(Radiographics. 2001;21:767-779.)
© RSNA, 2001
Imaging & Therapeutic Technology |
Principles and Applications of Echo-planar Imaging: A Review for the General Radiologist1
Mehdi Poustchi-Amin, MD,
Scott A. Mirowitz, MD,
Jeffrey J. Brown, MD,
Robert C. McKinstry, MD, PhD and
Tao Li, MD
1 From the Mallinckrodt Institute of Radiology, 510 S Kingshighway Blvd, St Louis, MO 63110. Presented as a scientific exhibit at the 1999 RSNA scientific assembly. Received March 17, 2000; revision requested May 18 and received July 30; accepted August 31. Address correspondence to M.P.A. (e-mail: mpamin@yahoo.com).
 |
Abstract
|
|---|
Echo-planar imaging is a very fast magnetic resonance (MR) imaging technique capable of acquiring an entire MR image in only a fraction of a second. In single-shot echo-planar imaging, all the spatial-encoding data of an image can be obtained after a single radio-frequency excitation. Multishot echo-planar imaging results in high-quality images comparable to conventional MR images. However, echo-planar imaging offers major advantages over conventional MR imaging, including reduced imaging time, decreased motion artifact, and the ability to image rapid physiologic processes of the human body. The use of echo-planar imaging has already resulted in significant advances in clinical diagnosis and scientific investigation, such as in evaluation of stroke and functional imaging of the human brain, respectively. The clinical indications for echo-planar imaging are expanding rapidly, and it can now be applied to many parts of the body, including the brain, abdomen, and heart. Today, with the availability of echo-planar imaging–capable MR imagers at many sites, the general radiologist can benefit from echo-planar imaging and its clinical applications.
Index Terms: Abdomen, MR, 70.121416 • Brain, MR, 10.121416 • Heart, MR, 50.121416 • Magnetic resonance (MR), echo planar • Magnetic resonance (MR), motion correction • Magnetic resonance (MR), rapid imaging
|
LEARNING OBJECTIVES FOR TEST 6
|
|---|
After reading this article and taking the test, the reader will be able to:
- Discuss the principles and characteristics of echo-planar imaging.
- Describe the major current clinical applications of echo-planar imaging.
- List the potential future clinical applications of echo-planar imaging.
|
Introduction
|
|---|
Echo-planar imaging (EPI) is capable of significantly shortening magnetic resonance (MR) imaging times. Echo-planar imaging allows acquisition of images in 20–100 msec. This time resolution virtually eliminates motion-related artifacts. Therefore, imaging of rapidly changing physiologic processes becomes possible. Echo-planar images with resolution and contrast similar to those of conventional MR images can be obtained by using multishot acquisitions in only a few seconds. Although primarily used for imaging the brain, echo-planar imaging can be applied to many anatomic regions of the body, such as the heart and abdomen. Echo-planar imaging–compatible MR imaging hardware and software is gradually being introduced and installed by commercial vendors throughout the world, thus making echo-planar imaging available at many MR imaging sites for clinical imaging.
In this article, we briefly review the basic principles of echo-planar imaging for the general radiologist and discuss its current and potential future clinical applications. Specific topics discussed are history, basic principles, special hardware requirements, characteristics, clinical applications, other uses for echo-planar imaging–capable systems, and comparison with other fast MR imaging techniques.
|
History
|
|---|
In 1977, Sir Peter Mansfield of the University of Nottingham described the general principles of echo-planar imaging (1). However, MR imaging hardware and computer limitations prevented the technique from being used outside of a laboratory setting. Mansfield’s group presented the first snapshot echo-planar images in 1981 and 1982 (2,3). Using an MR imager with a 12-cm-bore probe, they obtained six images of the heart of a sedated rabbit, which showed cardiac motion as a continuous loop. With improvements in MR imaging hardware and computer capabilities, this group was able to obtain cardiac images of an anesthetized piglet in 1983 (4). Finally, they obtained the first echo-planar images of a human in 1983 (Fig 1). These were images of the thorax of a 3-month-old infant and consisted of 32 sections with 16 images per section, for a total of 512 images (5). Each image was obtained in 35 msec, with a total imaging time of 4.5 minutes. The magnetic field strength was 940 G (0.094 T).
