90 % of cases. 75 % are in the LA. \200 may have a speckled, cystic appearance with frond-like projectio ns. \200 may be multiple. If they occur elsewhere, they may be mistaken for thrombus (ten d to be homogeneous in appearance) or other tumors. may cause valve obstruction. Fibroelastomas are benign tumors that attach to valve apparatus. They have a frond-like app earance and may mimic vegetations or myxoma. \200 Primary malignant cardiac tumors and se condary metastatic disease can in \201 ltrate the epicardium, myocardium, or endocardium, or present as intracavity m ass. Pericardial involvement in malignant disease frequently causes pericardial effusion. This page intentionally left blank 46 CHAPTER 1 Cardiac investigations TOE assessment of mitral valve prosthesis TOE is especially sensitive in detecting MR in as sociation with a prosthetic valve, because the left atrium is imaged in the foreground and is therefore devoid of the artifacts generated by the metal elements of the prosthetic valv e. Paraprosthetic mitral leak The sewing ring of the mechanical prosthetic valve appears as a bright ring attached to the mitral annulus. If a signi \201 cant circumference of the sewi ng ring has come away (e.g. from endocarditis), a rocking movement of the valve will be evi dent \200\224 this \200\230rocking prosthetic valve \200\231 is characteristic of a parti al dehiscence of the valve. Key views The mitral valve sewing ring should be identi \201 ed in and interrogated with c olor \202 ow and CW Doppler: \200 0 ME four chamber. \200 90 ME two chamber. \200 120 ME LAX views of the mitral valve. The motion of the prosthetic valve mechanism (e.g. tilting disc) should be assessed to ensure the disks or occluder are moving freely i n 120 ME LAX view. The presence of \202 ow outside the sewing ring (i.e. paraprosthetic) is not normal, alth ough small or mild jets may be seen early after valve implanta- tion. This represents failu re of the sutures attaching the valve ring to the annulus, either from technical failure or from the effect of endocarditis. Paraprosthetic leaks are more likely in the presence of a calci \201 ed mitral annulus. Transprosthetic mitral leak Mechanical prosthetic valves have regurgitation during closure of the valve, which arises within the sewing ring (i.e. intraprosthetic). These \200\230cl osing jets \200\231 are a normal feature and should not be mistaken as pathological. In gen eral they can be easily distinguished from pathological regurgitation (see Table 1.3 ). Cau ses for abnormal intraprosthetic leak include pannus or thrombus preventing the occluder fr om functioning properly. Tissue mitral prosthesis This should be assessed in the same way as a mechanical prosthesis . The prosthetic valve lea \202 et should be assessed for: \200 prolapsing lea \202 et. \200 \202 ail lea \202 et. \200 thickened cusps ( > 3 mm \200\224 higher risk of d ysfunction). \200 the Doppler signal for a \202 ail cusp shows a characteristic striate d \200\230zebra-stripe pattern \200\231. TOE ASSESSMENT OF MITRAL VALVE PROSTHESIS 47 Obstruction of the prosthetic mitral valve This may be due to thrombus or pannus formation. The diagnosis is made by \201 nding: \200 limited movement of the occluder or disc. \200 haemodynamic con \201 rmation by \201 nding a high peak pressure gradient across t he mitral valve( > 2.5 m/s) or a prolonged pressure half-time ( t > 200 ms). Features of normal closing jets \200 Short jets <3 cm length. \200 Narrow base <5 mm. \200 Early systolic rather than pansystolic leak. Table 1.3 Upper limit of pressure half-time (ms) for commonly used valves Maximum t (ms) Valve type Starr \200\223Edwards 170 St Jude 131 CarboMedics 117 Carpentier \200\223Edwards 171 Native valve 60 48 CHAPTER 1 Cardiac investigations TOE for aortic valve prosthesis \200 The orientation of the aortic valve means that the s ewing ring will obstruct the view of the ori \201 ce of the valve for mechanical and stented tissue aortic valves. Thus, assessment of aortic paraprosthetic leak on 2D imaging is limited compared t o mitral valve assessment. CW interrogation is not possible because the LVOT is orientated perpendicular to the ultras ound beam, when the aortic valve is assessed from the standard ME views. A combined TTE and TOE approach is required, because TTE is ideally suited to Doppler interrogation of the aortic valve. Deep transgastric view A deep transgastric TOE allows imaging of the aortic valve in the fo re- ground, and allows proper alignment of the Doppler signal from the aortic valve (0 d eep TG LAX and 120 TG LAX views). This view is useful to assess: \200 movement of the aortic prosthetic valve mechanism. \200 color \202 ow Doppler interrogation for aortic regurgitation. \200 CW assessment of aortic regurgitation. \200 CW measurement of the a ortic