Real-time adaptive motion correction in functional MRI

Functional magnetic resonance imaging (fMRI) of the brain is often degraded by bulk head motion. Algorithms that address this by retrospective re-registration of images in an fMRI time series are all fundamentally limited by any motion that occurs through-plane. Here, a technique is described that can account for such motion by prospective correction in real time. A navigator echo is used before every image acquisition to detect superior/inferior displacements of the head. The displacement information is then used to adjust the plane of excitation of the ensuing single-shot echo-planar fMRI axial image. These correction updates can be completed in 100 mm with motion sensitivity at least as small as 0.5 mm. The efficacy of this method is documented in phantom and human studies.

Magnetic resonance imaging of transverse acoustic strain waves

We describe a phase contrast based MRI technique with high sensitivity to cyclic displacement that is capable of quantitatively imaging acoustic strain waves in tissue-like materials. A formalism for considering gradient waveforms as basis functions to measure arbitrary cyclic motion waveforms is introduced. Experiments with tissue-like agarose gel phantoms show that it is possible to measure small cyclic displacements at a submicron level by an appropriate choice of the applied gradient basis function and to use this capability to observe the spatial and temporal pattern of displacements caused by acoustic strain waves. The propagation characteristics of strain waves are determined by the mechanical properties of the media. It is therefore possible to use this technique to noninvasively estimate material properties such as elastic modulus.

Algorithms for extracting motion information from navigator echoes

Algorithms to reliably detect motion in navigator echoes are crucial to many MRI motion suppression techniques. The accuracy of these algorithms is affected by noise and deformation of navigator echo profile caused by physiologic motion. This study compared the performance of algorithms based on correlation and least squares for extracting displacement information from motion-monitoring navigator echoes, using computer simulation and in vivo imaging. The least squares algorithm was determined to be of higher accuracy than the correlation algorithm against errors caused by noise and profile deformation.

Colorectal polyp detection with CT colography: two- versus three-dimensional techniques. Work in progress

PURPOSE

To compare detection of colorectal polyps with two-dimensional (2D) computed tomographic (CT) colography only, three-dimensional (3D) CT colography only, and a combination of 2D and 3D CT colography.

MATERIALS AND METHODS

A total of 11 computer-simulated polyps (1-10 mm) were placed randomly in five identical CT data sets for images of a 72-year-old man's polyp-free, rectosigmoid colon. Fifteen CT colographic data sets were produced: five with 2D CT images only, five with 3D CT images only, and five with 2D and 3D CT images. Two radiologists randomly, blindly, and independently evaluated all 15 data sets to detect the simulated polyps.

RESULTS

No polyps 2 mm or smaller were detected. No statistically significant differences in the detection of colorectal polyps were found between the three techniques. However, the combination of 2D and 3D CT colography resulted in polyp detection rates that were greater than or equal to those of 2D or 3D CT colography alone. Flat polyps were more difficult to detect than sessile polyps. Five false-positive findings occurred with 2D CT colography.

CONCLUSION

A combined display of 2D and 3D CT images likely provides the greatest rate of detection of colorectal polyps.

Variability of consecutive in vivo MR flow measurements in the main portal vein

OBJECTIVE

The variability of consecutive cine phase-contrast MR flow measurements could significantly affect their use for clinical decisions, especially during provocative testing. The purposes of this study were to determine the normal variability of flow and consecutive flow measurements in the main portal vein on MR images and to determine how intraobserver variability, interobserver variability, and MR imager variability affect these measurements.

SUBJECTS AND METHODS

MR flow measurements were acquired four consecutive times at the same location in the main portal vein of 12 subjects and three consecutive times at the same location in a nonpulsatile vessel model. All acquisitions were completed within 10 min. All main portal vein MR data sets were evaluated manually in a blinded review by two independent observers during three separate sessions spaced a mean of 4.5 weeks apart. Flow model data sets were evaluated during a single session by one observer. Variabilities were subsequently calculated by a components-of-variance analysis and by the coefficient of variation (SD/mean x 100).

RESULTS

Of the total variance, 90% was due to flow variability among subjects (intersubject), 6% to flow variability within one subject (intrasubject), 2% to intraobserver variability, and 2% to interobserver variability. The coefficient of variation of consecutive MR portal vein flow measurements within a single subject was 11% +/- 5% (range, 3-23%). Intra- and interobserver variabilities were 5% +/- 2% (range, 1-11%) and 4% +/- 4% (range, 0-17%), respectively. MR imager variability was 1% +/- 1% (range, 0-2%).

CONCLUSION

The mean variability of consecutive cine phase-contrast MR flow measurements in the main portal vein is 11% +/- 5% and could affect research and clinical protocols that employ this technique.

