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Resolution of MRI can be improved if FOV is reduced keeping other factors constant, but this reduces voxel volume, which in turn reduces SNR. How much SNR is enough for good imaging?
[Old question, but… ]
There is an old adage in MRI: High signal to noise, high resolution, short scan time… choose 2.
The question of "How much SNR is enough?" can't be answered without talking about specifics, primarily, what is the scan going to be used for?
For example, a typical MS diagnostic scan doesn't really need to be too high a resolution or signal to noise as typically the lesions are large and hyperintense. BUT with higher field scanners one is able to see smaller and smaller lesions.
If one is doing quantitative work off of an MRI scan, then it will depend on the type of quantification, algorithm etc.
So, in some ways the question is very broad and it depends, significantly, on a lot of other factors.
Doing some Google Scholar / Pubmed searches for SNR and MRI might help with specific areas you are interested in (e.g., https://scholar.google.com/scholar?hl=en&as_sdt=0%2C9&q=minimum+signal+to+noise+ratio+in+magnetic+resonance+imaging&btnG=)
where I = intrinsic signal intensity, f(QF) = coil quality, and f(B) = field strength.
Effects of Imaging Parameters on SNR, Spatial Resolution, and Acquisition Time
|Goal||Imaging Parameters||SNR Change *||Section-Encoding Direction||Frequency-Encoding Direction||Phase-Encoding Direction||Time of Acquisition|
|Higher SNR||NSA × 2||× 1.41||—||—||—||× 2|
|Higher SNR||BW/2||× 1.41||—||—||—||—|
|Higher||FOVphase/2||× 0.25||—||× 2||× 2||—|
|Higher resolution||Nfreq × 2||× 0.71||—||× 2||—||—|
|Higher resolution||Nphase × 2||× 0.71||—||—||× 2||× 2|
BW, bandwidth in frequency-encoding direction FOVfroq field of view in frequency-encoding direction FOVphase, field of view in phase-encoding direction Nfroq, number of pixels across FOVfroq (without interpolation) Nphase, number of phase-encoding steps NSA, number of signals averaged.
SNR for good MRI imaging - Biology
Molecular Imaging and Biology presents original research contributions on the utilization of molecular imaging in problems of relevance in biology and medicine. The primary objective of the journal is to provide a forum for the discovery of molecular mechanisms of health and disease through the use of imaging techniques.
Among the topics covered are molecular imaging investigations of macromolecular targets involved in significant biological processes design and evaluation of molecular probes used to investigate macromolecular targets and their functions and study of in vivo animal models of disease for the development of new molecular diagnostics and therapeutics.
The overall goal is to translate basic science discoveries into molecular imaging of disease in patients, both to investigate the biological nature of disease in actual patients and to establish new molecular imaging diagnostic procedures.
Molecular Imaging and Biology is the official journal of the World Molecular Imaging Society and the European Society for Molecular Imaging.
Molecular Imaging and Biology is ranked #15 in the Google Scholar H-5 Index of Nuclear Medicine and Radiotherapy, with a 2019 H-5 Index of 31.
Why publish with us
- We are the official journal of the World Molecular Imaging Society (WMIS), the European Society for Molecular Imaging (ESMI), and the Federation of Asian Societies for Molecular Imaging (FASMI).
- Our journal presents original research on the utilization of molecular imaging in the delineation and treatment of disease, and provides a forum for precision health through imaging technologies.
- With our swift peer review process averaging 30 days to first decision, we provide high levels of author satisfaction , with 100% of authors reporting that they would definitely or probably publish with us again.
Parallel imaging compressed sensing for accelerated imaging and improved signal-to-noise ratio in MRI-based postimplant dosimetry of prostate brachytherapy
Purpose: To investigate the feasibility of using parallel imaging compressed sensing (PICS) to reduce scan time and improve signal-to-noise ratio (SNR) in MRI-based postimplant dosimetry of prostate brachytherapy.
