Clinical implementation of a fully automated quantitative perfusion cardiovascular magnetic resonance imaging workflow with a simplified dual-bolus contrast administration scheme.

This study clinically implemented a ready-to-use quantitative perfusion (QP) cardiovascular magnetic resonance (QP CMR) workflow, encompassing a simplified dual-bolus gadolinium-based contrast agent (GBCA) administration scheme and fully automated QP image post-processing. Twenty-five patients with suspected obstructive coronary artery disease (CAD) underwent both adenosine stress perfusion CMR and an invasive coronary angiography or coronary computed tomography angiography. The dual-bolus protocol consisted of a pre-bolus (0.0075 mmol/kg GBCA at 0.5 mmol/ml concentration + 20 ml saline) and a main bolus (0.075 mmol/kg GBCA at 0.5 mmol/ml concentration + 20 ml saline) at an infusion rate of 3 ml/s. The arterial input function curves showed excellent quality. Stress MBF ≤ 1.84 ml/g/min accurately detected obstructive CAD (area under the curve 0.79; 95% Confidence Interval: 0.66 to 0.89). Combined visual assessment of color pixel QP maps and conventional perfusion images yielded a diagnostic accuracy of 84%, sensitivity of 70% and specificity of 93%. The proposed easy-to-use dual-bolus QP CMR workflow provides good image quality and holds promise for high accuracy in diagnosis of obstructive CAD. Implementation of this approach has the potential to serve as an alternative to current methods thus increasing the accessibility to offer high-quality QP CMR imaging by a wide range of CMR laboratories.

Accurate PET/MR quantification using time of flight MLAA image reconstruction.

PURPOSE: Quantification of positron emission tomography/magnetic resonance imaging (PET/MRI) studies is hampered by inaccurate MR-based attenuation correction (MR-AC). To date, most studies on MR-AC have been performed using PET/MR systems without time of flight (TOF). Maximum likelihood reconstruction of attenuation and activity (MLAA), however, has the potential to improve MR-AC by exploiting TOF. The purpose of this study is to assess the impact of MR-AC on PET image quantification for TOF-PET/MR systems and to evaluate PET accuracy when using TOF in combination with MLAA (TOF-MLAA). PROCEDURES: Simulations were designed to evaluate (1) the impact of MR-AC on PET quantification for different TOF windows (667, 500, 333 and 167 ps) and (2) use of TOF-MLAA for improving PET quantification. TOF-ordered subset expectation maximisation (OSEM) and TOF-MLAA reconstructions using MR-AC were compared with those obtained using TOF-OSEM with computed tomography-based AC (CT-AC). RESULTS: OSEM reconstructions without TOF showed a negative MR-AC-induced bias of -50 % in the bone. TOF-OSEM was able to reduce this bias down to -15 %, with more accurate results for better TOF. TOF-MLAA was able to reduce the bias to within 5 % but at the cost of a ∼40 % increase in image variance. CONCLUSIONS: TOF-MLAA can improve quantitative PET accuracy of PET/MR studies. Further improvements are anticipated with improving TOF performance.

Towards a noninvasive anatomical and functional diagnostic work-up of patients with suspected coronary artery disease.

Combining multidetector computed tomography and cardiovascular magnetic resonance imaging provides the clinician a strategy to comprehensively evaluate coronary morphology and function noninvasively. In the MARCC trial (Magnetic Resonance and CT in suspected CAD) a new noninvasive diagnostic work-up for patients with suspected coronary artery disease will be developed, involving the sequential use of both imaging techniques. (Neth Heart J 2010;18:270-3.).

'No-reflow' after acute myocardial infarction: direct visualisation of microvascular obstruction by gadolinium-enhanced CMR.

Cardiovascular magnetic resonance is considered the standard imaging modality in clinical trials to monitor patients after acute myocardial infarction. However, limited data are available with respect to infarct size, presence and extent of microvascular injury (MVO) and changes over time, in relation to cardiac function in optimally treated patients. In the current study we prospectively investigate the change of infarct size over time, and the incidence and significance of MVO in a uniform, optimally treated patient group after AMI. (Neth Heart J 2008;16:179-81.).

Feasibility of cardiovascular magnetic resonance of angiographically diagnosed congenital solitary coronary artery fistulas in adults.