valve gradient. The use of the deep transgastric TOE view is especially useful for as sess- ment of aortic prosthetic valve function when a mechanical mitral prosthesis is also present. This is because acoustic shadowing from the mitral valve obscures the LVOT in all ME TOE views of the aortic valve. However, this view is not possible in all patients. Regurgitation is most commonly transprosthetic in aortic tissue valves. The following may b e visible on TOE: cusp thickening ( > 3 mm predicts risk of valve failure), cusp tear, cusp prolapse, and \202 ail cusp. Rocking of the aortic prosthesis is due to dehiscence of th e valve (40 % circumfer- ence affected) and best seen in the 0 ME AV SAX and 120 ME A V LAX views. Key views The aortic valve prosthesis is assessed in: \200 0 ME AV SAX. \200 120 ME AV LAX. \200 0 deep TG LAX. \200 120 TG LAX. TOE FOR AORTIC VALVE PROSTHESIS 49 TOE assessment of normal and dysfunctional aortic prosthetic valves Normal peak gradient across mechanical aortic valve prosthesis \200 St Jude Medical. \200 Medtronic Hall. Signi \201 cant stenosis or obstruction likely in mechanical aort ic valve \200 Peak velocity of > 4 m/s or mean velocity of 3 m/s are suggestive of 24 7 mmHg. \200 21 7 mmHg. obstruction. Tissue aortic valves \200 Aortic valve area by continuity equation <1 cm 2 . \200 Mean gradient > 26 mmHg. \200 Ratio of LVOT to peak velocity across aortic valve <0.2. Stentless aortic valves \200 (Freestyle or Toronto valve) or aortic homograft s do not have the disadvantage of a sewing ring casting an acoustic shadow on the aortic va lve ori \201 ce, and TEE gives excellent views of valve cusps. Some of these valves are in distinguishable from native valves on TOE. 50 CHAPTER 1 Cardiac investigations Intraoperative TOE Indications AHA class I indications \200 Cardiac valve repair. \200 Haemodynamic compromise where LV function is unknown. \200 Complex endocarditis surgery w here perivalvular extension is suspected. Complex valve replacement such as homograft valve replacement with coronary reimplantation. Ascending aortic dissection with aortic valve involvement. \200 Posterior or loculated pe ricardial effusion. \200 Congenital heart surgery. \200 Hypertrophic obstructive cardio myopathy (HOCM surgery). AHA class II indications \200 Heart valve replacement. \200 Ca rdiac tumor removal. \200 Coronary surgery including \200\230off pump bypass \200\231. \200 Cardiac aneurysm repair. \200 LV or RV assist device implantation. \200 Ascending aortic dissection without aortic valve involvement. \200 Cardiac trauma. \200 Intracar diac thrombectomy or pulmonary embolectomy. \200 Heart and/or lung transplantation to ass ess anastomotic sites. Key views (see Fig. 1.11 ) LV assessment is made on these views: 0 ME four chamber. \200 90 ME two chamber. \200 120 ME LAX. \200 0 TG mid SAX. TOE in the intraoperative and perioperative period (in ITU) to assess haemodynamic instabil ity \200 Coronary artery bypass graft failure: examine LV in all 3 coronary distributions. The transgastric view shows all 3 coronary distributions in one view. Hypovolaemia: reduced LV cavity size and end-systolic cavity obliteration is a sign of hypovolaemia. Pericardial collection: any collection can be localized to, say, just the left atrium and need not be a global collection. Severe LV dysfunction. \200 Right ventricular failure. \200 Unsuspected severe mitral r egurgitation. \200 Unsuspected aortic dissection. INTRAOPERATIVE TOE 51 0 Four chamber 90 Two chamber 120 Long axis 0 Transgastric LAD CX RCA Fig. 1.11 Assessment of LV function by TOE. CX = circum \202 ex; LAD = left anterior desce nding artery; RCA = right coronary artery. Reproduced with permission from Shanewise JS. Jo urnal of the American Society Echocardiography 1999; 12 : 884 \200\223900. 52 CHAPTER 1 Cardiac investigations Basic principles of nuclear cardiology Nuclear medicine investigations involve the injectio n of a tracer (radi- opharmaceutical) labeled with a radionuclide which emits gamma (or X-r ay) photons. This is distributed within the body according to its chemical properties. This distribution can be imaged by single photon emission computed tomography (SPECT) using an Anger gamma camera. The most commonly performed nuclear cardiology investigation is myocard ial perfusion scintigraphy (MPS), which provides physiological information in known or susp ected coronary disease. Radionuclide ventriculography is an accurate method for quantifying left ventricular function, although it is now performed less commonly than in the past. Ca rdiac iodine-123-metaiodobenzylguanidine ( 123 I-MIBG) imaging is increas- ingly used to as sess sympathetic innervation in heart failure. Cardiac positron emission tomography (PET) i maging is less widely available than SPECT and uses different imaging