Methods and materials: Ten patients underwent low-dose-rate prostate brachytherapy with radioactive seeds stranded with positive magnetic resonance-signal seed markers and were scanned on a Siemens 1.5T Aera. MRI comprised a fully balanced steady-state free precession sequence with two 18-channel external pelvic array coils with and without a rigid two-channel endorectal coil. The fully sampled data sets were retrospectively subsampled with increasing acceleration factors and reconstructed with parallel imaging and compressed sensing algorithms. The images were assessed in a blinded reader study by board-certified care providers. Rating scores were compared for statistically significant differences between reconstruction types.
Results: Images reconstructed from subsampling up to an acceleration factor of 4 with PICS demonstrated consistently sufficient quality for dosimetry with no apparent loss of SNR, anatomy depiction, or seed/marker conspicuity when compared to the fully sampled images. Images obtained with acceleration factors of 5 or 6 revealed reduced spatial resolution and seed marker contrast. Nevertheless, the reader study revealed that images obtained with an acceleration factor of up to 5 and reconstructed with PICS were adequate-to-good for postimplant dosimetry.
Conclusions: Combined parallel imaging and compressed sensing can substantially reduce scan time in fully balanced steady-state free precession imaging of the prostate while maintaining adequate-to-good image quality for postimplant dosimetry. The saved scan time can be used for multiple signal averages and improved SNR, potentially obviating the need for an endorectal coil in MRI-based postimplant dosimetry.
Keywords: Compressed sensing Endorectal coil MRI Prostate brachytherapy SNR.
Copyright © 2018 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.
Anatomical Considerations on Hard Tissues
Hard tissues comprise all the tissues that show a mineralized component in their extracellular matrix, such as bone and teeth. The ratio of the mineral phase is not only different for each tissue but can vary with age, sex, gender, and site . Such tissue heterogeneity results in different T2 properties according to the amount of available free water (Table 1). In the following paragraphs, a general overview of the specific chemical components of bone and teeth is provided.
Bone is a mineralized tissue that performs several functions in the body. Bone protects the vital organs, provides sites for muscle attachment to allow for motion and locomotion, produces the blood cells, and serves as a reservoir for several important ions (e.g., calcium and phosphate). The adult human skeleton consists of two main components: compact bone (
80 %, also called cortical bone) and trabecular bone (
20 %, also called cancellous or spongy bone). The ratio of cortical and trabecular bone is different depending on the different locations in the skeleton. As the name implies, compact bone shows a dense structure that is almost solid (i.e., 10 % or less in porosity) and consists of parallel cylindrical units called osteons (or Haversian systems). Compact bone is present at the outer areas of long bone like femur and tibia, small bones like wrist and ankle, and flat bones like skull vault and other irregular bones. Trabecular bone is less dense than cortical bone, presents higher porosity usually between 50 and 90 %, and is located near the ends of long and small bones and in between the surfaces of flat bones. The outer surface of the bone is covered by a connective tissue, named the periosteum, which plays an important role in the skeletal development and bone healing (Fig. 5) [51,52,53].
Schematic representation of long bone morphology. From the outside towards the inside it is possible to distinguish the periosteum, compact bone (that consists of osteons) and the trabecular bone. Note that each tissue has different T2 values according to his composition. T2 values reported in the figure are measured on a 1.5 T MRI system.
The mineral component of the bone consists of hydroxyapatite (HA). Depending on the site, the mineral component of most bones represents 60 % to 70 % of the total dry weight, with an exception of the ossicles in the ear that show up to 98 % of mineral content. The remained component of the bone consists of an organic phase (20–30 %) and water (10–15 %). The main component of the organic phase is collagen type I (about 90 %), which is stiffened with the mineral phase, while non-collagenous proteins, lipids, and water are present in minor amount, i.e., 5 %, 3 %, and 2 % respectively [54,55,56].
Water content into the bone is found to be in three different forms. Firstly, water can be associated with the mineral phase secondly, water can be associated with the organic collagen phase thirdly, there is free bulk water located in the pores of the mineral phase [57, 58]. Naturally, the occurrence of both freely and tightly bound water results in two major relaxation components, which decay at different rate. Protons associated with the mineral phase and the organic phase are decaying quickly (i.e., T2 < 11.7 μs, and T2 = 320 μs, respectively), while bulk water decays slowly (T2 = 2.28 ms) as measured on a 3 T MRI system [59,60,61].