OBJECTIVE: To evaluate the use of cardiovascular magnetic resonance (CMR) to visualize angiographically-detected congenital coronary artery fistulas in adults. METHODS: CMR techniques were used to study 13 patients, recruited from the Dutch Registry, with previously angiographically diagnosed fistulas. RESULTS: Coronary fistulas were detected in 10 of 13 (77%) patients by CMR and, retrospectively, in two (92%) more. In 93% of these, it was possible to determine the origin and the outflow site of the fistulas. Cardiovascular magnetic resonance allowed demonstration of dilatation of the fistula-related coronary artery in all cases. Tortuosity of fistulas was detected in all visualized patients. Uni-or bilaterality of fistulas as seen on CAG was proven on CMR in all patients. Flow measurement could be performed in 8 patients. A fairly good correlation (r = 0.72) was found between angiographic (mean 6.2 mm, range 1-16) and cardiovascular magnetic resonance (mean 6.3 mm, range 3-15) measured fistulous diameters. CONCLUSIONS: Cardiovascular magnetic resonance of congenital fistulas with clinical significant shunting is feasible and can provide additional physiological data complementary to the findings of conventional coronary angiography.

Regional timing of myocardial shortening is related to prestretch from atrial contraction: assessment by high temporal resolution MRI tagging in humans.

Earlier studies have shown substantial nonuniformity in normal left ventricular (LV) myocardial function concerning both the degree of shortening and timing of shortening. We hypothesized that nonuniform LV function may be related to nonuniform prestretch induced by atrial contraction. Eleven healthy human subjects were studied using MRI myocardial tagging and strain analysis. The amount of circumferential prestretch was assessed in 30 LV segments. Prestretch was defined as the difference in strain between end diastole (at ECG R wave) and diastasis. Furthermore, both the degree of shortening (quantified as peak circumferential shortening, peak systolic shortening rate, and amount of postsystolic shortening) and timing of shortening (quantified as the onset time of shortening and time to peak shortening) were assessed. LV prestretch was found to be nonuniform, with the highest values in the lateral wall. The amount of segmental prestretch correlated significantly with peak shortening (r = 0.79), peak shortening rate (r = 0.50), amount of postsystolic shortening (r = 0.67), onset time of shortening (r = -0.57), and time to peak shortening (r = 0.71) (P < 0.001 for each of these relations). These relations may be explained by regional differences in wall stress or by a regional Frank-Starling effect. The correlation between timing of shortening and prestretch demonstrates that mechanical timing is not determined by electrical phenomena alone. In conclusion, regional variation in LV function correlates with the nonuniform prestretch from atrial contraction.

Quantitative differentiation between BOLD models in fMRI.

Several gradient-echo fMRI blood oxygenation level-dependent (BOLD) effects are described in the literature: extravascular spin dephasing around capillaries and veins, intravascular phase changes, and transverse relaxation changes of blood. This work considers a series of tissue compartmentalized models incorporating each of these effects, and tries to determine the model which is most consistent with the data. To isolate the different tissue contributions, a series of multi-echo inversion recovery (IR) fMRI scans were performed. Visual stimulation experiments were performed at 1.5 T, one interleaved six-echo and two IR six-echo EPI scans (the latter to suppress gray matter (GM) and cerebrospinal fluid (CSF)). The tissue and vascular composition of activated areas was analyzed using independent spin-echo IR MRI experiments and MR venography, respectively. This information was used to fit the multi-echo fMRI data to the BOLD models. The activated areas almost always included a venous vessel visible on the venogram and consisted of GM and CSF. The fMRI signal changes were best described by extravascular dephasing effects in both GM and CSF around a venous vessel, in combination with intravascular effects. The role of spin dephasing around capillaries in GM appears to be insignificant. Magn Reson Med 45:233-246, 2001.

Simultaneous noninvasive measurement of blood flow in the great cardiac vein and left anterior descending artery.