equipment and radioph armaceuticals, but provides similar information. The Anger gamma camera Crystal The key component of a gamma camera is a large \202 at cir cular or rec- tangular sodium iodide crystal, activated by non-radioactive thallium \200 \224 NaI(Tl). The crystal is a \200\230scintillator \200\231, i.e. absorption of a gamma p hoton via the photoelectric effect yields a burst of photons of visible light within 1 mm o f the interaction. Photomultiplier tubes The side of the crystal away from the patient is viewed by a hexagona l array of up to 100 photomultiplier tubes (PMTs). These convert the weak signal carried by photons of visible light leaving the crystal, into a detect- able electrical pulse. The pa ttern of activation of PMTs signals the location of the originating scintillation event, wi th the largest electrical pulse being generated in the PMT closest to the event. Collimator The side of the crystal facing the patient is shielded by a collimator, a lead d isc penetrated by thousands of uniform parallel channels separated by thin septa. Only phot ons traveling perpendicular to the collimator can penetrate the channels and enter the crys tal, while the remainder are absorbed by the lead septa. Thus gamma-photons originating fro m a par- ticular area of the heart can only enter a selected area of the NaI(Tl) crystal, m aintaining spatial information. SPECT image acquisition and display Planar acquisition Acquisition and summation of scintil lations by the head of a gamma camera for a few minutes yields a planar scan, which represe nts the 3D distribution of radiopharmaceutical within a patient as a 2D image. BASIC PRINCIPLES OF NUCLEAR CARDIOLOGY 53 SPECT acquisition SPECT imaging involves the acquisition of a series of planar projections at different angles as the head(s) of the gamma camera orbit(s) the patient. A dual-headed camera is preferred, with the heads positioned at 90 to one another to halve acquisition time. A typical MPS acquisition might involve 64 projections (32 per head) acquired over a 180 orbit and taking 16 minutes. Filtered back-projection or iterative reconstruction is used to produce a set of transaxial sections through the patient. The transaxial slices can then be reorientated to the axes o f the heart to produce vertical long- axis, horizontal long-axis, and short-axis slices. Th e count density of each pixel within the reorientated slices is displayed relative to the p ixel of maximal counts in the myocardium (0 \200\223100 % ), using a gray scale or color sp ectrum. Stress slices are displayed above corresponding rest slices to facilitate compariso n. Gated SPECT SPECT acquisitions can be \200\230gated \200\231 to the ECG. The R \200\223R i nterval is divided into 8 or 16 frames, and each planar projection is acquired as 8 or 16 c orresponding images. Each frame is reconstructed and reorientated separately to produce SPE CT slices that represent the left ventricle at a particular point in the cardiac cycle. Sta tic slices to assess perfusion can be obtained from the same acquisition, by summing the fr ames for each planar projection. Gated slices can be viewed in a looped cine format to assess regional function in terms of wall excursion and thickening. Commercially available software is used to \201 t endocard ial and epicardial boundaries and deter- mine end-diastolic volume, end-systolic volume, an d ejection fraction. Attenuation correction Gamma photons emitted from the heart are variably attenuated by soft tissue. This can produce spurious apparent perfusion abnormalities in the processed slices (attenuation artifacts). Many modern gamma cameras allow attenuation correction (AC). A tr ansmission acquisition is per- formed using one or more scanning gadolinium sources or X-ra y CT, and an attenuation map is reconstructed. This is used to correct the emission acquisi tion, which is acquired simultaneously (gadolinium approach) or separately (CT approach). 54 CHAPTER 1 Cardiac investigations Myocardial perfusion scintigraphy: technical Overview At rest, myocardial perfusion distal to a \202 ow-limiting coronary stenosis usually remains normal due to progressive arterio lar dilatation. During cardiac stress, there is less vasodilator reserve available, and per fusion will be lower than that downstream of an unobstructed vessel. A radiopharmaceutical perfusion tracer is injected during cardiac stress, and is taken up a nd retained by cardiac myocytes in relation to blood \202 ow. Subsequent SPECT imaging is performed, and the distribution of radio- nuclide re \202 ects myocardial viability and p erfusion at the time of tracer injection, i.e. during stress. A separate imaging study is p erformed after injection of tracer at rest (technetium-99m-labeled tracers) or following re distribution of the stress injection (thallium-201), and the distribution of radionuclide p rimarily re \202 ects viability. Cardiac stress Dynamic exercise stress (treadmill or bicycle) is the most physiological for m of stress and provides important clinical information that may com- plement imaging \201 ndings. Many patients referred for myocardial perfusion scintigraphy (MPS) are unable to exercise to their target heart rate, and pharmacological methods are required. The drugs used fall into two cat- egories: \200 primary vasodilators (dipyridamole or adenosine): \201 rst choice, unless contraindicated by airways disease or unpaced heart block. inotrope (dobutamine). Adenosine causes direct coronary vasodilatation via A 2a -receptors. Dipyridamole acts indirectly, increasing endogenous adenosine by inhib- iting its breakdow n and reuptake. Dobutamine, a synthetic beta-agonist, increases myocardial oxygen demand, c ausing secondary coronary vasodilatation. Radiopharmaceutical perfusion tracers The MPS perfusion tracers in routine clinical use are thallium-201 ( 201 Tl) and the newer technetium-99m ( 99m Tc)-based tracers, sestamibi and tetrofosmin. Thallium-201 201 Tl, given as thallous chloride, is a cyclotron-generated isotope. It enter s myocytes down the electrochemical gradient in a similar way to potas- sium. Following inj ection at peak stress, it gradually equilibrates between the intracellular and intravascula r compartments, and so immediate imaging is required. Redistribution imaging is performed 4 hours later, to assess perfusion at rest. For optimal assessment of myocardial viability, 201 Tl is sometimes reinjected at rest. MYOCARDIAL PERFUSION SCINTIGRAPHY: TECHNICAL 55 Technetium-99m-labeled tracers 99m Tc is produced from a generator and is complexed with an organic molecule (sestamibi or tetrofosmin). 99m Tc-sestamibi and 99m Tc-tetro- fosmin dif fuse passively into myocytes, where they bind to mitochondria. The absence of signi \201 c ant redistribution means that imaging can be per- formed conveniently 30 \200\22360 minutes after injection, but separate injections are required during stress and at rest. A 2-day o r a 1-day protocol may be used. A nitrate may be given prior to the resting injection, to m aximize uptake into viable myocardium. Comparison of tracers 201 Tl is a superior perfusion tracer physiologically, but the 99m Tc agents have several practical advantages. The images produced are of higher quality becaus e the photons are of higher energy, and can be gated to allow assessment of left ventricula r function. In addition, the shorter half- life (6 hours vs. 73 hours) reduces the radiatio n exposure to the patient. 56 CHAPTER 1 Cardiac investigations Myocardial perfusion scintigraphy: clinical Indications \200 Diagnostic and prognostic as sessment in suspected coronary disease when exercise ECG is likely to be unreliable: \200 women. \200 patients unable to exerc ise adequately. \200 patients with signi \201 cantly abnormal resting ECG. \200 Guidan ce of management in known coronary disease: post myocardial infarction. \200 post coronary angiography. \200 post coronary revascul arization. Assessment of myocardial viability, ischaemia, and function in ischaemic left ventricular dysfunction. Image interpretation \200 Perfusion defects: \201 xed (on both stress and rest acquisitions): suggests scar following myocardial infarction. reversible (on stress acquisition only): suggests inducible hypoperfusion of viable myocardium distal to \202 ow-limiting coronary stenosis. Characterized by location (coronary territory), extent, and severity (prognostic importance). may need to be distinguished from artifacts, e.g. anterior soft tissue attenuation in women, inferior attenuation in men. Other high-risk \201 ndings, e.g.: increased lung uptake of 201 Tl. \200 transient ischaemic dilatation (TID): left ventricu lar cavity larger post-stress than at rest. Left ventricular function on gated acquisitions. Clinical value of MPS Diagnosis or exclusion of obstructive coronary disease 1 For the dete ction of angiographic coronary stenoses, the sensitivity of MPS is approximately 90 % . The speci \201 city is usually lower at approximately 75 % , but may have been underestimated due to overinterpretation of artifacts and post-test referral bias. In practice, a more me aningful alter- native to speci \201 city is the normalcy rate (proportion of normal tests in a low-probability population), and this is typically 90 % or more. Prognosis MPS is a functional test, and hence cannot predict angiographic coro- nary anatom y with perfect accuracy. Nevertheless, MPS provides robust prognostic information that is i ndependent of clinical, exercise, and even angiographic data. 1 Underwood SR, Anagnostopoulos C, Cerqueira M, et al . (2004). Myocardial perfusion scinti g- raphy: the evidence. Eur J Nucl Med Mol Imaging 31 : 261 \200\22391. MYOCARDIAL PERFUSION SCINTIGRAPHY: CLINICAL 57 Normal MPS study predicts the annual rate of cardiac death or non- fatal MI of approximately 0.6 % . 