In 1983, these investigators summarized the advantages of echo-planar imaging as follows (5): "The EPI [echo-planar imaging] technique is devoid of known hazards compared with methods entailing ionizing radiation. Blood flow pattern may be observed without the injection of a potentially harmful contrast medium or radioisotope. EPI is quick and carries a prospect of a revolution in imaging methods in the thorax and elsewhere." In fact, the extreme speed of image acquisition was what differentiated these investigators’ work on echo-planar imaging from work on other MR imaging techniques in the early days of clinical MR imaging. Today, almost a quarter of a century after Mansfield’s description of the technique, echo-planar imaging is finally beginning to realize some of its early promise in the clinical arena.
|
Basic Principles
|
|---|
Before discussing the basic principles of echo-planar imaging or any other MR imaging technique, it is important to present the concept of k space. k space is a graphic matrix of digitized MR imaging data that represents the image prior to Fourier transform analysis. All points in k space contain data from all locations within an MR image. The Fourier transform of k space is the image (6). To understand echo-planar imaging, it is useful to compare it with conventional spin-echo (SE) imaging. In an SE pulse sequence, one line of imaging data (one line in k space or one phase-encoding step) is collected within each repetition time (TR) period (Fig 2). The pulse sequence is then repeated for multiple TR periods until all phase-encoding steps are collected and k space is filled. Therefore, the imaging time is equal to the product of the TR and the number of phase-encoding steps. For example, if the TR is 2 seconds and the number of phase-encoding steps is 256, the imaging time is 512 seconds or about 8.5 minutes. If multiple RF excitations are used for data averaging, the imaging time increases proportionally.
View larger version (74K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 2. Conventional SE imaging. Within each TR period, the pulse sequence is executed and one line of imaging data or one phase-encoding step is collected. The frequency-encoding gradient (Gx), phase-encoding gradient (Gy), and section-selection gradient (Gz) are shown during one TR period. RF = radio frequency.
|
|
In echo-planar imaging, multiple lines of imaging data are acquired after a single RF excitation. Like a conventional SE sequence, an SE echo-planar imaging sequence begins with 90° and 180° RF pulses. However, after the 180° RF pulse, the frequency-encoding gradient oscillates rapidly from a positive to a negative amplitude, forming a train of gradient echoes (Fig 3). Each echo is phase encoded differently by phase-encoding blips on the phase-encoding axis. Each oscillation of the frequency-encoding gradient corresponds to one line of imaging data in k space, and each blip corresponds to a transition from one line to the next in k space. This technique is called blipped echo-planar imaging (7). In the original echo-planar imaging method, the phase-encoding gradient was kept on weakly but continuously during the entire acquisition (8). Today, echo-planar imaging has many variants. In one of these variants, asymmetric echo-planar imaging, data are collected only during the positive frequency-encoding gradient lobe. The negative frequency-encoding gradient lobe is used to just traverse back to the other side of k space (9). This type of data collection strategy is also used in flow-compensated echo-planar imaging.
View larger version (78K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 3. Echo-planar imaging. Within each TR period, multiple lines of imaging data are collected. Gx = frequency-encoding gradient, Gy = phase-encoding gradient, Gz = section-selection gradient.
|
|
|
Special Hardware Requirements
|
|---|
For SE imaging, a conventional MR imaging system is usually equipped with gradient coils capable of a maximum amplitude of approximately 10 mT/m, a minimum rise time of 0.6 msec, a slew rate (amplitude/rise time) of 17 T/m per second, and a duty cycle of 8% (percentage of time the gradients are on during a hypothetical TR). However, in echo-planar imaging, the requirements for gradient strength, rise time, and duty cycle are markedly increased because all of k space can be traversed in a single RF excitation by using a rapidly oscillating frequency-encoding gradient. Echo-planar imaging is accomplished by using gradient coils capable of a maximum amplitude of 20 mT/m, a minimum rise time of 0.1 msec, a slew rate of 200 T/m per second, and a duty cycle of 50%–60%. This remarkable technical achievement was enabled by the marriage of resonant (rapidly switching) gradient technology to power supplies that are capable of catching a resonant mode at maximum amplitude and holding it there for a short time (10). However, the first generation of echo-planar imaging hardware (resonant systems) had some limitations, resulting in little effect on routine clinical imaging. With development of the second generation of echo-planar imaging hardware (nonresonant systems), there was extension of echo-planar imaging applications further into mainstream clinical MR imaging (11).
|
Characteristics
|
|---|
Echo-planar imaging can be performed by using single or multiple excitation pulses ("shots"). The number of shots represents the number of TR periods required to complete the image acquisition. The number of shots is equal to the total number of phase-encoding steps divided by the echo train length. For example, if the number of phase-encoding steps is 256 and the echo train length is eight, then the number of shots equals 256/8 or 32. In single-shot (snapshot) echo-planar imaging, all of the k-space data are acquired with only one shot. However, the image acquisition matrix is typically no larger than 128 x 128. To achieve higher resolution and reduce the image distortion and signal loss due to susceptibility differences, T2 relaxation, and main field inhomogeneities, multishot echo-planar imaging can be performed (12). In multishot echo-planar imaging, only a portion of the k-space data is acquired with each shot, and the shots are repeated until a full set of data is collected. It is the lack of RF refocusing pulses that is responsible for the unique qualities of echo-planar imaging, which include increased sensitivity to off-resonance effects (phase coherence loss), T2 or T2* imaging (effect of static magnetic field inhomogeneities), improved section efficiency (increased number of sections per TR), and snapshot imaging.
