Presentation on theme: "Dr Vivek Pillai. Senior resident in cardiology, Kozhikode medical college."— Presentation transcript:
Dr Vivek Pillai. Senior resident in cardiology, Kozhikode medical college.
REFERENCES. How does MRI work?-Introduction to physics and function of MRI-Dominik Weishaupt. MRI of the heart and vessels-Lombardi and bartolozi. MRI, basic principles and applications-Brown and Semelka. Mayo clinic guide to CMR-Mcgee and Williamson. Clinical cardiac MRI-Jan Bogaert,Dymarkowski and Taylor. Hurst’s – The Heart 13 th edn.
MRI PHYSICS All living objects are composed of atoms. Atoms- electrons, protons, neutrons. Proton is of interest in MRI. Positively charged protons in the nucleus continuously rotate around an axis and create their own magnetic field.
SPIN-intrinsic property of all elementary particles. i.e the proton rotates about its axis like a spinning top. The proton possesses angular momentum-acts like a spinning top that strives to retain the spatial orientation of its rotation axis. As a rotating mass with an electrical charge, the proton additionally has magnetic moment (B) and behaves like a small magnet.
PRECESSION-When an external force (typically the earth’s gravitational field G) acts on a spinning top and tries to alter the orientation of its rotational axis, the top begins to wobble.
LARMOR FREQUENCY-Precession of the nuclei occurs at a characteristic rate that is proportional to the strength of the applied magnetic field. ω0=γ0 X B0. ω0 is the Larmor frequency in Megahertz. γ0 is the gyromagnetic ratio specific to a particular nucleus. B0 is the strength of the magnetic field in Tesla. Protons have a gyromagnetic ratio of γ=42.58 MHz/T, resulting in a Larmor frequency of 63.9 MHz at 1.5 T.
Immediately after excitation, the magnetization rotates in the xy-plane and is now called transverse magnetization or Mxy. It is the rotating transverse magnetization that gives rise to the MR signal in the receiver coil.
TI RELAXATION. Transverse magnetization decays and the magnetic moments gradually realign with the z-axis of the main magnetic field B0. As transverse magnetization decays, the longitudinal magnetization, Mz – the projection of the magnetization vector onto the z-axis – is slowly restored. The time constant for this recovery is T1 and is dependent on the strength of the external magnetic field, B0, and the internal motion of the molecules (Brownian motion. Spin lattice interaction. “lattice”- surroundings.
T2: TRANSVERSE RELAXATION. Transverse relaxation is the decay of transverse magnetization because spins lose coherence (dephasing). Transverse relaxation - spins exchange energy with each other. Spin- spin interaction. “Phase”-refers to the position of a magnetic moment on its circular precessional path and is expressed as an angle
T2* RELAXATION. T2* refers to the effects of additional field inhomogeneities contributing to dephasing. These are intrinsic inhomogeneities that are caused by the magnetic field generator itself and by the very person being imaged. Most of the inhomogeneities that produce the T2* effect occur at tissue borders, particularly at air/tissue interfaces. The loss of the MR signal due to T2* effects is called free induction decay (FID).
T1 and T2 relaxation are completely independent of each other but occur more or less simultaneously. The decrease in the MR signal due to T2 relaxation occurs within the first 100–300 msec, i.e before there has been complete recovery of longitudinal magnetization Mz due to T1 relaxation (0.5–5 sec).
LONGITUDIN AL RECOVERY AND TRANSVERSE DECAY FOLLOWING A 90 DEGREES FLIP EXCITATION.
IMAGE CONTRAST 3 intrinsic features of a biological tissue contribute to its signal intensity or brightness on an MR image and hence image contrast: -Proton density -the no. of excitable spins per unit volume- determines the max. signal that can be obtained from a given tissue. -T1 time of a tissue-time it takes for the excited spins to recover and be available for the next excitation. -T1-weighted images-Images with contrast that is mainly determined by T1. T2-weighted images-Images with contrast that is mainly determined by T2.
Repetition Time (TR) and T1 Weighting Repetition time (TR)- is the interval between two successive excitations of the same slice. -is the length of the relaxation period between two excitation pulses and is therefore crucial for T1 contrast.
Short TR → strong T1 weighting. Long TR → low T1 weighting. Tissues with a short T1 appear bright because they regain most of their longitudinal magnetization during the TR interval and thus produce a stronger MR signal. Tissues with a long T1 appear dark because they do not regain much of their longitudinal magnetization during the TR interval and thus produce a weaker MR signal.
