123I-MIBG Imaging: Patient Preparation & Technologist's Role (PDF)

Downloaded from tech.snmjournals.org by Miami Dade College on June 12, 2018. For personal use only. CONTINUING EDUCATION 123I-MIBG Imaging: Patient Preparation and Technologist’s Role S. Seth Van Vickle, CN...

Downloaded from tech.snmjournals.org by Miami Dade College on June 12, 2018. For personal use only. CONTINUING EDUCATION 123I-MIBG Imaging: Patient Preparation and Technologist’s Role S. Seth Van Vickle, CNMT, NCT, and Randall C. Thompson, MD, FASNC Saint Luke’s Mid America Heart Institute and Saint Luke’s Cardiovascular Consultants, Kansas City, Missouri CE credit: For CE credit, you can access the test for this article, as well as additional JNMT CE tests, online at https://www.snmmilearningcenter.org. Complete the test online no later than June 2018. Your online test will be scored immediately. You may make 3 attempts to pass the test and must answer 80% of the questions correctly to receive 1.0 CEH (Continuing Education Hour) credit. SNMMI members will have their CEH credit added to their VOICE transcript automatically; nonmembers will be able to print out a CE certificate upon successfully completing the test. The online test is free to SNMMI members; nonmembers must pay $15.00 by credit card when logging onto the website to take the test. obtained during a hospitalization at another facility showed The radiopharmaceutical 123I-metaiodobenzylguanidine (MIBG) an ejection fraction of 33%. The patient is advised to con- was approved by the Food and Drug Administration in March sider an implantable cardioverter defibrillator but is reluc- 2013 for the assessment of myocardial sympathetic innervation tant because a relative has an implantable cardioverter in the evaluation of patients with heart failure and an ejection defibrillator that frequently fires inappropriately. fraction of no more than 35%. Almost any well-equipped nu- clear medicine or nuclear cardiology laboratory can perform this It is certainly clear that implantable cardioverter defib- test, although there is a need for special attention to patient rillators save lives; their prophylactic implantation in patients preparation, dose calibration, and proper timing of the image who have known heart failure with low ejection fractions is acquisition. This article reviews the role of the nuclear medicine considered standard therapy (2). However, these devices are technologist and some practical aspects of cardiac sympa- sometimes associated with complications and are expensive, thetic 123I-MIBG imaging of which the laboratory team needs and many are implanted for each life saved (3,4). to be mindful. For patients such as this one, who have borderline indi- Key Words: 123I-MIBG; cardiac sympathetic imaging; cardiac cations for implantable cardioverter defibrillators and are denervation; techniques for MIBG imaging reluctant to proceed, physicians have a few options: accept J Nucl Med Technol 2015; 43:82–86 that the patient does not want the device; try to convince the DOI: 10.2967/jnmt.115.158394 patient to have the device implanted; define the ejection fraction more exactly via another diagnostic medical imaging modality; or perform cardiac sympathetic imaging with 123I- MIBG for risk stratification. The heart-to-mediastinum ratio T he radiopharmaceutical 123I-metaiodobenzylguanidine (MIBG) (AdreView; GE Healthcare) was approved by the provided by 123I-MIBG imaging is a powerful prognostic in- dicator in these patients, and such information may be useful Food and Drug Administration in March 2013 for the as- in shared decision making by physicians and patients (5–8). sessment of myocardial sympathetic innervation in patients EQUIPMENT PREPARATION with New York Heart Association class 2 or 3 heart failure and an ejection fraction of less than 35% (1). The role of To properly acquire 123I-MIBG images, the camera must this test in clinical practice is currently evolving. This article allow for frontal planar imaging. Anterior frontal plane outlines the practicalities of the use of 123I-MIBG. images are a requirement, whereas SPECT images are op- The use of 123I-MIBG can be best demonstrated by a case tional but are usually obtained. The nuclear cardiology sys- presentation. A 65-y-old man presents with mild limitations tem should have low-energy, high-resolution collimation, due to shortness of breath on exertion. He had a heart attack and