Increased sensitivity to off-resonance effects in echo-planar imaging is due to the lack of RF refocusing pulses. In the absence of RF refocusing pulses, the spinning protons accumulate a phase error, which causes positioning errors in the phase-encoding direction, resulting in significant artifact. Chemical shift causes a common off-resonance artifact in echo-planar imaging due to fat protons precessing at 220 Hz off resonance. This problem is typically resolved by using fat suppression techniques, which are used routinely in all types of echo-planar imaging. A related artifact occurs due to water protons precessing off resonance, resulting in geometric distortion of the image. This effect is caused by magnetic field in-homogeneities and is particularly prominent in anatomic regions with air-tissue interfaces, such as the base of the skull and the lungs. Off-resonance effects in echo-planar imaging can be minimized or resolved by the following means: (a) Decrease the echo train time; (b) for single-shot echo-planar imaging, decrease the spatial resolution by using a coarser image matrix; (c) swap the phase- and frequency-encoding directions, resulting in the phase-encoding direction being oriented along the natural axis of symmetry of the imaged organ; and (d) use higher-performance imaging gradients (Figs 4,5).
View larger version (157K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 4a. Comparison between single-shot and multishot echo-planar imaging. Axial images were obtained with one shot (a), eight shots (b), 16 shots (c), and 32 shots (d). The geometric distortion of the anterior aspect of the brain (arrow) is reduced as the number of shots increases.
|
|
View larger version (134K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 4b. Comparison between single-shot and multishot echo-planar imaging. Axial images were obtained with one shot (a), eight shots (b), 16 shots (c), and 32 shots (d). The geometric distortion of the anterior aspect of the brain (arrow) is reduced as the number of shots increases.
|
|
View larger version (135K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 4c. Comparison between single-shot and multishot echo-planar imaging. Axial images were obtained with one shot (a), eight shots (b), 16 shots (c), and 32 shots (d). The geometric distortion of the anterior aspect of the brain (arrow) is reduced as the number of shots increases.
|
|
View larger version (127K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 4d. Comparison between single-shot and multishot echo-planar imaging. Axial images were obtained with one shot (a), eight shots (b), 16 shots (c), and 32 shots (d). The geometric distortion of the anterior aspect of the brain (arrow) is reduced as the number of shots increases.
|
|
View larger version (150K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 5a. Swapping of phase- and frequency-encoding directions. (a) Axial echo-planar image obtained with a left-to-right phase-encoding direction shows geometric distortion. (b) Axial echo-planar image obtained with the phase-encoding direction along the natural axis of symmetry of the brain (anterior to posterior) shows reduced geometric distortion.
|
|
View larger version (148K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 5b. Swapping of phase- and frequency-encoding directions. (a) Axial echo-planar image obtained with a left-to-right phase-encoding direction shows geometric distortion. (b) Axial echo-planar image obtained with the phase-encoding direction along the natural axis of symmetry of the brain (anterior to posterior) shows reduced geometric distortion.
|
|
|
Clinical Applications
|
|---|
Brain Imaging
The most prevalent clinical application of echo-planar imaging is imaging of the brain. Echo-planar imaging–based diffusion imaging is routinely used for evaluation of early cerebral ischemia and stroke. Contrast material–enhanced echo-planar imaging–based perfusion imaging is performed to evaluate cerebral ischemia and differentiate recurrent tumor from radiation necrosis. Perfusion imaging can also be performed without a contrast agent (13). Evaluation of cortical activation with echo-planar imaging or functional MR imaging is an active area of research in neuroscience. In uncooperative patients, single-shot echo-planar imaging of the whole brain (20 images) can be performed in 2 seconds (Fig 6). T2-weighted images can be acquired with multishot echo-planar imaging in about 1 minute, potentially replacing SE or fast SE T2-weighted images, which have average imaging times of 2–15 minutes.
