ECG-gated T1 images of the heart

Robust self-navigated body MRI using dense coil arrays

PURPOSE

To develop a robust motion estimation method for free-breathing body MRI using dense coil arrays.

METHODS

Self-navigating pulse sequences can measure subject motion without using external motion monitoring devices. With dense coil arrays, individual coil elements can provide localized motion estimates. An averaged motion estimate over all coils is often used for motion compensation. However, this motion estimate may not accurately represent the dominant motion within the imaging volume. In this work, a coil clustering method is proposed to automatically determine the dominant motion for dense coil arrays. The feasibility of the proposed method is investigated in free-breathing abdominal MRI and cardiac MRI, and compared with manual motion estimate selection for respiratory motion estimation and electrocardiography for cardiac motion estimation.

RESULTS

Automated motion estimation achieved similar respiratory motion estimation compared to manual selection (averaged correlation coefficient 0.989 and 0.988 for abdominal MRI and cardiac MRI, respectively), and accurate cardiac triggering compared to electrocardiography (averaged temporal variability 17.5 ms).

CONCLUSION

The proposed method can provide accurate automated motion estimation for body MRI using dense coil arrays. It can enable self-navigated free-breathing abdominal and cardiac MRI without the need for external motion monitoring devices. Magn Reson Med 76:197-205, 2016. © 2015 Wiley Periodicals, Inc.

Motion-gated acquisition for in vivo optical imaging

Wide-field continuous wave fluorescence imaging, fluorescence lifetime imaging, frequency domain photon migration, and spatially modulated imaging have the potential to provide quantitative measurements in vivo. However, most of these techniques have not yet been successfully translated to the clinic due to challenging environmental constraints. In many circumstances, cardiac and respiratory motion greatly impair image quality and/or quantitative processing. To address this fundamental problem, we have developed a low-cost, field-programmable gate array-based, hardware-only gating device that delivers a phase-locked acquisition window of arbitrary delay and width that is derived from an unlimited number of pseudo-periodic and nonperiodic input signals. All device features can be controlled manually or via USB serial commands. The working range of the device spans the extremes of mouse electrocardiogram (1000 beats per minute) to human respiration (4 breaths per minute), with timing resolution <or=0.06%, and jitter <or=0.008%, of the input signal period. We demonstrate the performance of the gating device, including dramatic improvements in quantitative measurements, in vitro using a motion simulator and in vivo using near-infrared fluorescence angiography of beating pig heart. This gating device should help to enable the clinical translation of promising new optical imaging technologies.

Atrial natriuretic peptide in stable and decompensated chronic obstructive pulmonary disease

BACKGROUND

Plasma levels of atrial natriuretic peptide (ANP) are elevated in patients with chronic obstructive pulmonary disease (COPD) and may have a role in preventing oedema formation in these patients.

METHODS

Plasma ANP levels were measured in 60 patients with COPD and these measurements were related to pulmonary haemodynamics, response to treatment during exacerbations, and clinical patterns of the stable disease.

RESULTS

Plasma ANP levels did not correlate significantly with right atrial or pulmonary arterial pressures but did correlate significantly with both the right ventricular end diastolic volume and right ventricular wall volume measured by magnetic resonance imaging. Oxygen (2 1/min by nasal prongs for 30 minutes) did not change the mean pulmonary arterial pressure or the level of plasma ANP. In 20 patients with an acute exacerbation of COPD plasma ANP levels were higher in those with oedema (302 (185) pg/ml) than in those without oedema (87 (43) pg/ml). Oxygen given for one hour had no effect on plasma levels of ANP. However, plasma ANP levels fell over the first three days during treatment in those with oedema, the fall correlating with the change in body weight. In a further 20 stable patients with hypoxic COPD, those with hypercapnia and previous episodes of oedema had higher levels of plasma ANP (120 (50) pg/ml) than normocapnic patients with no previous oedema (54 (15) pg/ml).

CONCLUSIONS

The level of ANP is high in the plasma of patients with COPD, particularly during exacerbations in those with oedema. The association of a high plasma ANP level and volume overload is shown by the fall in ANP levels with treatment of the oedema, and the correlation between levels of ANP and right ventricular end diastolic or wall volumes.