the camera should be able to obtain specific sequences 2 y previously and has been somewhat limited ever since. A from the imaging protocol, for example, static and SPECT recent myocardial perfusion imaging test showed left ven- imaging. Setting up and testing the imaging sequence be- tricular scarring but no ischemia and a left ventricular ejec- fore the day of dosing is often helpful and recommended. tion fraction of 36%. However, a recent echocardiogram The dose calibrator should also be inspected before the study is ordered. The dose calibrator should have a 123I-specific Received Mar. 26, 2015; revision accepted Apr. 16, 2015. setting, and a correction factor specific for the laboratory’s For correspondence or reprints contact: S. Seth Van Vickle, Saint Luke’s Cardiovascular Consultants, 4330 Wornall Rd., Kansas City, MO 64111. instrument must be obtained. It is strongly recommended that E-mail: [email protected] a commercial copper sleeve or copper tube be used during Published online May 8, 2015. COPYRIGHT © 2015 by the Society of Nuclear Medicine and Molecular dose calibration to lessen scatter counts from low-energy pho- Imaging, Inc. tons, which can give falsely low readings for 123I-MIBG (9). 82 JOURNAL OF NUCLEAR MEDICINE TECHNOLOGY Vol. 43 No. 2 June 2015  Downloaded from tech.snmjournals.org by Miami Dade College on June 12, 2018. For personal use only. kept close to the top central field of view and the base of the myocardium kept in the bottom of the central field of view. A 10-min anterior acquisition in a 128 · 128 · 16 matrix allows for adequate counting statistics. Photon acquisition should be centered on a 159-keV peak with a 20% window. PATIENT PREPARATION When a clinical laboratory is preparing a patient for cardiac 123I-MIBG imaging, most of the steps typical of nuclear cardiology imaging are followed. The imaging team should be mindful of the patient’s past medical history, in- cluding allergies, medications, and the general health history. There are also some medical issues particular to 123I-MIBG imaging. 123I-MIBG is contraindicated in patients with known hypersensitivity to iobenguane or iobenguane sulfate; how- ever, it is also customary to screen potential patients for aller- FIGURE 1. Anterior view of 123I-MIBG 3 h 50 min after injection. gies to iodine-containing materials by asking if they have had any prior reactions to iodinated contrast material, shellfish, Two static anterior planar acquisitions are obtained when strawberries, or cranberries. imaging with 123I-MIBG (at 15 min and 3 h 50 min after Particular attention must be paid to medication history. injection). With a dual-head camera, the detectors should be Table 1 provides a list of medications that affect 123I-MIBG positioned in the anterior and left lateral positions. The imaging. This list includes medicines that the patient may anterior field of view (FOV) will be focused on acquiring not record unless specifically queried, such as over-the-counter the myocardium and the mediastinal region for later pro- cold medicines and cocaine. Certain common cardiac medica- cessing (Fig. 1). The organs of interest should be positioned tions, including labetalol and calcium channel antagonists, also in the center of the field of view, with the apex of both lungs need to be stopped temporarily before 123I-MIBG imaging. TABLE 1 Medications That Affect 123I-MIBG Imaging (13) Mechanism of interference Discontinuation before Drug or class Examples (known or expected) 123I-MIBG scan (d) Opioid Uptake inhibition 7–14 Cocaine Uptake inhibition 7–14 Tramadol Uptake inhibition 7–14 Tricyclic Amitriptyline and derivatives, imipramine Uptake inhibition 7–21 antidepressants and derivatives, amoxapine, doxepin, others Sympathicomimetics* Phenylpropanolamine, ephedrine, Depletion of granules 7–14 pseudoephedrine, phenylephrine, amphetamine, dopamine, isoproterenol, salbutamol, terbutaline, fenoterol, xylometazoline Antihypertensive Labetalol Inhibition of uptake 21 or cardiovascular agents and depletion Reserpine Depletion and 14 transport inhibition Bretylium, guanethidine Depletion and 14 transport inhibition Calcium channel blockers (nifedipine, Increased uptake and 14 nicardipine, amlodipine) retention Antipsychotics Phenothiazines† (chlorpromazine, Uptake inhibition 21–28 promethazine, fluphenazine, others) Thioxanthenes (maprotiline, trazodone) Uptake inhibition 21–28 Butyrophenones (droperidol, haloperidol) Uptake inhibition 21–28 Loxapine Uptake inhibition 7–21 *Components of bronchodilators, decongestants, and diet aids. † Frequent components of antiemetic and antiallergic agents. 