flow (CBF) (mL/100 g-1/min-1). CASL-pMRI was used to assess CBF changes in major vascular distributions in patients (n = 10) prior to and 3 months after carotid endarterectomy (CEA). No significant change in the global baseline CBF before and after CEA was observed in the group as a whole (P = .81). In patients with reduced CBF prior to CEA (< 50 ml/100 g/min), a significant increase in global CBF following CEA was observed. An inverse relationship existed between percent change in CBF after CEA versus baseline CBF within the anterior circulation (r = -.78, P < .05) but not in the posterior distribution (r = .25, P = .63). CASL-pMRI may provide a convenient, inexpensive, noninvasive method for identifying CEA patients at risk for hyperperfusion following carotid revascularization.

Continuous Arterial Spin Labeled Perfusion Magnetic Resonance Imaging in Patients before and after Carotid Endarterectomy

Continuous Arterial Spin Labeled Perfusion Magnetic Resonance Imaging in Patients before and after Carotid Endarterectomy

Perioperative changes in cerebral blood flow after cardiac surgery: influence of anemia and aging

Background Stroke occurs in 2% to 5% and cognitive dysfunction occurs acutely in 60% to 80% of patients early after cardiac surgery. Both may have long-term consequences. Research into mechanisms behind these sequelae has been focused intraoperatively, although there is little reason to believe that injury is limited to this period. Aging prominently increases the incidence of these sequelae. Anemia with cardiac surgery is acute and severe, should cause an increase in cerebral blood flow (CBF), and may impact stroke and cognitive function in this setting. To better understand changes in perioperative CBF physiology we have measured changes in CBF and the influence of anemia and aging on these changes. Methods Cerebral blood flow was measured using the noninvasive continuous arterial spin labeling perfusion magnetic resonance imaging method. Cerebral blood flow, mean arterial pressure, hemoglobin, hemoglobin oxygen saturation, and cardiopulmonary bypass time were recorded in 12 subjects before and 6 ± 2 days after cardiac surgery. Results Cerebral blood flow increased from 44.6 ± 15.6 mL100 g −1min −1 to 64.4 ± 20.1 mL100 g −1min −1 after cardiac surgery, or 49.1% ± 26.7%, ( p < 0.0001). The absolute change in CBF (ΔCBF) was predicted by the following regression model: ΔCBF = −55 + 0.64(Age) + 0.53(CBF Pre) −3.3(ΔHgb); R 2 = 0.81; p = 0.003, where CBF Pre is the baseline preoperative CBF and ΔHgb is the change in hemoglobin from preoperative to postoperative periods. Conclusions Cerebral blood flow increases after cardiac surgery, and anemia appears to be an important cause. Age appears also to be an important covariate, with advancing age further increasing the magnitude of this hyperemia. The interrelationship of aging and anemia, in determining perioperative changes in CBF, and potentially cerebral oxygenation, may have important implications for the understanding of perioperative stroke and cognitive dysfunction after cardiac surgery.

Precision of the CASL-perfusion MRI technique for the measurement of cerebral blood flow in whole brain and vascular territories.

To analyze the precision of cerebral blood flow (CBF) measurements made with continuous arterial spin labeling(CASL) perfusion magnetic resonance imaging (MRI) over experimentally relevant intervals. CASL perfusion MRI measurements of CBF on a 1.5-T GE Signa magnet were repeated in young healthy male and female subjects at one hour and one week. Precision of the measurement was evaluated at both time intervals. CASL perfusion MRI measurements of CBF yielded within-subject coefficients of variation (wsCV) of 5.8% for global and 13% for individual vascular regions when measurements were repeated within one hour. Differences in these values represent the error in post-processing. Global and regional CBF measurements over one week yielded wsCVs of 13% and 14%, respectively. At one week, error secondary to physiologic variability affected global and regional measurements to the same degree and masked the software post-processing error seen at one hour. The magnitude of the difference in repeated measures correlated with the magnitude of the measurement. CASL perfusion MRI CBF measurements are accurate and precise. Variability over longer periods of time appears attributable to physiologic factors. Repeatability of the CASL measurement is sensitive to the magnitude of the measurement. This should be taken into account when studies requiring repeated measures involve subjects with significant variability in CBF.