ECHO- the signal induced in the receiver coil after phase coherence has been restored ECHO TIME (TE) is the interval between application of the excitation pulse and collection of the MR signal. Short TE → low T2 weighting. Long TE → strong T2 weighting. Short T2 → dark on T2- weighted images. Long T2 → bright on T2- weighted images.
Presaturation This technique employs an initial 90° or 180° inverting pulse that is delivered before the data for image generation is acquired. For enhancement of T1 contrast. A more pronounced T1 effect is achieved with a 180° inverting pulse than with a 90° pulse because a 180° pulse inverts all longitudinal magnetization. As a result, T1 relaxation begins at –1 rather than 0 and twice as much longitudinal magnetization is available. The operator can modulate the T1 effect by varying the time interval between the 180° inversion pulse and the excitation pulse (= inversion time, TI).
Saturation at Short Repetition Times SATURATION -When a series of excitation pulses is applied, the MR signal becomes weaker and weaker after each repeat pulse. Saturation is an important issue when fast or ultrafast MR techniques are used. Here the MR signal may become very weak due to the very short repetition times.
T1 WEIGHTED( LEFT) T2 WEIGHTED ( RIGHT)
COMPONENTS OF THE MRI SCANNER. -Coil that generates a static magnetic field (B0) to align the protons to the axis of the field – a RF pulse transmitter consisting in a coil that generates RF pulses for disturbing proton alignment along B0 – a RF signal receiver consisting in a coil that receives the energy emitted by the protons. –3 coils for generating magnetic field gradients. – a computerized system for the amplification, digitization, and processing of the MR signals to reconstuct for composing the final MR image..
GRADIENTS-Gradients are additional magnetic fields that are generated by gradient coils and add to or subtract from the main magnetic field. A. A.Orthogonal magnetic field gradients are used to localize the MR signal. B. B. Slice selection C. C. Frequency encoding. The MR signal from any given position has a unique frequency. In the example shown,
THE K- SPACE. It is a graphic matrix of digitized MR data that represents the MR image before Fourier transformation is performed. Each line in k-space corresponds to one measurement and a line is acquired for each phase- encoding step.
FOURIER TRANSFORM Transforming the frequency-encoded raw MR data recorded at the system can be done with Fourier Transformation. Fourier Transformation is a mathematical model transforming the frequency-encoded data from k-space to MR image domain.
ECG Gating. In many types of cardiac MRI, such as morphological imaging (e.g. coronary MR angiography) or tissue characterization (e.g. late Gd) a static image of the heart is required. Traditionally imaged during diastasis, when the myocardium is most at rest. Diastasis occurs during mid to-late diastole. -How long after the R-wave should imaging start? Weissler formula-trigger delay = [(R-R interval - 350)x0.3]+350.
Easier approach Perform a cine MRI scan with very high temporal resolution and find the start of diastasis. This approach reveals situations when diastasis is not the most quiescent period in the cardiac cycle. In children end- systole is often a better period to perform imaging, as diastole is short and filling is continuous.
RESPIRATORY GATING- BREATH HOLD MRI. Breath Hold Imaging-one of the main issues with breath hold scanning is patient specific optimization. ↑ ing either spatial or temporal resolution will lead to prolonged breath hold times.
NAVIGATOR GATING. These are simple MR measurements of diaphragmatic position that enable data acquisition to be restricted to certain points in the respiratory cycle. A navigator usually consists of a 2D RF pulse that excites a cylinder of tissue (a so-called pencil beam excitation) and a single readout along the length of the cylinder. The navigator is usually placed on the dome of the right hemi- diaphragm with the position of the diaphragm being the same as liver-lung interface
PRACTICAL SET UP- SAFETY ISSUES Attraction of ferromagnetic objects due to the static magnetic field is the most important consideration. Also RF-induced heating and peripheral nerve stimulation in the MR environment. Permanent pacemakers and ICD’s-heating and current induction in leads, significant image degradation. Stents, coils and filters- recommended waiting period of 6-8 wks, but shown to be safe on day of implantation.
Pulm. Artery catheters-potential for excessive heating, hence unsafe. Heart valve prostheses-safe during MRI, but compromise on image quality. Metallic cardiac occluders-safe for non ferromagnetic devices immediately after implant. Aneurysm clips,carotid artery vascular clamps,insulin or infusion pumps,bone growth/fusion stimulators,cochlear, otologic or ear implant- all contraindicated.
