This study will define the optimal Doppler-derived peak systolic velocity (PSV) and velocity ratio (VR) to identify >50% lesions in arteriovenous fistulas (AVF) and arteriovenous grafts (AVG). Visible narrowing on a color Doppler image accompanied by high-velocity color Doppler aliasing and poststenotic flow patterns are indicative of vertebral artery stenosis. In stenosis, a localized reduction in vascular radius increases resistance, causing increased PSV and EDV distal to the stenosed site 3,4. The scan may begin with either the longitudinal or transverse imaging of the CCA. 9.5 ). CCA , Common carotid artery . Peak systolic velocities Prior to intervention the PSV ECA in both groups was similar, 161.7 cm/s (CAS) versus 150.9 cm/s (CEA). First, it is well established that echocardiography underestimates the measurement of the LVOT annulus by 1 to 2 millimetres. be assessed by phase-contrast determination of peak systolic velocity combined with the modified Bernoulli equation [85]. Hypertension Stage 1 Normal human peak systolic blood flow velocities vary with age, cardiac output, and anatomic site. The vertebral artery is typically identified in the longitudinal plane, between the transverse processes of the cervical spine. Discordant grading is defined either by an AVA <1 cm while MPG is 40 mmHg/PVel <4 m/sec, or by an AVA 1 cm and an MPG 40 mmHg/PVel 4 m/sec, the first situation being much more common. [6] Among 1,704 patients with a valve area below 1 cm, 24% presented with discordant grading (AVA <1 cm and MPG <40 mmHg). EDV was slightly less accurate. Subsequent data from the NASCET reported improvement in outcome with CEA in patients with 50% to 69% stenosis, although the amount of improvement was far less than was the case with higher grade stenosis. Sex differences in aortic valve calcification measured by multidetector computed tomography in aortic stenosis. Measurement of LVOT diameter is probably the main source of error for the calculation of the AVA. Peak systolic velocity (PSV) and end-diastolic velocity (EDV) were measured in common and internal carotid artery. To an extent, an increased degree (%occlusion) of stenosis corresponds to increased PSV and EDV 4. Trials combining CEA with statin therapy started on hospital admission for surgery showed a decrease in neurologic events such as ischemic stroke and decreased mortality after CEA. There is wide variability in the peak systolic velocities seen in normal patients, with a range of 20 to 60cm/s, with an even wider range noted at the vertebral artery origin (also called segment V0). Spectral Doppler image confirms marked velocity elevation: PSV = 581 cm/s, end diastolic velocity ( EDV ) = 181 cm/s, and the PSV ratio is 8.2. (2003) Radiographics : a review publication of the Radiological Society of North America, Inc. 23 (5): 1315-27. As a result of improved high-resolution ultrasound imaging of the carotid arteries with supplemental imaging from MRA or CTA, the role of conventional angiography as a diagnostic technique has significantly decreased. Prof. Messika-Zeitoun: consultant for Edwards, Valtech, Mardil and Cardiawave. Research grants from Medtronic. Peak transmitral flow velocity in late diastole (peak A) was significantly higher, whereas peak transmitral flow velocity in early diastole (peak E), deceleration time (DT), and the ratio of early to late diastolic filling were significantly lower, in TS patients. Our mission: To reduce the burden of cardiovascular disease. The Velocity is taken with an angle for an accurate measurement.If an accurate angle (<60degrees) cannot be obtained then another measurement is taken with no angle so it can be compared to the renal artery at a stenosis site to do a renal artery:aorta ratio (RAR ratio). RVSP basically is the pressure generated by the right side of the heart when it pumps. Mean ratio peak systolic velocity in the DA-to-peak velocity across the pulmonary valve was 1.35 (SD 0.27). In addition to the fact that thresholds are different in males and females (approximately 2,000 and 1,250 AU, respectively), these results show that AS pathophysiology is different in males and females and, indeed, female leaflets are more fibrotic than those of males. For 70% ICA stenosis or greater, but less than near occlusion: An internal to common carotid PSV ratio 4.0. Ultrasound is the only imaging technique used in many facilities for selecting patients who might undergo carotid endarterectomy or stenting. 