2 Low risk persists for up to 5 years in patients without known coronary disease, though this \200\230warranty period \200\231 is shorter in patients with diabetes and those with known coronary disease. 3 The risk associated with an abnormal study increases in proportion to the degree of abnormality. 4 Cardiac death is primarily a function of left ventricular ejection fraction, while non-fatal MI is mainly predicted by the extent of inducible hypoperfusion. 5 The prognostic bene \201 t from revascularization may be limited to patients with inducible hypoperfusion amounting to > 10 % of the left ventricular myocardiu m. 6 Ischaemic left ventricular dysfunction MPS is of value in the assessment of patients with i schaemic left ventricular dysfunction to de \201 ne the extent of hibernating myocardium, which has the potential to recover function following revascularization. 7 Hibernating myocardium is: viable (tracer uptake > 50 \200\22360 % maximum counts on resting study), but \200 dysfun ctional at rest (hypokinetic or akinetic on gated study), with \200 exhausted vasodilator reserve (inducible hypoperfusion). 2 Iskander S, Iskandrian AE (1998). Risk assessment using single-photon emission computed t omographic technetium-99m sestamibi imaging. J Am Coll Cardiol 32 : 57 \200\22362. 3 Hacham ovitch R, Hayes S, Friedman JD, et al (2003). Determinants of risk and its temporal variati on in patients with normal stress myocardial perfusion scans. What is the warranty period o f a normal scan? J Am Coll Cardiol 41: 1329 \200\22340 . 4 Hachamovitch R, Berman DS, Kiat H, et al (1996). Exercise myocardial perfusion SPECT in patients without known coronary art ery disease: Incremental prognostic value and use in risk strati \201 cation. Circulation 93: 905 \200\22314 . 5 Sharir T, Germano G, Kang X, et al (2001). Prediction of myocardial infarction versus cardiac death by gated myocardial perfusion SPECT: Risk strati \201 cati on by the amount of stress-induced ischemia and the poststress ejection fraction. J Nucl Me d 42 : 831 \200\2237 . 6 Hachamovitch R, Hayes SW, Friedman JD, et al (2003). Comparison of the short-term survival bene \201 t associated with revascularization compared with medic al therapy in patients with no prior coronary artery disease undergoing stress myocardial p erfusion single photon emission computed tomography. Circulation 107 : 2900 \200\2236 . 7 B ax JJ, van der Wall EE, Harbinson M (2004). Radionuclide techniques for the assessment of m yocardial viability and hibernation. Heart 90 (suppl V): v26 \200\22333 . 58 CHAPTER 1 Cardiac investigations Other nuclear cardiology investigations Radionuclide ventriculography Equilibrium radionucl ide ventriculography (ERNV) was the \201 rst reliable non-invasive method of assessing le ft ventricular function. The blood-pool is labeled with 99m Tc-pertechnetate. A gated planar acquisition is obtained in a left anterior oblique projection, to minimize overlap of the left ventricle by other cardiac chambers. Regions of interest are drawn around the left ventricular cavity in end-d iastole and end-systole, together with an adjacent background region of interest, allowing calculation of the ejection fraction. This counts-based method makes no assumptions about l eft ventricular geometry, and the ejection fraction is accurate and reproducible ( + 5 % ). The growth of echocardiography has led to a sharp decline in the number of ERNV studies per formed, but it may still be used when a reproducible technique is required for serial asses sment of left ventricular ejection fraction (e.g. during chemotherapy). Cardiac 123 I-MIBG imaging Cardiac MIBG imaging is sometimes performed as a prognostic assess- ment in patient s with left ventricular dysfunction. MIBG is a false neurotransmitter, and is taken up by sympathetic nerve endings in the heart via the sodium- and energy-dependent \200\230uptake-1 \200\231 mechanism. It is labeled w ith cyclotron-generated 123 I. Following injection, planar acquisitions are obtained in an anterior projection at 15 minutes and 4 hours. The overall cardiac uptake of 123 I-MIBG can be quanti \201 ed as the heart-to-mediastinum (H/M) ratio. Reduced H/M ratio (e.g. <1.60 on delayed acquisition) with increased washout from early to delayed imaging is an independent predictor of an adverse prognosis. It has been suggested that this might \201 nd a role in the selection of patients for implantable de \201 bril lators. This page intentionally left blank 60 CHAPTER 1 Cardiac investigations Positron emission tomography scanning Radiopharmaceuticals used in PET emit positrons ( \200\230anti-electrons \200\231), which annihilate within a short distance by combining wit h an electron. This pro- duces two 511 keV gamma-photons traveling in opposite directions. A PET camera consists of a series of circular or hexagonal arrays of scintil- lation detect ors. If each of a pair of detectors simultaneously registers a scintillation event (coincid ence), positron annihilation is assumed to have occurred along a line between the two detec tors (coincidence detection). Coincidence detection is used to construct transaxial slices, which can be reorientated as in SPECT. PET allows quantitative assessment of various aspects of cardiac func- tion in different regions of the heart: \200 global and regional left ventricular functio n \200 myocardial blood \202 ow \200 myocardial metabolism: glucose and fatty acid me tabolism; myocardial oxygen consumption pharmacology: beta-adrenergic and muscarinic receptors; sympathetic innervation; myocardial angiotensin-converting enzyme (ACE) and angiotensin II receptors \200 myocardial gene ex pression. Clinical applications Identi \201 cation of myocardial viability The main clinical applica tion of PET scanning has been determination of the myocardial viability of patients with im paired left ventricular function secondary to coronary artery disease, who may bene \201 t from surgical or percutaneous coronary revascularization. These studies have demon- strate d that PET imaging has high sensitivity for predicting recovery of contractile function aft er revascularization, and have also provided major insights into the mechanisms underlying left ventricular dysfunction in patients with coronary artery disease. Research applications The large variety of compounds available for study using PET permits interrogation of many facets of cardiac function, providing important in vivo mechanistic i nformation in human disease. These measurements also allow for analysis of the mechanisms u nderlying the bene \201 t of estab- lished and novel therapeutic strategies. Examples incl ude: \200 myocardial blood \202 ow and microvascular function: ischaemic heart disease, hypertrophic cardiomyopathy, dilated cardiomyopathy, aortic stenosis, syndrome X. myocardial metabolism and cardiac energetics: ischaemic cardiomyopathy, dilated cardiomyopathy. cardiac autonomic function: hypertrophic cardiomyopathy, dilated cardiomyopathy, arrhythmogenic right ventricular dysplasia, autonomic neuropathy, long QT s yndrome. POSITRON EMISSION TOMOGRAPHY SCANNING 61 Comparison of PET over conventional cardiac nuclear modalities (gamma camera, SPECT) Advantages \200 Short half-life of radionuclides. \200 Better spatial resolution. \200 Accurate attenuation correction allowing absolute quanti \201 cation of radiopharmaeutical concentration. On-site cyclotron. Disadvantages \200 Expensive. \200 Limited access. \200 Predominan tly a research application. Adapted with permission from de Silva R and Camici PG (1994). R ole of positron emission tomography in the investigation of human coronary circulatory func tion. Cardiovasc Res 28 (11): 1595 \200\223612. 62 CHAPTER 1 Cardiac investigations Basic principles of cardiac MRI \200 Cardiac MRI (CMR) is a highly versatile modality tha t enables cardiac structure and function to be characterized with high spatial resolution a nd in any plane, without using ionizing radiation. The patient is placed within a strong static magnetic \201 eld, 1.5 \200\2233 Tesla (T) i n strength. This forces all the protons in the patient to align with the \201 eld and pre cess at a frequency determined by the \201 eld strength. A radiofrequency pulse is delivered at this resonant frequency, causing the magnetization of the protons to be \202 ipped away from the longitudinal dire ction. As it decays back towards the relaxed state, radiofrequency energy is emitted by the protons. The relaxation times for the recovery of longitudinal magnetization and the decay of transverse magnetization are in \202 uenced by the local environment of t he protons. The resulting signals are used to construct an image. Different sequences of pulses and varying magnetic \201 eld gradients can be used to emphasize diff erent tissue characteristics or highlight particular anatomical details. Tissues that lack protons, e.g. the air- \201 lled lung, are not well imaged by CMR in health. Major indications for CMR include: \200 evaluation of ventricular function (gold standard ). \200 cardiomyopathy assessment. \200 assessment of ischaemia and myocardial viabilit y. \200 assessment of cardiac iron status. \200 evaluation of congenital heart disease. \200 evaluation of valvular heart disease. \200 diagnosis and serial follow-up of aort ic disease. \200 assessment of pericardial disease and cardiac masses. This page intentionally left blank 64 CHAPTER 1 Cardiac investigations Making