MR CONTRAST AGENTS. MR contrast media fundamentally alter the intrinsic contrast properties of biological tissues in two ways: –directly by changing the proton density of a tissue. – indirectly by changing the local magnetic field or the resonance properties of a tissue and hence its T1 and/or T2 values. The local magnetic field strength is altered because the unpaired electron spins of the contrast medium (CM) interact with the surrounding hydrogen nuclei of the water, fat, or protein molecules in the tissue.
PARAMAGNETIC SUBSTANCES-have magnetic moment (resulting from individual spins) because they consist of atoms or molecules that have magnetic moment due to unpaired electron orbits in their outer electron shells. eg.s include Co2+, Co3+, Fe2+, Fe3+, Gd3+, Mn2+, Mn3+, and Ni3+. Most of the clinically available MR contrast media are paramagnetic metal ion compounds (gadolinium chelates, manganese, iron).
EXTRACELLULAR CONTRAST AGENTS. Low-molecular-weight, water-soluble compounds that distribute in the vascular and interstitial spaces following IV administration. Most MR contrast media used today belong to this gp of gadolinium(III) complexes. Gd-DTPA (gadopentetate dimeglumine = MagnevistR/linear ioniccomplex). Gd-DOTA (gadoterate meglumine = DotaremR/macrocyclic ionic complex), Gd-DTPA-BMA (gadodiamide = OmniscanR/linear nonionic complex),
IV administration of a standard dose of an extracellular contrast medium shortens T1, producing an increase in signal intensity in the vessels -first pass, and in the tissues due to tissue perfusion or disruption of the capillary barrier. Eliminated renally by glomerular filtration. Extracellular contrast media are administered intravenously as a bolus or drip infusion at a dose of 0.1–0.3 mmol/kg body weight.
ADVERSE EFFECTS Headache, nausea, or mild allergic reactions of the skin and mucosa occur in 1-2% of cases. Extravasated contrast medium can cause local pain and inflammatory reactions including tissue necrosis. Anaphylactic shock induced by an MR contrast agent is extremely rare (about 1:50,000 cases). Nephrogenic systemic fibrosis.
NEPHROGENIC SYSTEMIC FIBROSIS characterised by ↑ tissue deposition of collagen Thickening & tightening of skin - distal > proximal. Fibrosis- sk.muscles, lungs, pul.vasculature, heart, diaphragm Warrants cautious use in- CKD (GFR≤ 30mL/min/1.73 m 2 ) Peritoneal dialysis Hemodialysis ARF Hepatorenal synd Peritransplant period
MRI FOR HEART MORPHOLOGY Why MRI? -No window limitations, has a large field of view. -can provide 3D images. -accurate non-invasive assessment of Right ventricular (RV) mass and function (quantitative measurements of volumes). -MRI offers additional diagnostic information on characteristics of tissue and gives images with a high contrast between stationary tissues and circulating blood.
Study of heart morphology. The best morphological images are those obtained with fast techniques, requiring acquisitions in breath- hold,breath held in mid expiration. One of the prerequisites for an accurate morphological study of the heart by MRI is an efficient synchronization with ECG and the presence of sinus rhythm or, at least, the absence of uncontrolled arrhythmias.
As recommended by the American Heart Association (AHA), a correct evaluation of heart morphology requires that the images be obtained at oblique planes along the main axes of the heart. This involves the use of planes passing through the short axis of the left ventricle, and the long vertical and horizontal axes (Fig. 7.2), and oriented in space 90° one to the other. In Echo these planes correspond respectively to the short parasternal axis, to the apical projection in 2 chambers and 4 chambers.
The cardiac muscle can be visualised by using the 17 segment model. The slice thickness used for evaluating myocardial segments (for example, for locating ischemic areas) is generally between 6-8mm,.Slices less than 3 mm do not present particular advantages.
STRATEGY OF IMAGE AQUISITION. Take scout images – to locate the approx position of the heart and great vessels. Scout images are obtained in Gradient Echo, which has the advantage of being acquired in a few seconds (10-30 depending on the number of images and on heart rate). Classically, the study of the morphology is based on “black blood” sequences (Spin Echo, SE) that generate static images with an excellent spatial resolution.