1-3 Its -agonist effect is responsible for arterioconstriction, which is reflected clinically in a transiently increased arterial blood pressure. Its a single point and will always be a much higher number then the mean. [10] Interestingly, thresholds for severe AS were different between females and males. Mean peak oxygen consumption (VO 2 peak) at baseline was higher in the . Patients often present with nonlocalizing symptoms such as blurred vision, ataxia, vertigo, syncope, or generalized extremity weakness. John Pellerito, Joseph F. Polak. , and peak TR velocity > 2.8 m/sec. 7.3 ). The overall waveform has a sharp systolic upstroke and is characteristic of low-resistance flow. 9.3 ) on the basis of the direction of blood flow and the visualization of two vessels. 7.1 ). Symptoms High blood pressure that's hard to control. Technical success rates are lower at the origin of the left vertebral artery. Up to 20% to 30% of transient ischemic attacks and strokes may be due to disease of the posterior (vertebrobasilar) circulation. This is confirmed by a high-velocity measurement made on an angle-corrected Doppler waveform. Specific cut-points based on the arteriographic correlative studies need to use the NASCET/ACAS measurement approach ( Fig. 128 (16): 1781-9. We excluded velocity peaks from the isovolumetric phases with end systole defined by the closing of the aortic valve in the three chamber projection. Research grants from Edwards and Abbott. It is important to keep in mind that BSA correction should be only undertaken in patients with small and large stature (small, elderly lady or male, professional basketball player), and should be avoided in those who are obese. Note the dropout of color Doppler flow signals in the regions of acoustic shadowing (, Normal Doppler velocity waveform from the midsegment (V2) of a vertebral artery (, (A) This magnetic resonance angiogram of the right side of the neck shows a relatively small right vertebral artery (, (A) Color and spectral Doppler image at the origin of a normal vertebral artery. The normal superior mesenteric artery has a high-resistance waveform in the postprandial state and a peak systolic velocity of <2.75 m/s. Unable to process the form. With ACAS and NASCET, the degree of stenosis is measured by relating the residual lumen diameter at the stenosis to the diameter of the distal ICA. Normal doppler spectrum. This is why some have suggested combining CT (for the measurement of the LVOT area) and echocardiography for LVOT and aortic TVI in the calculation of the AVA. In general, for a given diameter of a residual lumen, the calculation of percent stenosis tends to be significantly higher using the pre-NASCET measurement method when compared with the NASCET method ( Fig. Thus, in the seminal paper from the Quebec team [4], the criterion used to differentiate groups was the stroke volume index. 2 ). In this setting, a significant reduction in post-stenotic flow velocity is termed trickle flow 5. 8 . We identified 622 patients with isolated, asymptomatic AS and peak systolic velocity > or =4 m/s by Doppler echocardiography who did not undergo surgery at the initial evaluation and obtained . Pulsatility is important to maintain blood flow around another stenotic or occluded vessel 7. Frequent questions. Prior to the 1990s, the degree of carotid stenosis was measured by angiography and estimated where the artery wall should be so that the local or relative degree of stenosis can be estimated. Since the E-wave is normally larger than the A-wave, the ratio should be >1. Between these anechoic and rectangular-shaped regions of acoustic shadowing lies an acoustic window where the vertebral artery can be seen. However, the gray-scale image will typically show the walls of the vertebral artery. Systolic BP of 140 or higher is Stage 2 hypertension, which can drastically increase the risk of stroke or heart attack, may require a prolonged regimen of medication. B., Egstrup K., Kesaniemi Y. 7.5 and 7.6 ). We will not discuss the assessment of AS severity in patients with depressed ejection, but will focus on patients with normal/preserved ejection fraction. Mean of maximum cerebral velocity readings are obtained, and results are classified . However, this approach can be difficult, if not technically impossible, in as many as one-third of patients because the clavicle interferes with the probe position necessary to see the origin of the vertebral artery and the V1 segment in the longitudinal plane. Blood flow velocity (which is what the test measures) is not exactly constant every time you measure. 