MR imaging of physeal bars

PURPOSE

To determine if three-dimensional (3D) rendered and projection images derived from magnetic resonance (MR) imaging data are advantageous for the preoperative assessment of physeal bars.

MATERIALS AND METHODS

Fifteen patients with suspected physeal bars were examined with MR imaging. Gradient-echo sequences were used with both two-dimensional and 3D MR imaging techniques. The 3D rendered and projection images were then derived from the MR imaging data. Patients were categorized on the basis of the effect of MR images on the decision to perform physeal bar excision.

RESULTS

MR findings had either a moderate (n = 7) or a major (n = 6) effect on the surgical treatment of 13 patients (87%). They had no effect in two patients (13%) and were misleading in none.

CONCLUSION

MR imaging is an excellent modality for imaging physeal bars. Imaging data can be processed to yield both 3D rendered and projection physeal maps that are particularly useful in preoperative planning.

Multiple breathhold 3D time-of-flight MR angiography of the renal arteries

A technique is described for angiographic imaging of the renal arteries with acquisition performed over several periods of suspended respiration. The 3D Fourier transform (FT) gradient-echo angiographic sequence uses magnetization preparation and appropriately chosen delay times for background nulling and time-of-flight enhancement of the vasculature. The sequence was applied to 10 volunteers, each of whom was imaged in three ways: (i) over a series of breathholds in which feedback was provided to enable reproducible breathholding; (ii) over a series of breathholds with no feedback; and (iii) over continuous respiration. Results were evaluated by measuring the transverse extent of the well-delineated renal vasculature and by noting the distal extent of the vasculature branching (main, segmental, and interlobar branches). The transverse extent of renal vasculature visible with breathhold feedback, breathholding, and free breathing was 6.1 +/- 0.9 cm, 5.0 +/- 1.8 cm, and 4.0 +/- 1.4 cm, respectively (mean +/- SD). Breathhold feedback enabled visualization of segmental renal arteries bilaterally in all 10 volunteers.

Navigator-echo-based real-time respiratory gating and triggering for reduction of respiration effects in three-dimensional coronary MR angiography

PURPOSE

To test the hypothesis that respiration effects in three-dimensional (3D) coronary magnetic resonance (MR) imaging can be reduced with navigator-echo-based gating or triggering according to the superior-inferior position of the diaphragm.

MATERIALS AND METHODS

Real-time respiratory gating and respiratory triggering (breath hold with feedback) were implemented with navigator echoes in a magnetization-prepared, segmented, 3D coronary imaging sequence. The two techniques were first tested with a motion phantom. An imaging protocol that compared real-time respiratory-gated acquisition, real-time respiratory-triggered acquisition, and continuous acquisition was then evaluated in six healthy subjects.

RESULTS

Real-time respiratory-gated and respiratory-triggered acquisition were superior to continuous acquisition with two signals averaged (P = .025). The performance of the gated acquisition was about the same as that of the triggered acquisition (P = .05).

CONCLUSION

Navigator-echo-based, real-time respiratory-gating and respiratory-triggering techniques are practical methods for effective reduction of respiration effects in coronary MR imaging.

Detection of colorectal polyps by computed tomographic colography: feasibility of a novel technique

BACKGROUND & AIMS

Computed tomographic colography (CTC) represents a novel technique for colorectal polyp detection. A prospective study was undertaken to determine the optimal CTC scanning parameters based on an artificial colon model and to assess the feasibility of CTC to detect clinically significant colorectal polyps.

METHODS

A colon model was scanned by helical computed tomography at multiple parameters. Reformatted two-dimensional and three-dimensional images were then graded for polyp detection and image quality. Subsequently, 10 patients with known colon polyps underwent CTC immediately before colonoscopy. The number of polyps detected by two radiologists using CTC were compared with colonoscopy results that served as the gold standard.

RESULTS

The optimal scanning parameters in the colon model were 5-mm collimation, 5 mm/s table speed, and 1-mm reconstruction interval. Ten patients had 30 polyps (range, 0.2-2.0 cm) by colonscopy, and all polyps > or = 0.5 cm were adenomas. Polyp detection by CTC for both observers was 100% (5 of 5) > or = 1 cm, 71% (5 of 7) between 0.5 and 0.9 cm, and 11%-28% (2-5 of 18) < 0.5 cm.

CONCLUSIONS

Based on this small, unblinded pilot study, CTC is feasible for colorectal polyp detection > or = 0.5 cm in diameter.