IMAGING STRATEGIES FOR OBTAINING QUANT. DATA ON VENTRICULAR CHAMBERS. Fig. 7.9 a-f. (a) Coronal scout; (b) end-diastolic image in axial projection; (c) end-diastolic image on long vertical axis; (d) end-diastolic image on long horizontal axis; (e) end-diastolic image on long vertical axis that is used in alternative for obtaining images on the short axis; (f) end-diastolic image on short axis at medium ventricular level that is used in alternative for obtaining images on the long horizontal axis
Application of Simpson’s rule is apt here where the volume of a complex structure – the ventricle in this case – is obtained by dividing the structure into subvolumes – to yield the total volume. Endocardial border of each slice – 2d area. Area× distance b/w 2 slices( slice thickness + interslice distance)= 3d volume.
CALCULATION OF MYOCARDIAL MASS. The endocardial and epicardial edges of each ventricle should be recognised, manually or automatically. The papillary muscles and the endocardial trabeculae should be included in the calculation of the mass. The mass is given by the volume of the myocardium X specific weight(i.e 1.05 g/cm3).
TIMELINE FOR CMR IMAGING STRESS TEST. - Adenosine (140micro g/ kg –1 min –1) Infused continously for 2 mts prior to initaiation of perfusion imaging.. - Gadolinium contrast ( o 0.10 mmol/kg body weight) is then administered followed by a saline flush (50 mL) at a rate of at least 3 mL/s by means of an antecubital vein. -Breath-holding starting from the appearance of contrast in the RV cavity. Once the contrast bolus has transited the LV myocardium, adenosine is stopped, and imaging is completed 5 to 10 seconds later. Typically, the total imaging time is 40 to 50 seconds, and the total time of adenosine infusion is 3 to 3.5 minutes.
Prior to the rest perfusion scan, a waiting period of approximately 15 minutes is required for gadolinium to sufficiently clear from the blood pool. For the rest perfusion scan an additional dose of to 0.10 mmol/kg gadolinium is given, and the imaging parameters are identical to the stress scan. Approximately 5 minutes after rest perfusion, delayed enhancement imaging can be performed.
MYOCARDIAL MR PERFUSION STUDIES. Myocardial MR perfusion imaging has several advantages- Higher spatial resolution. No radiation exposure. No attenuation problem related to overlying breast shadow, elevated diaphragm, or obesity. Myocardial MR perfusion imaging approaches are currently mainly based on the changes in myocardial signal intensity (SI) during the first pass of an intravenously injected contrast agent (first-pass imaging)
GENERAL REQUIREMENTS NEEDED FOR QUANTIFICATION A nondiffusable tracer. A complete washout of the tracer from the myocardium. Linear correlation between the tracer and the SI. - However these are not fulfilled by Gd-DTPA.
To circumvent the problems associated with quantitative analysis of myocardial perfusion, semiquantitative parameters have been used such as the - upslope. -mean transit time. -maximal SI. -time to 50% maximal SI.
The early part of the SI-time curve is mainly influenced by perfusion. The later parts are influenced by diffusion. A linear fit of the upslope has been shown to be the most reliable parameter for evaluating myocardial perfusion. The upslope is easy to determine, highly reproducible, with low inter- and intraobserver variability.
MPR INDEX.( MYOCARDIAL PERFUSION RESERVE INDEX.) MPR index is calculated as the relative difference of perfusion before and after vasodilatation with dipyridamole or adenosine. All semiquantitative parameters and the calculated MPR indices show an underestimation of perfusion estimates that seems to be less when evaluating the upslope.
A cut-off value of 1.5 (mean - 2 SD of normal segments) allowed discrimination of normal from ischemic segments with good sensitivity and specificity (90 and 83% respectively).
MYOCARDIAL MR PERFUSION SEQUENCE DESIGN. Assessment of myocardial perfusion can be obtained by means of fast techniques such as spoiled gradient- echo (GE), echoplanar imaging (EPI) techniques, and balanced steady-state free precession (b-SSFP).
GRADIENT ECHO SEQUENCE. GRE sequences employ the gradient coils for producing an echo rather than pairs of RF pulses. Utilise a smaller flip angle < 90 degrees, to optimize T1 weighting. This is done by first applying a frequency- encoding gradient with negative polarity to destroy the phase coherence of the precessing spins (dephasing). Subsequently, the gradient is reversed and the spins rephase to form a gradient echo.