View larger version (64K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 6. Hypertensive hemorrhage in the head of the left caudate nucleus in a 79-year-old patient. Left: Axial conventional SE image shows motion artifact. Middle: Axial diffusion-weighted echo-planar image shows the hemorrhage clearly (arrow), without motion artifact. Right: Axial gradient-echo (GRE) echo-planar image shows a blooming effect and extension of the hemorrhage into the left lateral ventricle (arrow), findings not visible on the conventional SE image.
|
|
Diffusion Imaging
Diffusion is the random thermal motion of molecules through a tissue compartment. The signal intensity at MR imaging is dependent among other factors on water motion, which intrinsically produces contrast. Diffusion of water molecules through a magnetic field gradient causes intravoxel dephasing and loss of signal intensity. Water molecules diffuse approximately at a rate of only 1:10 mm/sec. In diffusion-weighted echo-planar imaging, image acquisition is sensitized to the diffusion of water molecules by inserting very strong motion-sensitizing gradient pulses into the echo-planar imaging pulse sequence. Diffusion pulses cause the echo-planar imaging MR signal to dephase or to diminish in proportion to the random velocities of the diffusing water molecules, within the order of magnitude of the voxel size during an echo time (TE). These velocities have been referred to as intravoxel incoherent motion (14).
In the United States, many neuroradiologists routinely use diffusion-weighted imaging to localize a focal ischemic region during the acute phase of stroke (Fig 7). Minutes after the onset of an ischemic event, there is a decrease in the diffusion of water molecules through the ischemic region; this decrease manifests as an increase in signal intensity on diffusion-weighted echo-planar images, reflecting a decrease in the apparent diffusion coefficient. The working hypothesis to explain this effect is that impaired energy transport in the cell causes swelling or cytotoxic edema, which results in a decrease in the diffusion rate of water molecules. Echo-planar imaging–based diffusion imaging aids in characterization of suspected cerebral ischemia. It allows confirmation of the acute versus nonacute ischemic origin of the symptoms. This confirmation helps in triaging patients for thrombolytic or other aggressive therapies within the therapeutic window.
View larger version (147K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 7a. Infarct of the right superior cerebellar artery territory in a 14-year-old girl with sickle cell disease. (a) Contrast-enhanced coronal T1-weighted SE image shows only minimal enhancement in the region of the infarct (arrow). (b) Coronal diffusion-weighted echo-planar image shows the infarct as an area of markedly increased signal intensity.
|
|
View larger version (119K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 7b. Infarct of the right superior cerebellar artery territory in a 14-year-old girl with sickle cell disease. (a) Contrast-enhanced coronal T1-weighted SE image shows only minimal enhancement in the region of the infarct (arrow). (b) Coronal diffusion-weighted echo-planar image shows the infarct as an area of markedly increased signal intensity.
|
|
Diffusion in white matter is anisotropic and dependent on the direction of the diffusion-sensitizing gradient. Diffusion is faster along the length of an axon than across axons. Therefore, another potential application for diffusion-weighted echo-planar imaging is mapping white-matter development and anatomy to monitor myelination in infants.
Perfusion Imaging
Echo-planar imaging–based MR imaging has unique features and added values relative to positron emission tomography or single photon emission computed tomography in assessing tissue perfusion in the brain. It is minimally invasive and has high spatial resolution, and it can be made selectively sensitive to tissue microvasculature. Conventional SE and GRE sequences are not fast enough to capture the first-pass transit of contrast agent from multiple sections in the brain, which requires whole-brain imaging with a temporal resolution of 1–2 seconds. Currently, two echo-planar imaging–based methods are available for perfusion imaging: first-pass contrast-enhanced imaging and echo-planar imaging with signal targeting, in which alternating RF is used for spin labeling of blood protons without the use of a contrast agent.
First-pass contrast-enhanced imaging is dependent on the delivery of blood to the vascular bed of an organ. This technique involves intravenous injection of a gadolinium-containing contrast agent and rapid imaging with single-shot echo-planar imaging during the first passage of the contrast material bolus through the organ of interest. The magnetic susceptibility (T2 or T2*) contrast of gadolinium causes signal drop on perfusion images. The degree of signal drop depends on two factors: the concentration of the injected contrast agent and the cerebral blood volume. The shape of the transit curve depends on these two factors and on cerebral blood flow. A computer then analyzes the temporal behavior of the MR imaging signal to retrieve information related to delivery of blood to the organ. Next, the relative cerebral blood volume and cerebral blood flow are calculated. The relative cerebral blood volume is a measure of the average volume of blood residing in a region of cerebral tissue relative to the volume of blood in the surrounding tissue. The relative cerebral blood volume map is generated from the echo-planar imaging data.
Perfusion imaging is especially useful in detection of hypoperfused regions in patients with cerebral ischemia and is complementary to diffusion-weighted imaging (Fig 8). The combination of diffusion-weighted and perfusion imaging allows identification of the ischemic penumbra regions. Identification of these regions gives clinicians or the interventional neuroradiologist valuable information for intervention during acute stroke. With perfusion imaging, one can assess tumor vascularity, distinguish recurrent tumor from radiation necrosis, and demonstrate hypervascularity associated with an epileptic focus during a seizure. With the better computer software now offered by MR imaging vendors, which makes real-time online processing of data possible, perfusion imaging is on the way to becoming clinical routine at many academic MR imaging sites in the United States.