Abdominal phase-contrast MR angiography: breath-hold versus non-breath-hold techniques

Breath-hold magnetic resonance (MR) imaging is now replacing many non-breath-hold pulse sequences in the upper abdomen because of faster imaging times and improved image quality. The authors compared non-breath-hold cine phase-contrast (PC) and breath-hold 2D phase-contrast (2DPC) magnetic resonance (MR) angiograms of the main portal vein (MPV) and superior mesenteric artery (SMA) in 12 volunteers. All angiograms were graded in overall image quality, vessel conspicuity, and signal-to-noise ratios (SNR). In the MPV MR angiograms, the breath-hold 2DPC sequence produced better images than the non-breath-hold cine PC sequence as graded by overall image quality (P = .016) and SNR (P = .004). Conversely, in the SMA MR angiograms, the non-breath-hold cine PC sequence produced better images than the breath-hold sequence in terms of overall image quality (P = .008) and SNR (P = .008). By reducing the most significant cause of image artifacts, (ie, using a breath-hold 2DPC sequence to decrease respiratory misregistration of the MPV, and using a cardiac-gated cine PC sequence to minimize pulsatile artifacts of the SMA), one can clearly optimize the quality of MR angiography.

Orbital navigator echoes for motion measurements in magnetic resonance imaging

A single "orbital" navigator echo, that has a circular k-space trajectory, is used to simultaneously measure in-plane rotational and multi-axis translational global motion. Rotation is determined from the shift in the magnitude profile of the echo with respect to a reference echo. Displacements are calculated from the phase difference between the current echo and a reference echo. Phantom studies show that this technique can accurately measure rotation and translations. Preliminary results from adaptive motion correction studies on phantom and human subjects indicate that the orbital navigator echo is an effective method for motion measurement in MRI.

Magnetic resonance elastography by direct visualization of propagating acoustic strain waves

A nuclear magnetic resonance imaging (MRI) method is presented for quantitatively mapping the physical response of a material to harmonic mechanical excitation. The resulting images allow calculation of regional mechanical properties. Measurements of shear modulus obtained with the MRI technique in gel materials correlate with independent measurements of static shear modulus. The results indicate that displacement patterns corresponding to cyclic displacements smaller than 200 nanometers can be measured. The findings suggest the feasibility of a medical imaging technique for delineating elasticity and other mechanical properties of tissue.

Error in MR volumetric flow measurements due to ordered phase encoding in the presence of flow varying with respiration

Respiratory ordered phase encoding is often employed in MRI studies to reduce image artifacts due to breathing motion. The purpose of this work was to evaluate error caused by the use of respiratory ordering of phase encoding in MR cine phase-contrast (CPC) volumetric flow measurements when the flow rate is sensitive to respiration. It was hypothesized that this effect is due to the systematic biasing of a respiratory-induced phase modulation function in k-space. A theoretical model for the effects of respiration was developed and then tested in flow phantom studies and in normal volunteer studies. In phantom experiments, the use of respiratory ordering induced an error of as much as 13% in CPC volumetric flow measurements. In preliminary volunteer studies, error was as high as 26% in superior vena cava flow measurements versus less than 1% error in the ascending aorta. It is concluded that a potential for error exists in CPC volumetric flow measurements obtained with the use of respiratory ordering schemes. Volunteer studies with larger numbers are warranted. Clinical applications in which this effect may be important include flow measurements in vessels subject to variations in flow due to respiration, such as the venae cavae, pulmonary vasculature, and portal vein.

3D coronary MR angiography in multiple breath-holds using a respiratory feedback monitor

To reduce respiratory blur and ghosts in 3D coronary imaging, a data acquisition scheme using consistent multiple breath-holds was implemented. A navigator echo was acquired and processed in real time to dynamically measure diaphragm position. This information was provided as a visual prompt to the patient to maintain consistency in breath-hold levels such that the variation range of diastolic heart position was less than 2 mm. Preliminary results indicate that this multiple breath-hold acquisition scheme, compared with acquisition under respiration, can significantly reduce blur and ghost artifacts in 3D coronary imaging.

MR imaging of the abdomen with a phased-array multicoil: prospective clinical evaluation

PURPOSE

To prospectively compare use of a phased-array multicoil and a conventional body coil in abdominal MR imaging.

MATERIALS AND METHODS

Thirteen patients (seven men, six women; mean age, 55 years) underwent imaging with a phased-array multicoil and with a conventional body coil. Four pulse sequences were used: T2-weighted spin echo (SE), magnetization-prepared gradient-recalled echo (GRE), breath-hold fast SE, and echo planar (EP).

RESULTS

Lesion detection improved the most on fast SE, multicoil-acquired images. Signal-to-noise ratio (S/N) increased 64% with fast SE (P = .0005) and EP (P < .0109) sequences. Contrast-to-noise ratio (C/N) doubled (P < .05) with T2-weighted SE sequences. Lesion conspicuity improved on multicoil-acquired images with all fast sequences (magnetization-prepared GRE, P = .015; fast SE, P = .002; EP imaging, P = .013). There was little difference in respiratory and vascular artifact. Depiction of most abdominal structures improved (P < .01).