SPOILED GRADIENT ECHO. In order to obtain images in gradient Echo without artifacts, the transverse magnetization Mxy along the plane perpendicular to B0 needs to be completely null at the end of the TR interval so that only the longitudinal component Mz is left when the next RF pulse occurs. The amplitude of the signal that is obtained depends uniquely on the longitudinal relaxation.. “Spoiling” operation – from which the name of Spoiled Gradient Echo. This method spoils the phase coherence in transverse magnetization between successive TR intervals
ECHO PLANAR IMAGING. ADVANTAGE- ultrafast data acquisition,excellent for dynamic and functional MR imaging. DISADVANTAGE- poor image contrast, noisy and infield homogeneities.
SSFP- STEADY STATE FREE PRECESSION. Non zero steady state develops for both transverse and longitudinal components of magnetization. The MR signal will never completely decay- the spins in the transverse plane will never completely dephase. results in a higher SNR and hence better image contrast.
COVERAGE OF THE ENTIRE VENTRICLE. 3 short-axis slices covering the basal, mid and apical part of the LV are a strict minimum to appropriately evaluate regional myocardial perfusion. It is appropriate to acquire all image slices during one phase of the cardiac cycle-resting MBF is independent of the cardiac phase, adenosine-induced hyperemia yields significantly higher MBF and MPR in diastole than in systole.
MYOCARDIAL PERFUSION ANALYSIS Visual analysis-A true stress-induced myocardial perfusion defect has typical features that help to distinguish from- dark-rim artifacts diffuse microvascular ischemia fixed defects true ischemia at an infarct border.
IMPORTANT FEATURES OF A PERFUSION DEFECT. Onset of a myocardial perfusion defect coincides with the start of myocardial enhancement. Later than the occurrence of a dark-rim artifact. Most pronounced in the subendocardium (as is the dark- rim artifact) and the transmural extent is variable. The duration of the defect ranges from brief to prolonged (persistent till the second pass), while it resolves from the edge to the center of the perfusion defect, thus from subepi- to subendo-cardium. The defect obeys anatomic borders as well as the boundaries of the CA perfusion territories, whereas the dark-rim artifact does not.
Microvascular disease, presents as a circular subendocardial defect not respecting perfusion territories- hence difficult to differentiate from artifacts. Perfusion defects, caused by hemodynamically significant stenoses, are usually only visible during stress perfusion imaging.
Perfusion-like defects may occur in chronic, scarred infarcts in the absence of a coronary stenosis. This is due to the low capillary density in the scar compared to normal myocardium, simulating a perfusion defect on MPI. These defects are present also at rest ( ‘‘fixed defects’’), and the extent matches well with the findings on late gadolinium imaging.
DIAGNOSTIC PERFORMANCE OF PERFUSION MRI. On average, the sensitivity and specificity of perfusion MRI for detecting obstructive CAD were 83 percent (range, 44–93 percent) and 82 percent (range, 60–100 percent. The consensus report on CMR imaging classified perfusion imaging as a Class II indication for the assessment of CAD (provides clinically relevant information and is frequently useful).
NO REFLOW. In patients with a successfully reperfused acute myocardial infarction, MPI shows in 50% of patients a perfusion-like defect in the core of the infarct territory. This is caused by severe microvascular damage in the infarct core, a phenomenon also called no-reflow.
DARK RIM ARTIFACTS They typically occur at the interface between blood- pool and myocardium. Appear as soon as the contrast arrives in the LV cavity. More likely to occur with a higher contrast dose,balanced SSFP sequence. Occur on the basal half of the left ventricle, along the septal border and around the papillary muscles. Darker than a true perfusion defect.
EDEMA IMAGING. Increased free water in the infarcted myocardium prolongs T1- and T2- relaxation, and this prolongation is related to the duration of ischemia. T2-relaxation time linearly correlates with the % of free water, and infarcted myocardium is visible on T2-weighted MR sequences (T2w-imaging) as areas of increased signal intensity. In acute ischemia, the amount of free water increases not only in the irreversible but also in the reversible injured myocardium, leading to an overestimation of the true extent of myocardial necrosis. -reversible cell swelling. - increased capillary permeability in the surrounding ischemic rim. - myocardial edema
Healed infarcts, because of the lower water content in the fibrotic scar have decreased signal intensity compared with adjacent normal myocardium. Thus, T2w-imaging distinguishes b/w a recent and a healed MI. Acute myocarditis or transplant rejection may present equally focal or diffuse myocardial edema.