123I-MIBG IMAGING CONSIDERATIONS Van Vickle and Thompson 83  Downloaded from tech.snmjournals.org by Miami Dade College on June 12, 2018. For personal use only. no-shows, but the laboratory must have a written policy in place and it is good practice to have patients acknowledge the policy in writing. RADIOPHARMACEUTICAL PREPARATION The dose of 123I-MIBG is calibrated to be 370 MBq (10 mCi) at 12:00 PM on the day of administration. The dose arrives in a single-use vial containing 5 mL of 123I-MIBG (74 MBq/mL, or 2 mCi/mL). The radiophar- maceutical expires 36 h after the calibration time and should be stored at between 68!F and 77!F, following hospital safety protocols. Using aseptic technique and proper radiation safety, the technologist should draw 370 MBq (10 mCi) into a shielded syringe (Fig. 2). The syringe should be placed 123I-MIBG into the dose calibrator to verify the proper number of FIGURE 2. Proper technique for drawing a dose. megabecquerels (or millicuries) before injection. The dose can then be administered to the patient via a large periph- eral vein over 1–2 min. The intravenous line should be Two particularly relevant medical conditions include flushed with saline before isotope injection to verify Parkinson disease and kidney disease. Patients with Parkinson proper venous access and again after injection to remove disease have impaired cardiac uptake of 123I-MIBG (10). Re- any accumulated residual activity. During the timed ad- nal insufficiency is considered a relative contraindication for ministration, the patient should be continually monitored 123I-MIBG testing since the radiopharmaceutical is cleared for any hypersensitivity or other reactions to the medica- by the kidneys, although a recent report called into question tion, such as itching, burning, or tingling sensations at the this conventional position (11). injection site. A postinjection assay is performed on the Patients who have an increased risk for thyroid cancer syringe to determine residual radioactivity and the total should be considered for potassium iodide pretreatment in activity administered to the patient. order to block thyroid uptake and to lower the radiation dose THE 123I-MIBG ACQUISITION to the thyroid (1,12). Patients at increased risk would pre- sumably include young patients and those with a personal Because of the short injection-to-scan time, the technol- or family history of thyroid cancer. Potassium iodine was ogist should ensure that the camera is unoccupied before not routinely administered before 123I-MIBG in the pivotal administering the pharmaceutical. Preparing the acquisition ADMIRE-HF trial (5). setup before injection is also helpful. Patients should be informed of key elements of the 123I- Exact timing of the acquisition is important; a stop- MIBG test beforehand to help alleviate their uncertainty watch should be used. An early 10-min anterior planar and apprehension. Nuclear medicine tests can sound fright- image is obtained starting at 15 min after injection, and ening to some people, so it is important to outline the pro- a late 10-min anterior planar image is obtained starting cedure, time requirements, and reasons for the test. Patients at 3 h 50 min after injection. These combined images are generally instructed to be well hydrated before the test, but care should be taken with heart failure patients who are on fluid restriction. Radiation dosimetry should also be mentioned to the patient. The effective dose for an adult is 5.07 mSv for the injection of 370 MBq (10 mCi) of 123I (13.7 mSv/MBq · 370 MBq, or 0.507 mSv/mCi · 10 mCi) (1). There is also a small amount of radiation exposure from attenuation cor- rection. The total exposure is relatively low compared with other procedures in nuclear cardiology and is comparable to about 15 mo of average background radiation. The cost and logistics of this radiopharmaceutical cause the no-show policy to be more important than usual. 123I-MIBG is a particularly expensive radiopharmaceutical, and the dose travels a long distance before administration. Health insurance companies, including the Centers for Medicare and Medicaid FIGURE 3. Association of cardiac death with heart-to- Services, allow nuclear cardiology facilities to bill patients for mediastinum (H/M) ratio. 