Assessment of the right ventricle by magnetic resonance imaging in chronic obstructive lung disease

Right ventricular wall and chamber volume were measured by magnetic resonance imaging in 16 patients with stable chronic obstructive lung disease who subsequently underwent measurement of pulmonary haemodynamics by right heart catheterisation. The patients had a forced expiratory volume in one second of 0.7 (SD 0.3) litres, a forced vital capacity of 2.4 (1.0) l, an arterial oxygen tension (PaO2) of 6.5 (1.3) kPa, an arterial carbon dioxide tension (PaCO2) of 6.5 (1.0) kPa, and a mean pulmonary arterial pressure 30 (10) mm Hg. The mean right ventricular free wall volume was 57.1 (22.6) cm3, compared with a mean value of 115.0 (44.3) cm3 for the left ventricle and interventricular septal volume. The right ventricular chamber volume at end systole was 44.8 (23.4) cm3, whereas the left ventricular end systolic chamber volume was 51.1 (35.1) cm3. The right ventricular free wall volume correlated with the right ventricular chamber volume (r = 0.71), systolic (r = 0.74) and mean (r = 0.72) pulmonary arterial pressure, pulmonary vascular resistance (r = 0.67), and PaCO2 (r = 0.56). There was no significant correlation between the right ventricular free wall volume and PaO2 or the right ventricular ejection fraction, measured by radionuclide ventriculography. Assessment of the right ventricle by magnetic resonance imaging may help to better define patients with cor pulmonable and assess the long term effects of treatment in such patients.

A system for cardiac and respiratory gating of a magnetic resonance imager

A cardiac and respiratory gating system is described which minimises MRI distortion by using a commercially available pneumatic capsule for respiration sensing, and by isolating the ECG and respiration detector circuitry with a fibre-optic link. Operator use is facilitated by displaying, on the ECG trace, the time during the cardiac cycle when images are acquired, and by displaying the respiratory signal. The system is presently in use with a MD 800 MRI system (M&D Technology Ltd, Aberdeen) operating at 0.08 tesla.

Suppression of radiofrequency interference in cardiac gated MRI: a simple design

A simple design is proposed to suppress the noise pickup in the ECG leads from RF and gradient pulses during NMR imaging. The ECG signal is passed through a low-pass filter and a common-mode-rejection amplifier to reduce erroneous signal generated by the electrical ground loops. The output is gated through a CMOS switch to blank the preamplifier from the NMR during data acquisition.

Serial changes in the T1 magnetic relaxation parameter after myocardial infarction in man

A low field resistive nuclear magnetic resonance imaging system (0.08 Tesla) was used to study the in vivo changes in the relaxation parameter T1 of the left ventricular myocardium from the first day to six months after acute myocardial infarction in 41 consecutive patients admitted to a coronary care unit. T1 maps were constructed from transverse and coronal images at various times after infarction. Thrombolytic treatment had been successful in 28 patients. Thirty three of the 34 patients studied within two weeks of infarction had a significantly increased T1 value but this developed only after the third day in four. At day 1-3 the mean (1 SD) maximum T1 was 413 (29) ms (n = 23) compared with 430 (41) ms (n = 22) at day 4-7, 433 (35) ms (n = 24) at day 8-14, 420 (34) at one month (n = 22), 388 (39) (n = 20) at three months, and 361 (24) (n = 14) at six months. The number of regions of interest with an increased T1 followed a similar time course. Although the increase in T1 measured at three months correlated with the initial maximum creatine kinase and with the left ventricular ejection fraction measured at one month, the number of regions with abnormal T1 from day 4 through to one month correlated best with left ventricular ejection fraction. There was no significant difference in T1 between patients with or without reperfusion. The rise in T1 over the first few days together with the prolonged time course of T1 increase suggests that the increase in T1 may reflect cellular infiltration as much or more than tissue oedema.

Demonstration of pulsatile cerebrospinal-fluid flow using magnetic resonance phase imaging

The study of pulsatile cerebrospinal-fluid (CSF) flow may be useful in diagnosis of certain forms of intracranial disease. Previous techniques used to study CSF flow either are invasive or do not allow accurate measurement. Magnetic resonance imaging (MRI) offers a non-invasive method of studying the CSF pathways. Our technique uses MR phase images and allows quantitative measurement of flow velocities and volume-flow rates. Four volunteers were studied at the level of the second cervical vertebra (C2). The MRI pulse sequence was gated from the R-wave of the subject's electrocardiogram and 12 scans were taken corresponding to different times in the cardiac cycle. The variation in flow velocity throughout the cycle was plotted, and maximum caudad and cephalad flow velocities and flow rates were calculated. Good agreement was found between three of the four volunteers. The mean maximum caudad velocity was 2.91 cm s-1 occurring at a mean time of 190 ms after the R-wave. This corresponds to a mean maximum flow rate of 4.13 ml s-1. The total imaging time for each study was about 1 h. Technical developments, allowing simultaneous acquisition of several images throughout the cardiac cycle, will reduce this time significantly.

Cardiac gating in nuclear magnetic resonance imaging

In order to obtain NMR images of the heart, and to measure blood flow by NMR, the signal acquisition must be synchronized with the patient's cardiac cycle. Some of the problems of detecting the cardiac cycle within a strong uniform magnetic field without introducing RF interference into the imaging system are discussed. Some general applications of cardiac gated NMR imaging are presented.