Mammalian teeth consist of four main tissues: enamel, dentin, cementum, and pulp. Enamel, dentin, and cementum represent three differently mineralized tissues that are tightly attached to each other, while the pulp is the only soft tissue of the tooth (Fig. 6). The enamel is a highly mineralized structure (up to 96 %) forming the outer shell of the crown and is mechanically the hardest substance in the body. The remained 4 % consists of water and other organic proteins called amelogenin, ameloblastins, and enamelins. The dentin is formed by 70 % mineral phase, 20 % of organic phase, mainly collagen type I, and 10 % water. The cementum is a thin layer present between dental root and the periodontium. The thickness of the cementum is raging from 50 to 1500 μm depending on different locations and teeth. Cementum consists of 45–50 % of HA as inorganic phase, 50–55 % organic phase (mainly collagen type I), and for the rest of water [62,63,64,65,66,67]. Because of very low water content relaxation times for dentin and enamel are very short, i.e., T2 < 1 ms and 70 μs, respectively (data measured on a 1.5 T system) .
Schematic representation of human molar anatomy. From the outside to the inside it is possible to distinguish enamel, dentin, and dental pulp. T2 values reported in the figure are measured on a 1.5 T MRI system. Adapted from www.charlesfamilydental.com.
Imaging coordinates and planes
The XYZ magnetic resonance coordinate system is not used in clinical imagers for collection and presentation of images. The anatomic coordinate system is used instead of this. The axes are referenced to the body as shown in Figure 4.7. The three axes are left-right (L/R), anterior-posterior (A/P), and superior-inferior (S/I).
Similarly, on clinical imagers the terminology XY, YZ, and XZ are not commonly used to indicate the imaged planes. Instead they are called as axial or transverse, sagittal and coronal respectively as shown in Figure. An axial plane is an imaged plane, which is perpendicular to the long axis (Z-axis) of the body. L/R and A/P are the sides of this plane. A plane that bisects the left and right sides of the body is called a sagittal plane. The sagittal plane is perpendicular to the X-axis and parallel to the field of gravity (g). S/I and A/P are the sides of this plane. A coronal plane is a plane that bisects the front of the body from the back. L/R and S/I are the sides of this plane. This plane is mutually perpendicular to both axial and sagittal plane.
Figure 4.7 Anatomical coordinates and planes
Magnetic Resonance Imaging (MRI)
Magnetic Resonance Imaging (MRI) is a non-invasive imaging technology that produces three dimensional detailed anatomical images. It is often used for disease detection, diagnosis, and treatment monitoring. It is based on sophisticated technology that excites and detects the change in the direction of the rotational axis of protons found in the water that makes up living tissues.
MRIs employ powerful magnets which produce a strong magnetic field that forces protons in the body to align with that field. When a radiofrequency current is then pulsed through the patient, the protons are stimulated, and spin out of equilibrium, straining against the pull of the magnetic field. When the radiofrequency field is turned off, the MRI sensors are able to detect the energy released as the protons realign with the magnetic field. The time it takes for the protons to realign with the magnetic field, as well as the amount of energy released, changes depending on the environment and the chemical nature of the molecules. Physicians are able to tell the difference between various types of tissues based on these magnetic properties.
To obtain an MRI image, a patient is placed inside a large magnet and must remain very still during the imaging process in order not to blur the image. Contrast agents (often containing the element Gadolinium) may be given to a patient intravenously before or during the MRI to increase the speed at which protons realign with the magnetic field. The faster the protons realign, the brighter the image.
MRI scanners are particularly well suited to image the non-bony parts or soft tissues of the body. They differ from computed tomography (CT), in that they do not use the damaging ionizing radiation of x-rays. The brain, spinal cord and nerves, as well as muscles, ligaments, and tendons are seen much more clearly with MRI than with regular x-rays and CT for this reason MRI is often used to image knee and shoulder injuries.
In the brain, MRI can differentiate between white matter and grey matter and can also be used to diagnose aneurysms and tumors. Because MRI does not use x-rays or other radiation, it is the imaging modality of choice when frequent imaging is required for diagnosis or therapy, especially in the brain. However, MRI is more expensive than x-ray imaging or CT scanning.