CONCLUSION

Use of the phased-array multicoil provides better MR images of the abdomen than does use of a conventional body coil.

Spatial-frequency-tuned markers and adaptive correction for rotational motion

A common type of motion present in clinical magnetic resonance imaging examinations is rotational motion, such as that due to voluntary motion during head examinations. The correction scheme presented in this work offers a method for eliminating the effects of rotations within the imaging plane. Integral to the implementation of this technique is the concept and design of spatial-frequency-tuned markers, which are used to track the rotational motion. These studies showed that it is possible to accurately track the motion, measuring both axis and angle of rotation, and use this information to retrospectively correct the acquired images. These markers can also provide information about any translational motion present. The resulting images show a marked decrease in artifacts and improved clarity.

Respiratory motion of the heart: kinematics and the implications for the spatial resolution in coronary imaging

Respiratory motion is a major limiting factor in improving image resolution and signal-to-noise ratio in MR coronary imaging. In this work the effects of respiration on the cardiac position were studied quantitively by imaging the heart during diastole at various positions of tidal respiration with a breath-hold segmented fast gradient echo technique. It was found that during tidal breathing the movement of the heart due to respiration is dominated by superior-inferior (SI) motion, which is linearly related to the SI motion of the diaphragm. The motion of the heart due to respiration is approximately a global translation. These results provide motivation for employing adaptive motion correction techniques to reduce image blurring in nonbreath-hold coronary MR imaging.

MR measurements of mesenteric venous flow: prospective evaluation in healthy volunteers and patients with suspected chronic mesenteric ischemia

PURPOSE

To quantify portal vein (PV) and superior mesenteric vein (SMV) flow before and after a standardized meal in healthy volunteers and to prospectively evaluate patients with a clinical suspicion of chronic mesenteric ischemia on the basis of magnetic resonance (MR) measurement of flow in the mesenteric venous system in volunteers.

MATERIALS AND METHODS

Cine phase-contrast flow measurements were acquired in 10 asymptomatic volunteers and in 10 patients.

RESULTS

In volunteers, the difference between the fasting and post-prandial flows in the SMV and PV was significant (P < .001), with a peak flow augmentation of 245% +/- 74 and 70% +/- 29, respectively. Postprandial augmentation of peak flow in the SMV was significantly less in patients with mesenteric ischemia compared with volunteers (64% +/- 28; P = .02). SMV flow augmentation in patients without mesenteric ischemia did not differ significantly from that in volunteers (206% +/- 36; P = .31).

CONCLUSION

Measurement of postprandial flow augmentation in the SMV with MR imaging shows promise as a noninvasive screening test for chronic mesenteric ischemia.

Comparison of breath-hold fast spin-echo and conventional spin-echo pulse sequences for T2-weighted MR imaging of liver lesions

PURPOSE

To evaluate a breath-hold fast spin-echo (SE) technique for T2-weighted magnetic resonance (MR) imaging of liver lesions.

MATERIALS AND METHODS

A fast SE technique was developed that enabled six sections to be imaged per 16-second breath hold with a single echo. Resulting images were compared with those obtained with the first echo of a conventional dual-echo T2-weighted SE sequence (16 minutes 55 seconds for 18 sections). Thirty-one patients with malignant focal hepatic lesions were studied prospectively. The images were compared quantitatively and qualitatively.

RESULTS

Quantitatively, the contrast and contrast-to-noise ratios for the fast SE images were 20% +/- 5 and 19% +/- 8 greater, respectively, than those for the conventional T2-weighted SE images of the 54 representative lesions. Qualitatively, fast SE images had less image artifact, enabled comparable or better lesion sizing, and greatly improved depiction of extrahepatic structures compared with conventional T2-weighted SE images.

CONCLUSION

The fast SE technique with breath holding provides diagnostically useful liver images in a greatly decreased acquisition time.

The effect of respiration on the contrast and sharpness of liver lesions in MRI

This work demonstrates the effects of through-plane motion due to respiration on contrast and sharpness of liver lesions in MRI. The effects of slice coverage with and without such respiratory motion is also reported. This work is comprised of two parts: a theoretical prediction of liver-lesion contrast and blur with and without respiration and an experimental validation using gel phantoms of the predicted results. Both theory and experiment show a loss of contrast, increasing with amplitude of the peak-to-peak motion. The loss of contrast for a 5-mm lesion at normal respiration of 15 mm peak-to-peak superior-inferior motion is approximately 10% with a low order sorted respiratory ordered phase encoding acquisition and approximately 50% for an unsorted acquisition. Lesion blur is greatest for the low order sorted acquisition while the unsorted and high sort acquisitions maintain edge definition. Breath-hold imaging is potentially superior to nonbreath-hold imaging in liver lesion contrast and edge definition, but is more sensitive to inadequate slice coverage.