CMR requests \200 As with any other imaging modality, provide as much information as possible about the clinical question that is to be answered by the scan. The versatility of MRI provides the operator with a gamut of sequences and techniques that can be applied. A detailed clinical question will help to facilitate the construction of an appropriate imagi ng protocol. \200 Assess for absolute contraindications prior to making a request, and de tail any concerns or doubts in the referral letter (see below). \200 Cardiomyopathy and i schaemia assessments are greatly facilitated by providing a resting ECG with the referral. Requests to assess potential hypertrophic cardiomyopathy should detail any possible alternative causes for hypertrophy, and in particular the severity, chr onicity, and control of hypertension. When requesting viability studies, provide details of any interventions, angiogram \201 ndings, or graft details if available, as this can greatly facilitate repo rting. For valve studies, provide any available ECHO data. \200 Ask patients to bring along a fa vorite music CD as this can be played during the scan to minimize anxiety. Claustrophobic patients in particular bene \201 t fro m this. Where language may be a barrier, ensure that someone able to interpret attends the scan, or provide warning that an interpreter may be required. Without cooperation from the patient, particularly with regard to breath-holding instructions, ima ge quality may be signi \201 cantly degraded. Patients must be able to lie \202 at for 1 hour and breath hold. \200 Patients attendi ng for adenosine stress perfusion imaging should be advised to abstain from caffeine in the 24 hours prior to the study. Those likely to receive dobutamine should be similarly advise d to withhold beta-blockers or rate-limiting calcium-channel antagonists. Provide details of the patient \200\231s renal function, as this will enable appropriate precautions and consent to be taken in renal impairment if gadolinium contrast is to be administered. For obese patients, provide details of the patient \200\231s weight to assess whether the M RI table and scanner can safely accommodate the patient. In patients with suspected metallic foreign bodies in the eyes/orbits (absolute contraindication to MRI), arrange for orbital X-ray imaging to be undertaken befo re making the request if there is any doubt or the object has not been removed. Absolute contraindications to CMR \200 Pacemaker/implantable cardioverter de \201 brilla tor (ICD) or retained/ fractured pacing leads. Metallic foreign bodies in the eye/orbit. \200 Ferromagnetic cerebrovascular aneurysm cli ps. \200 Cochlear implants. \200 Insulin pumps. MAKING CMR REQUESTS 65 There is no documented harm from MRI in pregnancy. Nonetheless, adhering to the precautionary principle, it should be avoided, particularly in the \201 r st trimester, unless there is likely to be compelling clinical bene \201 t and the scan ca nnot be deferred. As a precaution, women who are breast-feeding should be advised to express and discard brea st milk for 24 hours after receiving gadolinium contrast media. 66 CHAPTER 1 Cardiac investigations CMR safety \200 Used with the appropriate precautions, CMR is a very safe diagnostic tech nique. However, the strong magnetic \201 elds employed, typically 1.5 T or 3 T can conver t any ferromagnetic object into a projectile with potentially lethal effect. All patients u ndergoing CMR examination should be screened for ferromagnetic implants, and equipment not speci \201 cally deemed MR safe , i.e. safe in all MRI environments, should be excluded. The radiofrequency pulses used as part of imaging can generate heating effects in wires or implants. Limb-to-limb skin contact can allow current loops to develop with thermal injury at the points of contact. Similar heating and burns can result from such apparently benign objects as standard ECG stickers. These must be removed from the patient prior to the scan, and monitoring carried out with carbon-based MR-safe stickers. In addition to the main magnetic \201 eld, gradient \201 elds are employed and switched on and off rapidly during imaging to generate various sequences and to allow spatial local ization of MR data. These generate very loud noise ( > 100 dB) and can cause hearing loss. All patients and anyone present in the scanner room should wear protective headphones or ea rplugs. Rapid gradient switching can also cause unpleasant peripheral nerve stimulation. Implanted prostheses \200 The precautions required depend on the material composition of the device and implant, and where doubt exists, the manufacturer should be consulted. Where no data exist to provide guidance, the risks and bene \201 ts of the scan must be careful ly weighed. For weakly ferromagnetic devices, a major