Magnetic resonance (MR) flow mapping can be used to quantify flow velocity and volume flow in the coronary vessels noninvasively. The close anatomic relationship of the left anterior descending artery (LAD) with the great cardiac vein (GCV) allows imaging of both in one view. We examined the feasibility to discriminate between these two vessels based on the flow pattern and to measure the flow quantitatively. Eleven individuals with a normal LAD and 8 patients with a diseased LAD underwent MR imaging. From MR angiograms using connectivity to the aortic root, differentiation between the LAD and GCV was obtained. Perpendicular to both vessels, phase-contrast velocity mapping was performed to measure phasic and mean volume flow. After correction for cardiac motion of the vessel, GCV flow was found to be mainly systolic and pointing in the inverse direction as the predominantly diastolic flow in the LAD. These criteria appeared valid in all subjects, even in cases of highly stenotic arteries. The volume flow measurements corrected for body surface area were 31+/-15 ml/min/m2 in the normal LAD (n = 11) and 21+/-10 ml/min/m2 in the diseased LAD (n = 7). The volume flow measurements in the GCV corrected for body surface area were 23+/-19 ml/min/m2 in the normal vessels and 19+/-16 ml/min/m2 in the diseased vessels. In the patient with an occluded LAD and collaterals, the volume flow in the GCV was 7 ml/min/m2. MR is a unique tool for noninvasive simultaneous measurement of the flow pattern and volume flow in the GCV and the LAD, showing a clear distinction between arterial and venous flow.

High-resolution segmented EPI in a motor task fMRI study.

A high-resolution gradient echo, multi-slice segmented echo planar imaging method was used for functional MRI (fMRI) using a motor task at 1.5 Tesla. Functional images with an in-plane resolution of 1 mm and slice thickness of 4 mm were obtained with good white-gray matter contrast. The multi-shot approach, combined with a short total readout period of 82 ms, limits blurring effects for short T(2)(*) tissues (such as gray matter), assuring truly high-resolution images. In all subjects, motor functions were clearly depicted in the contralateral central sulcus over several slices and sometimes activation was detected in the supplementary motor area and/or ipsilateral central sulcus. The average signal change of 11+/-3% was much higher than in standard low-resolution fMRI EPI experiments, as a result of larger relative blood fractions.

Morphologic and functional evaluation of coronary artery bypass conduits.

There is clear evidence in the literature that conventional spin-echo and gradient-echo magnetic resonance imaging (MRI) is capable of assessing patency of coronary artery vein grafts. With more recently introduced breath-hold two-dimensional (2D) and contrast-enhanced 3D techniques, the predictive accuracy has further improved, with sensitivities and specificities in the 90% range. Limitations arise with regard to assessing obstructive disease and evaluating distal segments of sequential grafts, due to insufficient spatial resolution, low signal-to-noise ratio, and cardiac motion. Imaging of arterial grafts is complicated by the metallic clip artifacts. Adding information on graft flow patterns and flow reserve using velocity-encoded cine MRI may help to reduce some of the problems. Clinically, these functional measurements may become of use in non-invasive monitoring of gradually increasing graft narrowing. However, apart from a few exceptions, most patients undergo evaluation of their grafts because they are considered for a re-intervention by angioplasty or coronary artery bypass graft surgery. In these cases information on the status of the native coronary arteries is required. A broader clinical use of MRI in the evaluation of patients with coronary artery bypass grafts may therefore only be expected with further improvement in MR techniques for coronary angiography. J. Magn. Reson. Imaging 1999;10:734-740.

Sub-millimeter fMRI at 1.5 Tesla: correlation of high resolution with low resolution measurements.

Functional magnetic resonance imaging of the visual cortex with an in-plane resolution of 0.4 x 0.4 mm2 was performed using a simple visual stimulus resulting in clear maps of activation. A collapsing filter was used to compare these high-resolution images with low-resolution images collected during the same session. A good correspondence between the high- and low-resolution functional maps was found with respect to the center of localization of activation. However, only 20% of the size of activated areas in the low-resolution experiment was observed at high resolution, which was partly caused by the difference in signal-to-noise ratio. The high-resolution images produce signal changes much higher than the low-resolution images due to reduced partial volume effects. Additionally, the high-resolution functional maps were compared with detailed anatomical and venous information. The activated areas were predominantly observed at venous vessels within the sulci with a diameter on the order of the pixel size.

Blood pool agent strongly improves 3D magnetic resonance coronary angiography using an inversion pre-pulse.