One kind of specialized MRI is functional Magnetic Resonance Imaging (fMRI.) This is used to observe brain structures and determine which areas of the brain “activate” (consume more oxygen) during various cognitive tasks. It is used to advance the understanding of brain organization and offers a potential new standard for assessing neurological status and neurosurgical risk.
Although MRI does not emit the ionizing radiation that is found in x-ray and CT imaging, it does employ a strong magnetic field. The magnetic field extends beyond the machine and exerts very powerful forces on objects of iron, some steels, and other magnetizable objects it is strong enough to fling a wheelchair across the room. Patients should notify their physicians of any form of medical or implant prior to an MR scan.
When having an MRI scan, the following should be taken into consideration:
- People with implants, particularly those containing iron, — pacemakers, vagus nerve stimulators, implantable cardioverter- defibrillators, loop recorders, insulin pumps, cochlear implants, deep brain stimulators, and capsules from capsule endoscopy should not enter an MRI machine.
- Noise—loud noise commonly referred to as clicking and beeping, as well as sound intensity up to 120 decibels in certain MR scanners, may require special ear protection.
- Nerve Stimulation—a twitching sensation sometimes results from the rapidly switched fields in the MRI.
- Contrast agents—patients with severe renal failure who require dialysis may risk a rare but serious illness called nephrogenic systemic fibrosis that may be linked to the use of certain gadolinium-containing agents, such as gadodiamide and others. Although a causal link has not been established, current guidelines in the United States recommend that dialysis patients should only receive gadolinium agents when essential, and that dialysis should be performed as soon as possible after the scan to remove the agent from the body promptly.
- Pregnancy—while no effects have been demonstrated on the fetus, it is recommended that MRI scans be avoided as a precaution especially in the first trimester of pregnancy when the fetus’ organs are being formed and contrast agents, if used, could enter the fetal bloodstream.
- Claustrophobia—people with even mild claustrophobia may find it difficult to tolerate long scan times inside the machine. Familiarization with the machine and process, as well as visualization techniques, sedation, and anesthesia provide patients with mechanisms to overcome their discomfort. Additional coping mechanisms include listening to music or watching a video or movie, closing or covering the eyes, and holding a panic button. The open MRI is a machine that is open on the sides rather than a tube closed at one end, so it does not fully surround the patient. It was developed to accommodate the needs of patients who are uncomfortable with the narrow tunnel and noises of the traditional MRI and for patients whose size or weight make the traditional MRI impractical. Newer open MRI technology provides high quality images for many but not all types of examinations.
Replacing Biopsies with Sound
Chronic liver disease and cirrhosis affect more than 5.5 million people in the United States. NIBIB-funded researchers have developed a method to turn sound waves into images of the liver, which provides a new non-invasive, pain-free approach to find tumors or tissue damaged by liver disease. The Magnetic Resonance Elastography (MRE) device is placed over the liver of the patient before he enters the MRI machine. It then pulses sound waves through the liver, which the MRI is able to detect and use to determine the density and health of the liver tissue. This technique is safer and more comfortable for the patient as well as being less expensive than a traditional biopsy. Since MRE is able to recognize very slight differences in tissue density, there is the potential that it could also be used to detect cancer.
New MRI just for Kids
MRI is potentially one of the best imaging modalities for children since unlike CT, it does not have any ionizing radiation that could potentially be harmful. However, one of the most difficult challenges that MRI technicians face is obtaining a clear image, especially when the patient is a child or has some kind of ailment that prevents them from staying still for extended periods of time. As a result, many young children require anesthesia, which increases the health risk for the patient. NIBIB is funding research that is attempting to develop a robust pediatric body MRI. By creating a pediatric coil made specifically for smaller bodies, the image can be rendered more clearly and quickly and will demand less MR operator skill. This will make MRIs cheaper, safer, and more available to children. The faster imaging and motion compensation could also potentially benefit adult patients as well.