View larger version (124K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 8a. Large right middle cerebral artery infarct in a 61-year-old woman. The infarct was seen on a cerebral angiogram; MR imaging was performed approximately 5 hours after onset of the stroke. (a, b) Axial T1-weighted (a) and T2-weighted (b) images show only mild sulcal effacement and gyral edema on the right side of the brain (arrow). (c, d) Axial diffusion-weighted (c) and perfusion (d) images show the extent of the infarct clearly. (Courtesy of Katie D. Vo, MD, and Weili Li, PhD, Mallinckrodt Institute of Radiology, St Louis, Mo.)
|
|
View larger version (138K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 8b. Large right middle cerebral artery infarct in a 61-year-old woman. The infarct was seen on a cerebral angiogram; MR imaging was performed approximately 5 hours after onset of the stroke. (a, b) Axial T1-weighted (a) and T2-weighted (b) images show only mild sulcal effacement and gyral edema on the right side of the brain (arrow). (c, d) Axial diffusion-weighted (c) and perfusion (d) images show the extent of the infarct clearly. (Courtesy of Katie D. Vo, MD, and Weili Li, PhD, Mallinckrodt Institute of Radiology, St Louis, Mo.)
|
|
View larger version (134K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 8c. Large right middle cerebral artery infarct in a 61-year-old woman. The infarct was seen on a cerebral angiogram; MR imaging was performed approximately 5 hours after onset of the stroke. (a, b) Axial T1-weighted (a) and T2-weighted (b) images show only mild sulcal effacement and gyral edema on the right side of the brain (arrow). (c, d) Axial diffusion-weighted (c) and perfusion (d) images show the extent of the infarct clearly. (Courtesy of Katie D. Vo, MD, and Weili Li, PhD, Mallinckrodt Institute of Radiology, St Louis, Mo.)
|
|
View larger version (147K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 8d. Large right middle cerebral artery infarct in a 61-year-old woman. The infarct was seen on a cerebral angiogram; MR imaging was performed approximately 5 hours after onset of the stroke. (a, b) Axial T1-weighted (a) and T2-weighted (b) images show only mild sulcal effacement and gyral edema on the right side of the brain (arrow). (c, d) Axial diffusion-weighted (c) and perfusion (d) images show the extent of the infarct clearly. (Courtesy of Katie D. Vo, MD, and Weili Li, PhD, Mallinckrodt Institute of Radiology, St Louis, Mo.)
|
|
Functional MR Imaging
Localization of brain activity is perhaps the most dynamic field of study in neuroscience today. Functional MR imaging has emerged as the primary experimental tool in this field due to its superior spatial resolution compared with that of positron emission tomography. Most functional MR imaging techniques are based on the susceptibility effects of paramagnetic contrast agents or of oxyhemoglobin (ie, blood oxygen level–dependent [BOLD] imaging).
Task activation functional MR imaging provides information that can be used in planning therapy of or the surgical approach to brain tumors (Fig 9). Functional maps of the cerebral cortex obtained with preoperative functional MR imaging have been shown to correlate well with results of the Wada test, which involves intra–carotid artery injection of amobarbital, as well as with results of intraoperative electrocortical stimulation mapping in patients with epilepsy who were referred for temporal lobectomy (15). In typical task activation imaging, the patient is asked to perform a simple task such as finger movement for a short period while multiple echo-planar images of the motor cortex are acquired. The subject then discontinues the task for a short period while imaging continues. Task-on and task-off imaging is performed for several cycles. The resulting data are processed to generate a map of the regions in the brain that showed repeatedly increased or decreased blood oxygen levels and hence T2* during task-on and task-off imaging.
View larger version (110K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 9. Functional MR imaging with blood oxygen level-dependent imaging. Axial echo-planar image shows increased activity around the central sulcus (top arrow) and an area of the brain with no increased activity (bottom arrow).
|
|
According to published research data (16), each time during cortical activation of the brain, an increase in cerebral blood flow is seen without a proportional increase in tissue oxygen extraction. Blood oxygen level–dependent imaging relies on this physiologic effect. During activation due to a prescribed task, there is an increase in focal cerebral arterial oxyhemoglobin, which has a longer T2* than venous blood. The oxyhemoglobin flows with cortical activation from the arterial side to the venous bed. The result is lengthening of the T2* of venous blood and increased signal intensity on T2*-weighted images. Blood oxygen level–dependent images can be acquired with a GRE or asymmetric SE echo-planar imaging sequence, which is a modified SE echo-planar imaging sequence with a deliberately miscentered 180° pulse to introduce some T2* weighting. Echo-planar imaging has the advantages of less motion sensitivity and greater multisection capability (17).