The ability of a blood pool contrast agent to enhance MR coronary angiography was defined. The proximal coronary vessels of pigs were imaged before and after administration of Gd-DTPA bound covalently to bovine serum albumin (0.2 mmol/ kg). The contrast agent resulted in a reduction of the blood T1 value to 33+/-5 msec, as determined in vivo with a Look-Locker technique. Both 2D and 3D imaging techniques were performed. An inversion pulse suppressed the signal of nonblood tissue postcontrast. After contrast agent administration, in the 3D data set the signal-to-noise ratio (SNR) of blood and contrast-to-noise ratio (CNR) of blood to myocardium were improved by factors of 2.0+/-0.2 and 15+/-8, respectively (P < 0.05). Postcontrast, the 3D acquisition was superior to the 2D technique in terms of spatial resolution, SNR of blood, and CNR of blood to myocardium. The high contrast of the 3D data set allowed for direct and rapid display of coronary arteries using a "closest vessel projection."

Quantification of in-plane motion of the coronary arteries during the cardiac cycle: implications for acquisition window duration for MR flow quantification.

Motion of the coronary arteries during the heart cycle can result in image blurring and inaccurate flow quantification by MR. This condition applies particularly for longer acquisition windows that are typical of breath-hold coronary flow measurements. To determine the sensitivity of the technique to in-plane motion of different coronary arteries, the temporal variation in coronary position was measured in a plane perpendicular to the proximal portion of the vessel. The results indicated the presence of substantial displacement of the coronary arteries within the cardiac cycle, with a magnitude of motion approximately twice as large for the right as for the left coronary arteries. An estimation of the resulting vessel blurring was calculated, showing that the duration of the acquisition window for high spatial resolution coronary flow acquisitions should be less than 25 to 120 msec, depending on the specific coronary artery studied. In addition, these data specify optimal acquisition window placement for high resolution coronary angiography.

Clinical utility of two-dimensional magnetic resonance angiography in detecting coronary artery disease.

AIMS: The accuracy of magnetic resonance angiography in detecting proximal coronary artery stenoses is unclear. We postulated that fast magnetic resonance angiography is capable of (1) imaging proximal coronary arteries, and (2) detecting stenoses of > or = 50% of their luminal diameter. METHODS AND RESULTS: Thirty-five patients, referred for analysis of angina pectoris, underwent both conventional angiography and magnetic resonance angiography of coronary arteries. A fast k-space segmented gradient-echo technique was used during breath-holds. Two observers, blinded to the results of conventional angiography, independently analysed the magnetic resonance studies for (1) length of visualized segments, and (2) presence of signal voids indicative of stenoses. From 140 proximal arteries, 15 (11%) were excluded because of incomplete imaging or degraded image quality. Mean length of the visualized segments was 9 +/- 4 mm for the left main, 62 +/- 16 mm for the left anterior descending, 21 +/- 9 mm for the left circumflex and 89 +/- 32 mm for the right coronary artery. Sensitivity for detecting > or = 50% luminal diameter stenoses was 0.00 for the left circumflex, 0.53 for the left anterior descending coronary artery, 0.71 for the RCA and 1.00 for the left main artery. Specificity varied from 0.73 for the left anterior descending coronary artery to 0.96 for the left circumflex. Inter-observer agreement was 0.90. CONCLUSION: Thus, segmented magnetic resonance angiography is capable of non-invasive imaging of proximal coronary anatomy. Its good accuracy in detecting left main coronary artery disease, intermediate accuracy in detecting right coronary artery and left anterior descending coronary artery stenoses, and low accuracy in detecting left circumflex lesions fit within a range of sensitivities and specificities found by others. Further technical advances are necessary to make the technique clinically robust.

Three-dimensional respiratory-gated MR angiography of coronary arteries: comparison with conventional coronary angiography.