A no.of acute MI patients do not show homogeneous bright signal on T2w-imaging in the jeopardized myocardium-show a central hypo-intense zone- the breakdown of hemoglobin into deoxyhemoglobin- shortening of T2-relaxation times. Useful to detect post reperfusion myocardial hemorrhage.
CONTRAST ENHANCED MRI. Paramagnetic gadolinium chelated contrast agents, mainly Gd-diethylenetriamine pentaacetic acid (DTPA) have been used for MI imaging in both the acute and chronic setting. Infarcted/ scarred myocardium appear bright.
SALVAGEABLE MYOCARDIUM. Area of increased T2 signal- area of LGE.
Contrast Enhanced MRI in Acute Myocardial Infarction Early after reperfusion, the hyperemic response in the reperfused myocardium results in an ↑ ed delivery and higher gadolinium concentrations in the jeopardized myocardium as compared to normal myocardium. The supply of contrast agent to the infarcted region is dependent -on the patency of the infarct related vessel. -collaterals to jeopardised myocardium. -patency of microcirculation of the infarcted myocardium..
The optimal time window for infarct imaging should be somewhere between 10 and 25 min post-contrast administration. Because of the delayed or late period of imaging, this kind of imaging is called - (gadolinium) (DE/DGE) or late (gadolinium) enhancement (LE/LGE).
DEMRI- DELAYED ENHANCEMENT MRI Simultaneous study of viability and infarction. The goal of DEMRI is to create images with high contrast between abnormal myocardial tissue, which generally accumulates excess gadolinium and normal tissue in which gadolinium concentration is low. Imaging -approx 5 mts after rest perfusion-imaging OR mts after a one-time intravenous gadolinium dose of 0.15 to 0.20 mmol/kg if stress-rest perfusion imaging is not performed.
Following an iv bolus, gadolinium distributes throughout the intravascular and interstitial space, while simultaneously being cleared by the kidneys. In normal myocardium- myocytes are densely packed, tissue volume is mostly intracellular (~75–80 percent of the water space)- gadolinium is unable to penetrate intact sarcolemmal membranes- the volume of distribution is small- viable myocytes exclude gadolinium media.
In acute MI, myocyte membranes are ruptured, allowing gadolinium to passively diffuse into the intracellular space -↑ed gadolinium vol of distn. And ↑ed tissue conc. Compared to normal myocardium. Chronic infarction, as necrotic tissue is replaced by collagenous scar, the interstitial space is expanded, and gadolinium tissue concentration is increased.
PHYSIOLOGIC INSIGHTS OF DEMRI VIABLE MYOCARDIUM- black. INFARCTED MYOCARDIUM- white. REMOTE ZONE- max. viability. DE-MRI – shows only a 1.5 mm subendocardial infarction. Direct method – shows predominant viability. Compared to indirect method( nuclear scintigraphy) of assessing viability.
EXTENT OF HYPERENHANCEMENT AND SUCCESSFUL REVASCULARISATION. Transmural extent of hyperenhancement reflects the transmural extent of scar- irreversible loss of contractile fn. Post revasc. studies have indicated that > 75% transmural enhancement predicts nonreversibility of contractile fn. < 25 % enhancement predictive of post revascularisation improvement.
PATTERNS OF ENHANCEMENT ON LATE GD IMAGING. Patterns 1 (subendocardial infarct) Pattern 2 (transmural infarct) Pattern 3 the presence of microvascular obstruction (no-reflow) Pattern 4 epicardial coronary artery obstruction(occlusive infarct).
HYPERENHANCEME ENT PATTERNS IHD- always involves the subendocardium. Isolated midwall or epicardial hyperenhancement strongly suggests a non ischemic pathology. Also globally present subendocardial hyperenhancement is unlikely even in diffuse CAD, to consider amyloidosis, systemic sclerosis.
CHRONIC ISCHEMIC CARDIOMYOPATHY. The dysfunctional ventricle or myocardium in CAD often contains a mixture of different ischemic substrates (i.e. stunned, ischemic, hibernating, necrotic, scarred myocardium) within the same perfusion territory,characterization of these ischemic substrates, especially viability- important to determine success of revascularisation. MRI has the unique capability to visualize even subtle amts of scar formation.
MYOCARDIAL VIABILITY ASSESMENT First step is to measure segmental end-diastolic wall thickness(EDWT). Infarct healing with scar formation leads to a wall thinning of the infarcted region. Thinned myocardium <6 mm has a low likelihood of functional recovery after revascularization and accurately reflects scar tissue.