|
Abdominal Imaging
|
|---|
The primary contribution of echo-planar imaging to abdominal imaging has been the increased gradient capabilities of echo-planar imaging–capable systems. The result has been shorter TEs and improved GRE sequences for breath-hold imaging and three-dimensional MR angiography. There are several potential uses for echo-planar imaging in abdominal imaging. Multishot echo-planar imaging can be used to acquire breath-hold T2-weighted images, potentially replacing SE and fast SE T2-weighted sequences. In the uncooperative patient, single-shot echo-planar imaging of the upper abdomen can be performed in about 2 seconds. Abdominal diffusion and perfusion imaging with echo-planar imaging has not been adopted for clinical use but is under investigation.
Conventional SE images of the abdomen are often degraded by respiratory motion artifacts. Image quality has been improved with the use of fast SE imaging, but most multishot fast SE sequences cannot be performed during a breath hold. Single-shot techniques, such as half-Fourier acquisition single-shot fast SE (HASTE) imaging or single-shot fast SE (SSFSE) imaging, can be used for breath-hold imaging but are limited by image blurring and suboptimal contrast resolution. Spoiled GRE pulse sequences can produce high-quality T1-weighted images of the abdomen but are less useful for T2-weighted image acquisition. The echo-planar imaging pulse sequence can potentially combine the tissue contrast advantages of conventional SE sequences and the speed of single-shot fast SE sequences (Figs 10–13). A multisection study of the upper abdomen can be completed in as little as 2 seconds with single-shot T2-weighted SE echo-planar imaging.
View larger version (112K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 10. Comparison between single-shot and multishot echo-planar imaging of the upper abdomen. Axial images were obtained with eight shots (top left), four shots (top right), two shots (bottom left), and one shot (bottom right). With a decrease in the number of shots, there is an increase in geometric distortion.
|
|
View larger version (106K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 11a. Colon cancer metastatic to the liver. (a) Axial fast SE image shows one faint lesion (arrow). (b) Axial single-shot fast SE image also shows only one lesion (arrow). (c) Axial echo-planar image shows many lesions (arrows).
|
|
View larger version (99K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 11b. Colon cancer metastatic to the liver. (a) Axial fast SE image shows one faint lesion (arrow). (b) Axial single-shot fast SE image also shows only one lesion (arrow). (c) Axial echo-planar image shows many lesions (arrows).
|
|
View larger version (100K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 11c. Colon cancer metastatic to the liver. (a) Axial fast SE image shows one faint lesion (arrow). (b) Axial single-shot fast SE image also shows only one lesion (arrow). (c) Axial echo-planar image shows many lesions (arrows).
|
|
View larger version (104K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 12a. Biopsy-proved renal cell cancer. (a) Axial fat-saturated T1-weighted spoiled GRE image obtained with intravenous gadolinium contrast material shows a focal lesion in the left renal cortex (arrow). (b) Axial eight-shot echo-planar image shows the lesion clearly (arrow) without use of intravenous gadolinium.
|
|
View larger version (64K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 12b. Biopsy-proved renal cell cancer. (a) Axial fat-saturated T1-weighted spoiled GRE image obtained with intravenous gadolinium contrast material shows a focal lesion in the left renal cortex (arrow). (b) Axial eight-shot echo-planar image shows the lesion clearly (arrow) without use of intravenous gadolinium.
|
|
View larger version (86K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 13. Benign prostatic hyperplasia. Echo-planar imaging of the pelvis was performed by using eight shots and a 7-mm section thickness. The imaging time was only 18 seconds for 21 images. Axial echo-planar image shows hypertrophy of the central zone of the prostate (small arrow). The bright rim around the central zone is the peripheral zone of the prostate (large arrow). This example illustrates the potential of echo-planar imaging to combine the tissue contrast advantages of conventional SE imaging and the speed of single-shot fast SE imaging.
|
|
With its purely T2-weighted images, echo-planar imaging can potentially provide better soft-tissue contrast than fast SE imaging; this characteristic may prove useful for detection and evaluation of focal liver lesions (eg, solid vs nonsolid lesions). However, benign and malignant liver lesions show overlap in their T2 characteristics (18–21). In the literature, there are some reports on use of echo-planar imaging and its calculated T2 values to distinguish benign from malignant adrenal masses in patients with known malignancy (22,23). Echo-planar imaging methods are also being developed for MR cholangiopancreatography (24). Diffusion-weighted echo-planar imaging of the abdomen and dynamic echo-planar imaging of the intestine have been shown to be feasible (25,26); however, further investigation is required to determine the appropriate clinical roles for these techniques.