9.7 ). In the coronal plane, a heel-toe maneuver is used to image the CCA from the supraclavicular notch to the angle of the mandible. In addition, results in symptomatic patients were conflicting with more studies arguing against CAS in patients with symptomatic stenosis and high medical risk. (B) Rounded upstroke and decreased velocities (tardus-parvus) in the mid-upper right vertebral artery. {"url":"/signup-modal-props.json?lang=us"}, O'Shea P, Rasuli B, Hacking C, et al. a. potential and kinetic engr. They are usually classified as having severe AS. If the Doppler sample is positioned too far from the aortic orifice, it will be responsible for an overestimation of AS severity. Otherwise, the findings must be regarded as suggestive of hemodynamic significance, and confirmation must be sought with other imaging approaches. The former study used the traditional method of grading stenosis, whereas the latter used the NASCET/ACAS approach. Normal human peak systolic blood flow velocities vary with age, cardiac output, and anatomic site. Why Is Aortic Pressure High. The solution - The second lesion should be sought. The mean elimination half-life in single-dose studies ranged from 2.8 to 7.4 hours. RESULTS . Symptoms of posterior circulation ischemia are typically varied, making it difficult to determine the potential contribution of vertebral-basilar insufficiency ( Table 9.1 ). Circ Cardiovasc Imaging. The systolic pressure falls between 10 and 30 mmHg, and the diastolic pressure falls between 5 and 10 mmHg. Significant stenosis of the vertebral arteries tends to occur at the vertebral artery origin. b. potential and gravitational energy c. gravitational and inertial energy d. inertial and kinetic energy, Which statement about pressure in the vascular system is correct? At angles >60o, the cosine function curves much more steeply,leading to a significant reduction in the accuracy of angle correction, and thus the accuracy of blood velocity indices such as PSV and end-diastolic velocity (EDV)1. Calculation of the AVA relies on the measurement of three parameters; error measurement may occur in all three. Up to 60% of patients have a dominant vertebral artery (i.e., with a larger diameter and higher blood flow velocity than the contralateral side [see Fig. Intervention is recommended in symptomatic patients with proven severe AS, as in classic severe AS. The current management of carotid atherosclerotic disease: who, when and how?. It should be noted that the ECST continued to rely on the conventional method of stenosis measurement, and, although both the original NASCET and ECST confirmed the effectiveness of CEA, their methods of measuring ICA stenosis were quite different. With the use of computed tomography in the workup evaluation before TAVI, the anatomy of the aortic annulus has been well described. Once this image has been obtained, a slight lateral rocking motion of the probe will bring the vertebral artery into view. Visualization of the vertebral artery is easiest in the V2 segment, the segment that extends from vertebral bodies C 6 to C 2 . In addition, direct . 9.3 ). What could cause peak systolic velocity of right internal carotid artery to be elevated to 130cm/s but no elevation in left ica & no stenosis found? In addition, ulcerated plaque that demonstrates a focal depression or break within the plaque is also more prone to plaque rupture and subsequent embolic event ( Fig. The ascending aorta has the highest average peak velocities of the major vessels; typical values are 150-175 cm/sec. Because of tortuosity, nonlaminar blood flow is commonly seen in the proximal vertebral artery, and kinking of the vessel may occur, causing an elevated peak systolic velocity. This can be quantified using the pulmonary velocity acceleration time (PVAT). However, Hua etal. If the elevated thoracic pressure is maintained, blood pressure will be insufficient to support . Elevated Elevated blood pressure is when readings consistently range from 120-129 systolic and less than 80 mm Hg diastolic. The ascending aorta has the highest average peak velocities of the major vessels; typical values are 150-175 cm/sec. Transversely, the CCA is imaged from its proximal to distal aspects with gray-scale and color Doppler imaging. what does elevated peak systolic velocity mean. This study confirms the high prevalence of patients with discordant grading and also shows that most often these patients presented with normal flow. Calcification can be seen with both homogeneous and heterogeneous plaques. - (A) Normal upstroke and velocity in the mid left vertebral artery. The spectral Doppler system utilizes Fourier analysis and the Doppler equation to convert this shift into an equivalently large velocity, which appears in the velocity tracing as a peak2. 