84 JOURNAL OF NUCLEAR MEDICINE TECHNOLOGY Vol. 43 No. 2 June 2015  Downloaded from tech.snmjournals.org by Miami Dade College on June 12, 2018. For personal use only. are recorded. For the mediastinal region of interest, a hori- zontal line is drawn at the apices of the lungs. A vertical line is then drawn about equidistant between the right and left lungs. At the intersection of the horizontal and vertical lines, counts 15 pixels and below are marked. Starting with the fourth pixel below the intersection, the area with the lowest counts is identified and, if there are multiple areas, the most superior selected. At the lowest pixel, a 7 · 7 pixel region of interest is drawn, and the number of counts in this 49-pixel region of interest is recorded. The heart-to- mediastinum ratio is equal to the counts per pixel in the cardiac region of interest divided by the counts per pixel in the mediastinal region of interest (Fig. 4). THE SPECT ACQUISITION The 4-h SPECT image dataset allows the clinician to visualize myocardial uptake and retention of 123I-MIBG in the classic short-, vertical-, and horizontal long-axis views. Our laboratory uses a circular orbit with 180! of acquisi- FIGURE 4. Heart-to-mediastinum (H/M) ratio obtained with tion, and we acquire 64 projections using 32 stops of approx- 123I-MIBG. imately 25 s/stop. An attenuation correction map is acquired simultaneously. The study is acquired in a 64 · 64 · 16 allow for calculation of the washout ratio of the isotope matrix and is nongated. (1). Between these acquisitions, patients are allowed to eat IMAGE PROCESSING and are encouraged to stay well hydrated but not exces- Sample anterior planar images are shown in Figure 5. We sively so if they have heart failure. Patients should also be customarily display the planar images in a 2-view panel advised that they are radioactive and to avoid small children (one image displaying the raw image and the other display- and going through some airport security systems. SPECT ing the heart-to-mediastinum ratio along with the regions of images are acquired after the late planar acquisition. interest). The primary prognostic parameter supplied by 123I- The SPECT reconstruction parameters may vary depend- MIBG imaging is the heart-to-mediastinum ratio on the ing on the site-specific routine, and the technologist should delayed images, which has been shown to predict the 2-y consistently use the reconstruction parameters best suiting cardiac mortality rate (Fig. 3) (5). The regions of interest the laboratory. The 123I-MIBG images are reconstructed in must be drawn around the heart and in the mediastinum in a manner similar to that of myocardial perfusion SPECT the prescribed fashion according to the product package images. Either standard filtered backprojection or iterative insert. In brief, an irregular region of interest is drawn reconstruction can be used. Typically, a low-pass filter is around the whole heart, defining the epicardial border. used, with a cutoff of 0.50 and an order of 5.00. The images The number of counts and pixels in the region of interest are then displayed in a fashion similar to that of SPECT FIGURE 5. Classic SPECT myocardial display of 123 I-MIBG images after reconstruction. 123I-MIBG IMAGING CONSIDERATIONS Van Vickle and Thompson 85  Downloaded from tech.snmjournals.org by Miami Dade College on June 12, 2018. For personal use only. myocardial perfusion images. Creating reconstruction 3. Vollmann D, Luthje L, Vonhof S, et al. Inappropriate therapy and fatal pro- arrhythmia by an implantable cardioverter-defibrillator. Heart Rhythm. 2005; defaults can help to standardize methods. 2:307–309. In 2010 Flotats et al. proposed standardization of 123I- 4. Uslan DZ, Sohail MR, St Sauver JL, et al. Permanent pacemaker and implantable MIBG cardiac sympathetic imaging on behalf of the Euro- cardioverter defibrillator infection: a population-based study. Arch Intern Med. 2007;167:669–675. pean Association of Nuclear Medicine Cardiovascular 5. Jacobson AF, Senior R, Cerqueira MD, et al. Myocardial iodine-123 meta-iodo- Committee and the European Council of Nuclear Cardiol- benzylguanidine imaging and cardiac events in heart failure: results of the pro- ogy (13). Their publication provides more details about the spective ADMIRE-HF (AdreView Myocardial Imaging for Risk Evaluation in Heart Failure) study. J Am Coll Cardiol. 