OBJECTIVE: MR coronary angiography is most often performed using two-dimensional techniques. Although three-dimensional (3D) acquisitions do have important advantages, they take too long for a single breath-hold and are thus susceptible to respiratory motion artifacts. The purpose of this study was to investigate the accuracy of a unique respiratory-gated 3D MR angiographic technique in identifying the proximal coronary arteries in patients suspected of having coronary artery disease. In addition, we investigated the capability of this technique to detect proximal stenoses. SUBJECTS AND METHODS: We performed a prospective blinded study in 20 patients who were referred for conventional coronary angiography. A cardiac-gated 3D gradient-echo sequence with fat suppression was used. Retrospective respiratory gating was performed using navigator echoes of the diaphragm position. Using multiplanar reformatting, two independent readers blindly analyzed the data sets for visualization of major coronary arteries, lengths of imaged segments, and detection of significant stenoses (> 50% occlusion of the luminal diameter by conventional angiography). RESULTS: Seventy-seven of 80 (96%) coronary arteries were positively identified. In one patient, an anomalous coronary anatomy was readily identified and confirmed by conventional angiography. The average lengths of the imaged segments of the right, left main, left anterior descending, and left circumflex coronary arteries were 58 +/- 13 mm, 9 +/- 5 mm, 59 +/- 16 mm, and 24 +/- 10 mm, respectively. Overall sensitivity for the detection of stenoses was low (38%), with a specificity of 95%. Interobserver agreement was 0.92, with a kappa value of 0.65. CONCLUSION: Respiratory-gated 3D MR angiography allows accurate identification of proximal coronary arteries and may be valuable for 3D imaging of coronary anomalies. Further technical improvements are required to enhance the value of the technique in detecting stenoses.

Assessment of flow in the right human coronary artery by magnetic resonance phase contrast velocity measurement: effects of cardiac and respiratory motion.

Flow in the human right coronary artery was determined using magnetic resonance phase contrast velocity quantification. Two methods were applied to reduce respiratory motion: Imaging during breath holding, which is fast, and retrospective respiratory gating, which has a high temporal resolution (32 ms) in the cardiac cycle. Vessel cross-sectional area, through-plane velocity, and volume flow were determined in six healthy subjects. In-plane vessel displacement during the cardiac cycle, caused by cardiac contraction, was about 2-4 mm within a time frame of 32 ms in systole and early diastole. The motion resulted in blurring of images obtained during breath holding caused by the large acquisition time window (126 ms) within the cardiac cycle. Therefore, only with a high temporal resolution correct velocity images over the entire cardiac cycle could be obtained. The time- and cross-sectionally averaged velocity was 7 +/- 2 cm/s, and the volume flow was 30 +/- 10 ml/min.

Transesophageal cardiac pacing during magnetic resonance imaging: feasibility and safety considerations.

The feasibility and safety of transesophageal cardiac pacing during clinical MRI at 1.5 Tesla is considered. An MRI compatible pace catheter was developed. In vitro testing showed a normal performance of the pulse generator, image artifacts that extended less than 11 mm from the catheter, and a less than 5% increase in noise. Cardiac stimulation induced by MRI was not observed and, theoretically, is not expected. Potentially, tissue around the catheter tip may become heated. This heating (delta tau) was monitored. Eight dogs were exposed to MRI during pacing. For low RF radiation exposure, a time-averaged squared B1 field below 0.08 p tau 2 (SAR < 0.03 W/kg), delta tau was below 1 degree C. For high RF radiation exposure, but at normal RF radiation specific absorption rate (0.4 W/kg) delta tau was 5 degrees C. Thus, transesophageal atrial pacing during MRI at low RF exposure seems to be possible to perform cardiac stress studies or to correct unstable heart rates.

Quantification of coronary artery bypass graft flow by magnetic resonance phase velocity mapping.

OBJECTIVES: Determination of the true coronary artery bypass graft function requires quantification of the flow rate within the graft. The purpose of the present study was to assess the feasibility of characterizing and quantifying graft flow by magnetic resonance phase velocity mapping. MATERIALS AND METHODS: Twenty-seven patients with 41 angiographically patent coronary artery bypass grafts underwent electrocardiographically gated magnetic resonance phase velocity mapping. Imaging was performed at 0.6 Tesla using a surface coil. Velocity maps of the bypass grafts were obtained throughout the cardiac cycle with a temporal resolution of 50 ms and a spatial resolution of 1.9 x 1.2 x 5 mm3, allowing calculation of phasic and mean graft flow. RESULTS: Adequate flow measurements were obtained in 84% (41 out of 49) of the grafts. Coronary artery bypass graft flow was characterized by a biphasic pattern with a first peak during systole and a second peak during diastole. Average maximum systolic and diastolic velocities over the cross-section of the grafts were 14 +/- 8 cm/s and 15 +/- 9 cm/s, respectively. Mean coronary artery bypass graft cross-sectional area was 0.28 +/- 0.13 cm2. Mean volume flow was 87 +/- 59 ml/min. CONCLUSION: Flow in coronary artery bypass grafts can be characterized and measured noninvasively by magnetic resonance phase velocity mapping.