9,14 Classic Signs Post date: March 22, 2013 Low gradient severe aortic stenosis with preserved ejection fraction: reclassification of severity by fusion of Doppler and computed tomographic data. (C) Magnetic resonance angiogram (MRA) shows a high-grade origin stenosis (, Click to share on Twitter (Opens in new window), Click to share on Facebook (Opens in new window), Click to share on Google+ (Opens in new window), on Ultrasound Assessment of the Vertebral Arteries, Ultrasound Assessment of the Vertebral Arteries, Ultrasound Assessment of Lower Extremity Arteries, The Role of Ultrasound in the Management of Cerebrovascular Disease, Anatomy of the Upper and Lower Extremity Arteries, Dizziness or vertigo (accompanied by other symptoms). Therefore one should always consider the gray-scale and color Doppler appearance of the carotid segment in question including the plaque burden and visual estimates of vessel narrowing to determine whether all diagnostic features (both visual and velocity data) of a suspected stenosis are concordant. Check for errors and try again. Ability to use duplex US to quantify internal carotid stenoses: fact or fiction? It is the interval between the onset of flow and peak flow. The identification of carotid artery stenosis is the most common indication for cerebrovascular ultrasound. Circulation, 2011, Mar 1. These vessels exhibit high diastolic flow and EDV 4. Introduction to Vascular Ultrasonography. Example of Sensitivity and Specificity for Internal Carotid Artery Peak Systolic Velocity Cut Points Corresponding to a 70% Diameter Stenosis. The second source of error is the measurement of the aortic valve TVI obtained using continuous Doppler. Using semi-automatic software, areas that are considered as calcification (defined by a tissue density >130 Hounsfield units) are highlighted in red. Peak systolic velocity ( PSV ) exceeds 317 cm/s. The arteries of the hand have many anatomic variants and their evaluation may require a high level of technical expertise. The mean exercise capacity achieved was 87%22% of predicted. (A) The approximate locations of the V1 and V2 segments of the vertebral artery are shown. The ultrasound criteria for estimating ICA stenosis severity are largely based on the results of the NASCET and ECST. 10 Jan 2018, Association for Acute CardioVascular Care, European Association of Preventive Cardiology, European Association of Cardiovascular Imaging, European Association of Percutaneous Cardiovascular Interventions, Association of Cardiovascular Nursing & Allied Professions, Working Group on Atherosclerosis and Vascular Biology, Working Group on Cardiac Cellular Electrophysiology, Working Group on Pulmonary Circulation & Right Ventricular Function, Working Group on Aorta and Peripheral Vascular Diseases, Working Group on Myocardial & Pericardial Diseases, Working Group on Adult Congenital Heart Disease, Working Group on Development, Anatomy & Pathology, Working Group on Coronary Pathophysiology & Microcirculation, Working Group on Cellular Biology of the Heart, Working Group on Cardiovascular Pharmacotherapy, Working Group on Cardiovascular Regenerative and Reparative Medicine, E-Journal of Cardiology Practice - Volume 15, e-Journal of Cardiology Practice - Volume 22, Previous volumes - e-Journal of Cardiology Practice, e-Journal of Cardiology Practice - Articles by Theme. 9.9 ). MPG and PVel are highly correlated (collinear) and can be used almost interchangeably. 9.2 ). Symptoms and Signs of Posterior Circulation Ischemia. The SRU panel concluded that elevated PSV in the ICA and the presence of flow-limiting plaque are the primary parameters determining the severity of ICA stenosis. This vertebral artery segment does not have any adjacent blood vessels except for the vertebral vein ( Fig. Introduction. Occasionally (in 3% to 5% of cases) the left vertebral artery has its origin from the aorta and not from the left subclavian artery. The vertebral artery is readily identified by the prominent anatomic landmarks of the transverse processes of the cervical spine, which appear as bright echogenic lines that obscure imaging of deeper-lying tissues because of acoustic shadowing ( Fig. The NASCET (North American Symptomatic Carotid Endarterectomy Trial) demonstrated that CEA resulted in an absolute reduction of 17% in stroke at 2 years when compared with medical therapy in symptomatic patients with 70% or greater stenosis. In addition, when statins were started on asymptomatic patients prior to CEA, the incidence of perioperative stroke and early cognitive decline also decreased. (2000) World Journal of Surgery. The ascending aorta has the highest average peak velocities of the major vessels; typical values are 150-175 cm/sec. Plaque that contains an anechoic or hypoechoic focus may represent intraplaque hemorrhage or deposits of lipid or cholesterol. 