2010;55:2212–2221. acquisition and reporting of 123I-MIBG imaging. 6. Tamaki S, Yamada T, Okuyama Y, et al. Cardiac iodine-123 metaiodobenzyl- guanidine imaging predicts sudden cardiac death independently of left ventric- ular ejection fraction in patients with chronic heart failure and left ventricular CONCLUSION systolic dysfunction: results from a comparative study with signal-averaged Cardiac sympathetic 123I-MIBG imaging can be performed electrocardiogram, heart rate variability, and QT dispersion. J Am Coll Cardiol. 2009;53:426–435. by almost all well-equipped nuclear medicine and nuclear car- 7. Agostini D, Verberne HJ, Burchert W, et al. I-123-mIBG myocardial imaging for diology laboratories. The procedure requires special attention assessment of risk for a major cardiac event in heart failure patients: insights to patient preparation, dose calibration, and timing of image from a retrospective European multicenter study. Eur J Nucl Med Mol Imaging. 2008;35:535–546. acquisition. As opportunities to use this technique grow, its 8. Nakata T, Nakajima K, Yamashina S, et al. A pooled analysis of multicenter exact clinical role should become clear. The test is expected cohort studies of 123I-mIBG imaging of sympathetic innervation for assess- to provide clinicians with useful prognostic information about ment of long-term prognosis in heart failure. JACC Cardiovasc Imaging. 2013;6:772–784. cardiovascular risk in patients with heart failure. 9. Kowalsky RJ, Johnston RE. Dose calibrator assay of iodine-123 and indium-111 with a copper filter. J Nucl Med Technol. 1998;26:94–98. 10. Braune S, Reibhardt M, Schnitzer R, et al. Cardiac uptake of (123I) MIBG DISCLOSURE separates Parkinson’s disease from multiple system atrophy. Neurology. 1999;53: No potential conflict of interest relevant to this article 1020–1025. 11. Malhotra S, Wang L, Bunker CH, et al. Renal dysfunction does not affect the was reported. prognostic value of myocardial iodine-123 meta-iodobenzylguanidine imaging in heart failure. Nucl Med Commun. 2014;35:58–63. 12. Friedman NC, Hassan A, Grady E, et al. Efficacy of thyroid blockade on REFERENCES thyroid radioiodine uptake in 123I-mIBG imaging. J Nucl Med. 2014;55: 1. AdreView" iobenguane I 123 injection. GE Healthcare website. http://www3. 211–215. gehealthcare.com/en/products/categories/nuclear_imaging_agents/adreview. Published 13. Flotats A, Carrió I, Agostini D, et al.; EANM Cardiovascular Committee; February 6, 2015. Accessed April 20, 2015. European Council of Nuclear Cardiology. Proposed standardization of 123I-meta- 2. Passman R, Goldberger JJ. Predicting the future: risk stratification for sudden iodobenzylguanidine (MIBG) cardiac sympathetic imaging by the EANM Car- cardiac death in patients with left ventricular dysfunction. Circulation. 2012;125: diovascular Committee and the European Council of Nuclear Cardiology. Eur J 3031–3037. Nucl Med Mol Imaging. 2010;37:1802–1812. 86 JOURNAL OF NUCLEAR MEDICINE TECHNOLOGY Vol. 43 No. 2 June 2015  Downloaded from tech.snmjournals.org by Miami Dade College on June 12, 2018. For personal use only. 123I-MIBG Imaging: Patient Preparation and Technologist's Role S. Seth Van Vickle and Randall C. Thompson J. Nucl. Med. Technol. 2015;43:82-86. Published online: May 8, 2015. Doi: 10.2967/jnmt.115.158394 This article and updated information are available at: http://tech.snmjournals.org/content/43/2/82 Information about reproducing figures, tables, or other portions of this article can be found online at: http://tech.snmjournals.org/site/misc/permission.xhtml Information about subscriptions to JNMT can be found at: http://tech.snmjournals.org/site/subscriptions/online.xhtml Journal of Nuclear Medicine Technology is published quarterly. SNMMI | Society of Nuclear Medicine and Molecular Imaging 1850 Samuel Morse Drive, Reston, VA 20190. (Print ISSN: 0091-4916, Online ISSN: 1535-5675) © Copyright 2015 SNMMI; all rights reserved.  SNMMI Learning Center * Indicates Required Field / Question Search 1. 