A substantial % of segments with preserved wall thickness, however,do not improve in function following revascularization. The most likely explanation is the presence of subendocardial infarction with preserved wall thickness. Thinned myocardial segments may undergo a process of ‘‘reverse remodeling’’ after successful myocardial revascularization with recovery of function and regain in regional wall thickness.(John AS, et al2005 Images in cardiovascular medicine. Reversible wall thinning in hibernation predicted by cardiovascular magnetic resonance. Circulation 111:e24–e25)
In patients with chronic Lv dysfunction, dysfunctional but viable (hibernating) segments improve contractility during low-dose (5–10 microgm/kg body weight) dobutamine infusion (i.e. increased systolic wall thickening> 2mm). Contractility in non-viable scarred segments remain unchanged or worsen during stress imaging Contrast enhanced MRI using late Gd imaging, the third approach, detects scar tissue but not viability.
MR OF CORONARIES- ANGIOGRAPHIC APPROACH The diagnostic capacity of MR is based on the administration of a contrast agent and the acquisition of 3D images (CEMRA). Coronary imaging is difficult- -small dimensions of the structure < 5mm. -variable coursing. -rotational and translational activity caused by cardiac and respiratory activity.
TECHNIQUES “Black blood” or “white blood”. 2 or 3 D. Breath hold or free breathing. With or without contrast medium.
BLACK BLOOD TECHNIQUES Completely cancel-out the signal from blood through the application of a double inversion pulse (double inversion recovery). The first pulse inverts the magnetization field throughout the body, blood included. The second re-inverts the magnetization of the tissues on the scanning plane. The final result in the image is the annulment of the signal emitted from blood, whereby the lumen of the vessel is black in contrast to the more or less intense signal of the wall. Significant limitation is that this is a 2D method.
WHITE BLOOD TECHNIQUES. Can be obtained by several acquisition techniques( Gradient echo, echo planar imaging) that are faster, with excellent contrast b/w vessel lumen and surrounding fat which can be easily cancelled. ADVANTAGE- 3D technique, easy for operator to centre the coronary vessel more easily. DISADVANTAGE- cannot visualize arterial wall, only lumen.
PLAQUE CHARACTERISATION BY MRI. Better validated in carotid arteries. The lipid plaques have both a short T1 and a short T2, therefore they are hyperintense in T1-weighted images and hypointense in T2. Fibrous plaques have a quite similar signal intensity in T1- and T2-weighted images; the signal intensity is lower compared to lipidic plaques in T1-weighted images.
STUDY OF CORONARY RESERVE The coronary reserve expresses the vasodilatation capacity of coronary vasculature in response to cardiac metabolic demand. Coronary flow can increase up to 5 times the baseline value. Coronary flow decreases only in the presence of a stenosis over 85%,while the coronary reserve begins to decrease, in presence of a stenosis of about 35-40%. When stenosis is over 90% the reserve goes down to zero.
It is possible to measure coronary flow using a Phase Contrast sequence(principle- signal from moving blood will undergo a phase shift relative to stationary tissue if a magnetic field is applied in that direction.) The values of coronary reserve as measured by PC images (ratio of flow in maximum vasodilatation obtained with adenosine over baseline flow) correlate well with severity of stenosis and CFR measured invasively. The sensitivity and specificity for the diagnosis of coronary stenoses >70%,are respectively 100 and 83%.
VENC- MRI ( Velocity encoded MRI) ECG-gated velocity-encoding MRI. Analogous to cine imaging, each velocity-encoded image (top row of images) corresponds to a cardiac phase, and gating to the electrocardiogram is necessary. On the images, white represents maximal velocity (in this case across the aortic valve, red arrow). Black represents flow in the opposite direction (in the descending thoracic aorta, blue arrow). Grey represents no flow. The bottom row demonstrates the corresponding cineMRI images.
BYPASS AND STENTS The application either of SE or GRE sequences, and especially CEMRA, allows an accurate evaluation of the morphology of both arterial and venous grafts. It is possible to visualize 98% of bypasses and give their accurate description with an 88% specificity. The measurement of blood flow by means of the Phase Contrast sequence can provide further information on the hemodynamic effect of a stenosis of the graft. The flow in the bypasses can also be measured after adenosine for the evaluation of the flow reserve. Reliable in identifying coronary artery aneurysm in Kawasaki disease. Imaging of anomalous coronary arteries.