115 (22): 2856-64. Can you tell me what this could possibly mean? Vertebral artery dissection is not commonly associated with elevated blood flow velocities in the absence of significant narrowing in either the true or the false lumen ( Fig. The operator 'just' has to select the area that is considered as belonging to the aortic valve. It is critical to underline that a 1 mm change in measurement of the LVOT diameter results in 0.1 cm difference in AVA calculation. This is probably related to both a true increase in velocity as blood accelerates around a curve and difficulty in assigning a correct Doppler angle. Importantly, this study also showed that the subset of patients with discordant grading (AVA <1 cm, MPG <40 mmHg) and a low flow had the worst prognosis (Figure 2). Not using other views leads to the underestimation of AS severity in 20% or more of patients. Severe calcification and poor echogenicity are important challenges to measure the LVOT diameter accurately. Magnetic resonance angiography (MRA) and computed tomographic angiography (CTA) have shown high accuracy, with duplex ultrasound having moderate accuracy, for the diagnosis of vertebral-basilar disease. Methods: This retrospective analysis includes patients with both DUS and fistulogram within 30 days. The patient is supine and the neck is slightly extended with the head turned slightly to the opposite side. Finally, the origin and proximal segment of the vertebral artery may be confused with other large branches arising from the proximal subclavian artery, such as the thyrocervical trunk. Thus, among patients with an AVA below 1 cm, four groups can be identified (Figure 1). Peak systolic velocity ranged from 1.2 to 3.3 cm/s, and peak diastolic velocity ranged from 1.6 to 4.5 cm/s. There are a number of other hemodynamic conditions that might lead to elevated vertebral peak systolic velocities. Multivariable linear and logistic regression were used to evaluate the relationship of cognitive function with carotid flow velocities and BP. Although the surgical treatment of vertebral artery disease can be successful and relatively safe, patient selection may require consideration of internal carotid artery disease because symptoms of posterior circulation ischemia frequently improve following carotid artery endarterectomy or reconstruction. Although ultrasound evaluation of the vertebral arteries is recognized as a routine part of the extracranial cerebrovascular examination by various accrediting organizations, this assessment is typically limited to documenting the absence, presence, and direction of blood flow. A study by Lee etal. More specifically, CT has clearly demonstrated that the LVOT and the aortic annulus are not circular but oval. The recommendation is to move the Doppler sample up and down in order to obtain a nice Doppler trace with a closure click (possibly missing in very severe AS) without the opening click. Ideally, these parameters should be concordant, with severe AS being defined by a peak velocity >4 m/sec, an MPG >40 mmHg and an AVA <1 cm (Table 1). In contrast, high resistance vessels (e.g. Peak Velocity is the highest velocity attained during the same concentric lift phase. The first two parameters are directly measured using continuous wave Doppler, while the last one is calculated based on the continuity equation and measurement of the left ventricular outflow tract (LVOT) diameter, LVOT time-velocity integral (TVI) and aortic TVI. 7.2 ). Secondary parameters such as elevated EDV in the ICA and elevated ICA/CCA PSV ratios further support the diagnosis of ICA stenosis if present. Thus, extremely low LVOT VTI may predict heart failure patients at highest risk for mortality. Posted on June 29, 2022 in gabriela rose reagan. Elevated peak systolic velocity at the stenosis with pansystolic spectral broadening. Third, in no study combining CT measurement of the LVOT area was a reference (if not a gold standard) method used. All rights reserved. In near occlusion (>99%), flow velocity indices become unreliable (may be high, low or absent) 4. At the time the article was created Patrick O'Shea had no recorded disclosures. Each bin represents an average of PSV values over a 10% stenosis range (i.e., the 45% point represents the average between 40% and 50% stenosis). On a Doppler waveform, the peak systolic velocity corresponds to each tall peak in the spectrum window 1. . The majority of stenotic lesions occur in the proximal internal carotid artery (ICA); however, other sites of involvement in the carotid system may or may not contribute to significant neurologic events. The most appropriate way of classifying patients is first to consider whether AVA and MPG are concordant, and secondly to consider the flow (stroke volume index). 