Cardiac Imaging
Echo-planar imaging facilitates rapid evaluation of cardiac function and anatomy. Cine imaging of the heart is performed with GRE echo-planar imaging over multiple cardiac cycles. When single-shot echo-planar imaging is used, electrocardiographic gating is not required. Therefore, this technique is particularly helpful in patients with arrhythmias. GRE echo-planar imaging can also be used for T2*-weighted perfusion studies. Cardiac perfusion imaging involves first-pass imaging after administration of a contrast material bolus and is used for evaluation of myocardial ischemia. Echo-planar imaging offers sufficient spatial and temporal resolution to distinguish perfusion patterns in the subendocardial, middle, and subepicardial zones of the left ventricular myocardium. Echo-planar imaging of the heart requires good shimming over the volume of the heart. However, such shimming is difficult to achieve because breathing and cardiac motion cause fluctuation of the shimming signal, which is difficult to distinguish from the alternating shim currents.
MR imaging of the coronary arteries is currently an area of great interest. Echo-planar imaging has been used for this purpose (Fig 14) and will most likely play a role in further development of the technique. Cardiac diffusion imaging with echo-planar imaging pulse sequences has also been studied (27). In the pediatric population, cardiac echo-planar imaging offers the potential to image congenital heart disease without the need for sedation.
View larger version (33K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 14. Three-dimensional breath-hold imaging of the coronary arteries with segmented echo-planar imaging. Double oblique image (left image) and axial images (all other images) show the right coronary artery (RCA) and the left anterior descending artery (LAD). (Courtesy of Piotr Wielopolski, PhD, Dr Daniel den Hoed Kliniek, Rotterdam, The Netherlands, and Debiao Li, PhD, Northwestern University, Chicago, Ill.)
|
|
Miscellaneous Applications
With single-shot echo-planar imaging, there is relatively little flow-related dephasing even without flow compensation. Ghost artifacts are also absent. Echo-planar imaging has potential value as an ultrafast method for MR angiography and flow quantification (17). Echo-planar imaging freezes fetal motion and has been used to identify fetal anomalies (28). Echo-planar imaging can also be used to guide minimally invasive surgery performed with focused ultrasound or a thermal laser (29). Diffusion-weighted imaging allows real-time temperature monitoring during tissue coagulation or freezing performed for tumor ablation. Kinematic studies of joint motion can be performed with echo-planar imaging.
Rapid MR imaging of hyperpolarized helium performed with echo-planar imaging has been experimentally demonstrated in the human lung (30). This technique could be useful in the future in the study of emphysema (Fig 15), particularly as related to lung volume reduction surgery. The screening potentials of rapid whole-body MR imaging with the echo-planar imaging sequence for benign and malignant diseases are under investigation (Fig 16). In the near future, it will be possible to perform MR fluoroscopy (real-time MR imaging) and echo volume imaging, which captures three-dimensional snapshot images in 80–100 msec.
View larger version (51K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 15. Hyperpolarized helium MR images of ventilation in a healthy 53-year-old male volunteer obtained with echo-planar imaging. Moving in time from left to right, each column of images is the same two-section set consisting of an upper and lower axial section (each 10 mm thick). Two of the 10 levels imaged are shown. Each image was obtained in 40 msec. The spatial resolution is low due to a coarse matrix size (32 x 64), with an in-plane resolution of 6.25 x 6.25 mm and a large field of view (200 x 400 mm). At end inspiration (End-insp), more gas fills the lungs, causing increased signal intensity. In this example, some spatial resolution loss is traded for better temporal resolution and more efficient use of a single bolus of hyperpolarized gas. (Courtesy of David S. Gierada, MD, Mallinckrodt Institute of Radiology, St Louis, Mo, and Brian Saam, PhD, Washington University, St Louis, Mo.)
|
|
View larger version (123K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 16a. Neurofibromatosis type 1 in a 14-year-old girl. (a) Axial short inversion time inversion-recovery (STIR) image shows a cutaneous neurofibroma (arrow). (b) Whole-body coronal echo-planar image clearly shows the extension of the neurofibroma (arrow).
|
|
View larger version (115K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 16b. Neurofibromatosis type 1 in a 14-year-old girl. (a) Axial short inversion time inversion-recovery (STIR) image shows a cutaneous neurofibroma (arrow). (b) Whole-body coronal echo-planar image clearly shows the extension of the neurofibroma (arrow).
|
|
|
Other Uses for Echo-planar Imaging–capable Systems
|
|---|
Echo-planar imaging–capable MR imaging systems provide improved performance of non–echo-planar imaging sequences due to their enhanced gradient capabilities (Fig 17). TE values as short as 1–2 msec can be achieved by using the faster and more powerful gradients required for echo-planar imaging. GRE images acquired with these short TE values show minimal or no evidence of flow-related dephasing. Ultrashort TE values may also be useful for imaging of lung parenchyma (17).