123 I­MIBG is indicated for detection of primary or metastatic pheochromocytoma or neuroblastoma, as well as for congestive heart failure patients with the following condition A. Previous heart­to­mediastinum ratio above 1.6. B. New York Heart Association class II or class III heart failure and left ventricular ejection fraction ≤ 35%. gu Physicians | Technologists | Scientists | Media | Healthcare Provider | Patients | Int C. New York Heart Association class II or class III heart failure and left ventricular ejection fraction ≥ 35%. NEWS & D. PUBLICATIONS MEMBERSHIP Patients with no prior EDUCATION history of coronary disease MEETINGS but with recent angina & EVENTS symptoms. ISSUES & ADVOCACY 89 QUALITY & PRACTICE RESEARCH ABOUT SNM HOME > EDUCATION > ACTIVITY Quick Links: Online Education | My Activities | My Transcript 2. Collimation to be used for obtaining a heart­to­mediastinum ratio with 123 I­MIBG imaging (with typical Anger SPECT setups) should be… * A. Pinhole collimation. 8 Preparation and Technologist’s Role JNMT (June 2015) 123 I-MIBG Imaging: Patient B. Low­energy all­purpose collimation. C. Low­energy high­resolution collimation. JNMT (June 2015) 123 I­MIBG Imaging 19 D. Diverging collimation. Activity Overview Activity Material Post­Assessment Evaluation Certificate 3. For cardiac imaging, the recommended adult dose of 123I­MIBG is… * Post­Assessment(s) A. 370 MBq (10 mCi). Credit Inform page B. 185 MBq (5 mCi). SNMMI > All required C. 740items must MBq (20 be completed. To complete an assessment, please select Launch. To view a completed mCi). Availab Claime assessment, please select View. If an assessment is NOT available, your data is currently being processed and you may Provided By: Society of Nuclear Medicine and M return D. at Weight­based. a later time to complete it. Release Date Expiration Date: Certificate Status: In 4. The estimated effective dose received by an adult after the recommended dose of 123I­MIBG is… * Attempts Name Status Required Score Action A. 13.7 microsieverts/MBq (0.507 millisieverts/mCi). Remaining B.(June JNMT 13.7 2015) mrem/MBq (0.507Not Assessment mrem/mCi). Started Yes gi Launch C. 13.7 microsieverts/MBq (0.507microsieverts/mCi). SNMMI > VOICE A D. None of the above. Not Started Yes My 3 5. Contraindications for 123I­MIBG include… * A. None. % B. All patients who are on lipid­lowering medications. C. Patients unable to achieve adequate exercise. da D. Patients with known hypersensitivity to iobenguane or iobenguane sulfate. 1/1 6/12/2018 SNMMI Learning Center SNMMI Learning Center Donate | Join | Store | View Cart | Member Directory | My SNMMI | Welcome, Lorens 6. It is recommended that a copper sleeve be used when checking the 123 I­MIBG in the dose calibrator in order to… * A. Lessen scatter counts from low­energy photons. Search Dal B. Increase scatter counts from low­energy photons. C. Lessen scatter counts from high­energy photons. D. Increase scatter counts from high­energy photons. Physicians | Technologists | Scientists | Media | Healthcare Provider | Patients | Int NEWS7.&Regarding the technique PUBLICATIONS for injecting 123I­MIBG, MEMBERSHIP the dose should be EDUCATION delivered… MEETINGS * & EVENTS ISSUES & ADVOCACY QUALITY & PRACTICE RESEARCH ABOUT SNM A. Tightly, in less than 5 s. B. Over a period HOME > EDUCATION of 1­2 min. > ACTIVITY C. At a rate of 1 mL/s. D. Over a period of 2­3 min. page Quick Links: Online Education | My Activities | My Transcript JNMT (June 2015) 123 I-MIBG Imaging: Patient Preparation and Technologist’s Role 8. InJNMT (June what year 2015) did 123and the Food I­MIBG DrugImaging Administration approve 123I­MIBG imaging for assessment of myocardial sympathetic innervation? * A. 1989. Activity B. Overview 2001. Activity Material Post­Assessment Evaluation Certificate C. 2013. Post­Assessment(s) D. 2014. Firsta Credit Inform SNMMI > All required items must be completed. To complete an assessment, please select Launch. To view a completed Availab Claime assessment, 9. Patientsplease should select View. Iftoan be instructed doassessment is NOTbetween all of the following available, your data 123I­MIBG is currently imaging beingexcept… acquisitions, processed * and you may Provided By: Society of Nuclear Medicine and M return at a later time to complete it. A. Avoid small children. Release Date B. Eat as needed. NameC. Stay well hydrated. Status Required dasScore Action Attempts Expiration Date: Certificate Status: In D. Abstain from food. De Remaining JNMT (June 2015) Assessment Not Started Yes Launch SNMMI > VOICE A Not Started Yes 3 10. The primary prognostic information (in the form of the heart­to­mediastinum ratio) is achieved during which portion of the imaging sequence? * A. 3­h 50­min anterior planar. Ogg B. 15­min anterior planar. C. 4­h SPECT. 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