Another NIBIB-funded researcher is trying to solve this problem from a different angle. He is developing a motion correction system that could greatly improve image quality for MR exams. Researchers are developing an optical tracking system that would be able to match and adapt the MRI pulses to changes in the patient’s pose in real time. This improvement could reduce cost (since less repeat MR exams will have to take place due to poor quality) as well as make MRI a viable option for many patients who are unable to remain still for the exam and reduce the amount of anesthesia used for MR exams.
Determining the aggressiveness of a tumor
Traditional MRI, unlike PET or SPECT, cannot measure metabolic rates. However, researchers funded by NIBIB have discovered a way to inject specialized compounds (hyperpolarized carbon 13) into prostate cancer patients to measure the metabolic rate of a tumor. This information can provide a fast and accurate picture of the tumor’s aggressiveness. Monitoring disease progression can improve risk prediction, which is critical for prostate cancer patients who often adopt a wait and watch approach.
THE NEXT DECADE OF CLINICAL MRI
Although clinical challenges abound for MRI, the opportunities are equally expansive. Looking forward, the most pressing clinical needs are shortening MRI acquisition times, optimizing image quality and content, automating analyses, perfecting fusion imaging, and enabling whole-body imaging. Approaches to achieve these goals will likely be similar to those described above. Deep learning (DL) algorithms are likely to play a central role in image acquisition (sub-Nyquist sampling strategies using DL), reconstruction (AUTOMAP), and automated image post-processing. More seamless integration of imaging results (including structured reporting and alerts of significant findings) into electronic medical records will be essential. Currently, this remains cumbersome because the different imaging technologies and platforms are not optimized to interact with and learn from each other. Below we discuss established, emerging, and still-needed clinical MRI applications for cancer and other diseases.
Cancer imaging using MRI for initial cancer diagnoses, staging, serial imaging in therapeutic trials, and recurrence/progression monitoring is already established. In general, MRI has been shown to accurately stage cancer triage patients to appropriate therapy and support patient follow-up, particularly for colorectal, gynecologic, and prostate cancers. It is generally accepted that MRI is the most sensitive imaging method for identifying early metastatic disease in the liver and brain. MRI is also routinely used to establish the extent of bone marrow involvement and to identify skin or satellite lesions in bone malignancies. Furthermore, MRI is increasingly relied upon to phenotype cancers by extraction of quantitative imaging features (radiomics) (9). Several newer applications in cancer imaging include the following: screening for breast cancer in high-risk populations carrying BRCA mutations planning radiation treatment, whereby superior soft tissue contrast permits accurate boundary delineation and dose painting and predicting and monitoring patient response to chemotherapy. Another example is using whole-body MRI in oncology staging, particularly for lymphoma in younger patients for whom radiation exposure should be limited. Intraoperative MRI will likely play an increasing role in neurosurgical oncology by providing real-time information about the precise spatial relationship between tumors and adjacent areas in the brain to optimize surgical resection while limiting inadvertent damage to healthy cerebral parenchyma. The continued evolution of open-bore scanners will further enable MRI-guided procedures in interventional oncology.
Of the many potential clinical advances likely to result from expanding imaging technologies, several hold notable potential. A key interest in neurologic imaging is to translate emerging ultrahigh-field MRI (>3 T), which increases the signal-to-noise ratio, into clinical practice. This will allow better anatomic and ultrastructural imaging and will likely open new doors to disease characterization. Although a few such ultrahigh-field systems are operational in clinical research settings, practical challenges will need to be overcome for the approach to be adopted more broadly. Improved signal-to-noise ratio and increased imaging sensitivity as a function of higher field strength have had profound effects on advanced techniques such as blood oxygen level–dependent (BOLD) imaging, which may help uncover the neurobiology of complex processes in addiction and the locoregional actions of drugs targeting the central nervous system. For musculoskeletal imaging, a central goal is the ability to perform MRI with stress loading on joints. This may seem simple but will likely require new magnet designs. In cardiology, fast data acquisition will allow routine real-time imaging in patients with arrhythmias. Similarly, advanced thoracic imaging will require the development of image acquisition methods during quiet breathing rather than the current approach of breath holding, which often is difficult for patients. Important aspirations in abdominopelvic imaging include establishing MRI as a surrogate end point for metabolic disorders such as hemochromatosis and validating MRI as a readout in drug development and trial assessment (for example, in drug trials for nonalcoholic steatohepatitis).