concern is device/implant dislodgemen t. Particularly if the scan is not clinically urgent, it is generally recommended that scan s are deferred for 6 weeks after implantation to allow the healing process to take place an d ensure the device is adequately anchored. Coronary and vascular stents are generally only very weakly ferromagnetic. CMR studies can be performed safely in the majority of cases. Similarly, the majority of heart valve prostheses and annuloplasty rings have been labeled as MR safe. Ot hers have been deemed MR conditional , i.e. no hazard identi \201 ed under speci \201 c i maging conditions. The same applies to closure or occlusion devices, IVC \201 lters, or e mbolization coils. Sternal wires and epicardial pacing leads are safe; however, they can generate artifacts that hamper image interpretation. Pacemakers/automated ICDs (AICDs) \200 These pose potentially signi \201 cant hazards an d there have been reported deaths. In addition to device migration, radiofrequency (RF) heating in leads can result in cardiac injury. The magnetohydrodynamic effects of the strong \201 elds on the ECG can lead to inappropriate device behavior, and direct effects on device programming/res ponse can occur. New MR-safe pacemakers are now available, but currently CMR SAFETY 67 AICDs remain an absolute contraindication to CMR. Retained, particularly fractured device l eads pose a signi \201 cant theoretical safety risk and are regarded as an absolute contra indication to CMR. The need for a device should be carefully anticipated and, if indicated, CMR undertaken bef ore implantation if possible, as up to 75 % of such patients have or will develop an indica tion for CMR in their lifetime. Implantable loop recorders have been safely scanned and can be deemed MR conditional. They generate considerable artifact and, in theory, their performanc e can be in \202 uenced by strong magnetic \201 elds. They should therefore be interroga ted before a scan is undertaken and again afterwards to clear the device buffers of any ina ppropriate artefactual data that may have accumulated during the scan. Gadolinium contrast media \200 The two principal risks of gadolinium contrast media are a naphylaxis to the gadolinium chelate or one of its excipients and nephrogenic systemic \201 brosis (NSF). MR contrast media are completely distinct from the iodinated contrast agents used in X-ray- based imaging, so allergy to one does not imply allergy to the other unless it is to a comm on excipient. \200 MR contrast media do not induce renal failure or contrast nephropathy; however, in patients with signi \201 cant renal impairment (estimated glomeru lar \201 ltration rate (eGFR) <30 mL/min/1.73 m 2 ), there is a risk of nephrogenic syste mic \201 brosis \200\224 a condition characterized by the development of dermal and mult i-organ \201 brosis, typically within days to weeks of contrast administration. 5 % of p atients experience a fulminant malignant course. It has only ever been reported in the context of renal impairment and is thought to be due to the release of free toxic Gd 3 + ions from their chelate. In th is context, cyclic gadolinium chelates appear to be signi \201 cantly safer than older lin ear agents. It is estimated to occur in 2.4 % of patients per study in the context of end-stage renal f ailure. In patients with an eGFR less than 30 L/min/ 1.73 m 2 , or in those who are on dial ysis, the risks and bene \201 ts of contrast administration should therefore be carefully weighed by both patient and physician. If contrast is to be administered, the lowest possib le effective dose should be given, and a cyclic gadolinium chelate should be employed. Pati ents who require renal replacement therapy should undergo haemodialysis immediately after t he scan. Caution should also be exercised in patients with any degree of renal impairment who are awaiting or who have recently had liver transplants, because of th e risk of hepatorenal syndrome. Unstable patients \200 Patients who are haemodynamically or otherwise unstable should not be taken into the scanner. There is limited scope for monitoring, and resuscitation equ ipment is not MR safe. Any patient requiring cardiopulmonary resuscitation (CPR) has to be transferred out of the scanner \201 rst. Together with limited access to the patient, the se constraints make CMR an impractical modality for the unstable patient. 68 CHAPTER 1 Cardiac investigations Cardiomyopathy assessment CMR has the unique ability in a single study to non-invasively, a ccurately, and reproducibly assess LV mass and wall thickness, LV and RV volumes, and myoca rdial perfusion, as well as the ability to detect myocardial \201 brosis and in \202 amm ation. It is therefore the modality of choice for the assessment and follow-up of patients with heart muscle disease.