16 (3): 339-46. Proceedings of Ranimation 2017, the French Intensive Care Society International Congress Computational modeling and drug design approaches can speed up the drug discovery and significantly reduce expenses aiming to improve the treatment of cardiomyopathy. Following the stenosis the turbulent flow may swirl in both directions. Diastolic flow augmentation may represent a novel target for development of reperfusion therapies. This should be less than 3.5:1. a. pressure is the highest at the carotid . The ICA Doppler spectrum typically shows a low-resistance pattern. SRU Consensus Conference Criteria for the Diagnosis of ICA Stenosis. Data from 202 patients showing changes in peak systolic velocity (PSV) sensitivity, specificity, and accuracy for the diagnosis of 70% or greater angiographically proven stenosis using NASCET grading system. In these same studies, after repetitive dosing, the half-life increased to a range from 4.5 to 12.0 hours (after less than 10 consecutive doses given 6 hours apart . There is no obvious cut point to indicate an ideal threshold. Uncommonly, increased peak systolic velocities can be seen in the vertebral artery V2 segment because of extrinsic compression by the spine or osteophytes in segment V2 and occasionally V3 ( Fig. doppler ultrasound examination of fetal. 331 However, these devices are often heavy and uncomfortable to use, with 64% patient discontinuation rates at 2 years 332 Trials among individuals with diabetes showed that vacuum . The most commonly used obstetrical applications are the peak systolic frequency shift to end-diastolic frequency shift ratio, (S/D) and the resistance index (RI), which represents the difference between the peak systolic and end-diastolic shift divided by the peak systolic shift. In one study, PSV and ICA/CCA PSV ratios performed almost identically with regard to the identification of ICA stenoses greater than 70% when compared with angiography ( Fig. Circulation, 2013, Oct 13. Modified from Grant EG, Benson CB, Moneta GL, etal. Error bars show one standard deviation about mean. The SRU consensus conference provided reasonable values that can be easily applied ( Table 7.1 ) and have been adopted by a large number of laboratories. The Doppler waveform should have a well-defined systolic peak with sustained blood flow signals throughout diastole as shown in Fig. 3. Previous studies have shown the importance of internal carotid plaque characterization (see Chapter 6 ). There is no need for contrast injection. Velocity magnitude and wall shear stress (WSS) were calculated during one cardiac cycle. We previously established a safeguard formula using the body surface area (BSA) (theoretical LVOT diameter = 5.7*BSA + 12.1). This was confirmed by Yurdakul etal. Low resistance vessels (e.g. The association of carotid atherosclerotic disease with symptomatic cerebrovascular disease (i.e., transient ischemic attacks), amaurosis fugax, and stroke, is well established. 9.2 ). In 20%-30% of patients, these parameters are discordant (usually AVA <1 cm and MPG <40 mmHg). In addition, the Doppler blood flow velocities should always be compared with the degree of plaque, if present. where they found a ratio of 2.2 to have the best accuracy for stenosis of 50% or more. The velocity criteria apply when atherosclerotic plaque is present and their accuracy can be affected by: ICA/CCA PSV ratio measurements may identify patients that for hemodynamic reasons (low cardiac output, tandem lesions, etc. 7.1 ). Uncommonly, increased peak systolic velocities can be seen in the vertebral artery V2 segment because of extrinsic compression by the spine or osteophytes in segment V2 and occasionally V3 ( Fig. Baumgartner H., Hung J., Bermejo J., Chambers J. Ritter JC, Tyrrell MR. The goal of this study is to determine the impact of 12 weeks of Lp299v supplementation (20 million cfu/day vs. placebo) on exercise capacity, circulating biomarkers of cardiac remodeling, quality of life, and vascular endothelial function in humans with heart failure and reduced ejection fraction (HFrEF) who have evidence of residual inflammation based on an elevated C-reactive protein level. Did you know that your browser is out of date? Reference article, Radiopaedia.org (Accessed on 05 Mar 2023) https://doi.org/10.53347/rID-78164, View Patrick O'Shea's current disclosures, see full revision history and disclosures, Factors that influence flow velocity indices, fetal middle cerebral arterial peak systolic velocity, end-diastolic velocity (Doppler ultrasound), iodinated contrast media adverse reactions, iodinated contrast-induced thyrotoxicosis, diffusion tensor imaging and fiber tractography, fluid attenuation inversion recovery (FLAIR), turbo inversion recovery magnitude (TIRM), dynamic susceptibility contrast (DSC) MR perfusion, dynamic contrast enhanced (DCE) MR perfusion, arterial spin labeling (ASL) MR perfusion, intravascular (blood pool) MRI contrast agents, single photon emission computed tomography (SPECT), F-18 2-(1-{6-[(2-[fluorine-18]fluoroethyl)(methyl)amino]-2-naphthyl}-ethylidene)malononitrile, chemical exchange saturation transfer (CEST), electron paramagnetic resonance imaging (EPR).