While acknowledging the advances made in decreasing scan time and optimizing image acquisition over the past decade, there remains considerable potential for improving the patient experience in the MRI suite. At present, advanced imaging techniques, such as functional and cardiovascular MR, require protracted scan times (sometimes up to or more than an hour). Consequently, patients often become uncomfortable during the examination, and motion artifacts remain a considerable problem. Although retrospective motion correction algorithms are useful, adaptive dynamic imaging, including prospective motion correction currently in development, is expected to expand the applications of cardiovascular and functional MRI. Within the field of cardiovascular imaging, efforts are being made to combine these prospective motion correction algorithms with free breathing techniques to further enhance the patient experience. Previously, complex sequence acquisition required electrocardiogram gating, breath-hold sequences, and respiratory gating to achieve artifact-free images. New efforts are emerging on MR “multitasking,” which continuously collects geometric data and resolves for these artifacts. Undoubtedly, the secondary gains from this progress will include enhanced image quality, shorter scan times, and improved throughput.
MRI biobanking programs by global initiatives seek to acquire multiorgan imaging from large cohorts of patients. Such biobank efforts also include genomic, proteomic, and metabolic outcomes and other patient data often collected at multiple time points. These large repositories (U.K. Biobank and The Cancer Imaging Archive OpenNeuro) will be invaluable to advance research, education, and training. Although many of these programs are just beginning, they present an exciting opportunity in population-based health care, where MRI will improve understanding of disease mechanisms. The breadth of information acquired by biobanks presents its own challenges and will very likely require automated techniques for data collection and storage.
The advances in MRI technologies described here have not been realized without growing pains, and a considerable amount of work must be done to improve them further. The aspirations of the field are ambitious and will require a community of basic and translational scientists, as well as physicians, to achieve them. Such efforts have been catalytic in the past, as evidenced by the expeditious development of MRI in the past decade. Many new applications will necessitate prospective clinical trials and cost-effectiveness analyses so that emerging techniques can become reimbursable. Further broadening our horizons in MRI may not simply extend to increasing field strengths or improving sequence technology but to providing ubiquitously available low-field MRI at the bedside. Commensurate advances in the allied fields of AI and MR physics will be necessary to facilitate all of these improvements. Collaborative efforts and public-private partnerships will be essential to driving these technologies and realizing the full potential of MRI in the years to come (10).
This is an article distributed under the terms of the Science Journals Default License.
Atherosclerosis is a prevalent disease affecting a large portion of the population at one point in their lives. There is an unmet need for noninvasive diagnostics to identify and characterize at-risk plaque phenotypes noninvasively and in vivo, to improve the stratification of patients with cardiovascular disease, and for treatment evaluation. Magnetic resonance imaging is uniquely positioned to address these diagnostic needs. However, currently available magnetic resonance imaging methods for vessel wall imaging lack sufficient discriminative and predictive power to guide the individual patient needs. To address this challenge, physicists are pushing the boundaries of magnetic resonance atherosclerosis imaging to increase image resolution, provide improved quantitative evaluation of plaque constituents, and obtain readouts of disease activity such as inflammation. Here, we review some of these important developments, with specific focus on emerging applications using high-field magnetic resonance imaging, the use of quantitative relaxation parameter mapping for improved plaque characterization, and novel 19 F magnetic resonance imaging technology to image plaque inflammation.
Phase-contrast MRI has an important role in modern diagnostic imaging. Derived from phase data that are intrinsic to all MRI signals, this velocity-measuring technique is critical in diagnostic imaging of the heart, evaluation of CSF disorders, angiography, and elastography. Knowledge of the basic physics underlying this examination is essential for the accurate acquisition and interpretation of these images.
Recipient of a Certificate of Merit award for an education exhibit at the 2018 RSNA Annual Meeting.
For this journal-based SA-CME activity, the authors, editor, and reviewers have disclosed no relevant relationships.