View larger version (95K):
[in this window]
[in a new window]
[Download PPT slide]
|
Figure 17. Maximum-intensity projection image from breath-hold gadolinium-enhanced three-dimensional MR angiography. An ultrashort TR/TE of 3.8/1.4 was used, with only 18 seconds of breath holding. An enhanced echo-planar imaging-capable gradient system was required to achieve such short TR/TE values.
|
|
|
Comparison with Other Fast MR Imaging Techniques
|
|---|
The imaging potential of MR imaging continues to evolve. In recent years, an increasing number of fast and ultrafast imaging strategies have been described (31). One promising method is spiral imaging with two oscillating gradients. These gradients describe a spiral k-space trajectory that early on covers the central part of k space, which determines image contrast. However, spiral imaging is quite sensitive to static magnetic field inhomogeneities (17). A successful implementation of this technique involves imaging of the coronary arteries (32). The advantages and disadvantages of spiral imaging versus echo-planar imaging require further study.
|
Conclusions
|
|---|
Echo-planar imaging–compatible MR imaging equipment is gradually being introduced by commercial vendors, and the hardware capabilities for ultrafast MR imaging, including various types of echo-planar imaging techniques, are being installed throughout the world. Echo-planar images with resolution and contrast similar to those of conventional MR images can be obtained by using multishot acquisitions in only a few seconds. However, in comparison with conventional imaging, single-shot echo-planar imaging offers major advantages, which include reduced imaging time with increased patient throughput, reduced motion artifact, and the ability to image rapid physiologic processes of the human body. These capabilities open up new research fields such as functional MR imaging and real-time imaging. Today, with the availability of echo-planar imaging–capable MR imagers at many sites, the general radiologist can benefit from echo-planar imaging and its clinical applications.
|
Footnotes
|
|---|
Abbreviations: GRE = gradient echo, RF = radio frequency, SE = spin echo, TE = echo time, TR = repetition time
|
References
|
|---|
-
Mansfield P. Multi-planar imaging formation using NMR spin-echo. J Phys C Solid State Phys 1977; 10:L55-L58.
-
Ordidge RJ, Mansfield P, Coupland RE. Rapid biomedical imaging by NMR. Br J Radiol 1981; 54:850-855.[Medline]
-
Ordidge RJ, Mansfield P, Doyle M, Coupland RE. Real time movie images by NMR. Br J Radiol 1982; 55:729-733.[Medline]
-
Doyle M, Rzedzian R, Mansfield P, Coupland RE. Dynamic NMR: cardiac imaging in a piglet. Br J Radiol 1983; 56:925-930.[Abstract]
-
Rzedzian R, Chapman B, Mansfield P, et al. Real-time nuclear magnetic resonance: clinical imaging in paediatrics. Lancet 1983; 12:1281-1282.
-
Mitchell DG. MRI principles Philadelphia, Pa: Saunders, 1999; 57-63.
-
Stehling MJ, Howseman AM, Ordidge RJ, et al. Whole-body echo-planar MR imaging at 0.5 T. Radiology 1989; 170:257-263.[Abstract/Free Full Text]
-
Mansfield P. Real-time echo-planar imaging by NMR. Br Med Bull 1984; 40:187-190.[Free Full Text]
-
Feinberg DA, Turner R, Kakab PD, von Kienlin M. Echo-planar imaging with asymmetric gradient modulation and inner-volume excitation. Magn Reson Med 1990; 13:162-169.[Medline]
-
Matwiyoff NA, Brooks WM. Instrumentation. In: Stark DD, Bradley WG, eds. Magnetic resonance imaging. Vol 1, 3rd ed. St Louis, Mo: Mosby, 1999; 15-32.
-
Weber DM. Non-resonant EPI gradients will enhance conventional MRI. MR 1993; September/October:29.
-
McKinnon GC. Ultrafast interleaved gradient-echo-planar imaging on a standard scanner. Magn Reson Med 1993; 30:609-616.[Medline]
-
Edelman RR, Siewert B, Darby DG, et al. Qualitative imaging of cerebral blood flow and functional localization with echo-planar MR imaging and signal targeting with alternating radio frequency. Radiology 1994; 192:513-520.[Abstract/Free Full Text]
-
Li Bihan D, Breton
Top
Abstract
LEARNING OBJECTIVES FOR TEST...
