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Computed-Tomography a Powerful Tool for Diagnosis of Pediatric and Adult Congenital Heart Disease
Methodology and Interpretation Guide
By Jami G. Shakibi, Mahmood Tehrai
AuthorHouseCopyright © 2015 Jami G. Shakibi, M.D., FACC
All rights reserved.
BASIC TECHNICAL INFORMATION FOR CARDIOLOGIST USING CT ANGIOGRAPHY.
For technical, and theoretical information regarding computed tomography, and its application to the cardiovascular system ie CT angiocardiography major texts and references should be consulted (See Chapter 5: References). In this work we shall concerntrate on the method to diagnose congenital cardiovascular defects. However certain technical points are reviewed briefly.
Certain practical technical points are mentioned below:
1-The CT angios shown in this work were obtained by multidetector series computed tomography (MDCT). Data acquisition is spiral (helical) mode, 64-detector system are obtained per rotation. The manufacturerer is Siemens.
2-The contrast medium used for cardiovascular CT angio, is Iodixanol, trade name Visipaque. The dose used for infants and children is 1.5-2ml/kg given IV push.
3-If coronaries, root of the aorta, ventricular, atrial or cardiac valves have to be studied EKG-gated CT angios are obtained. However static organs like the aortic arch, pulmonary veins, pulmonary artery system etc, could be satisfactorily studied by non-EKG-gated image acquisition.
4-For infants an anesthesiologist versed in pediatric care is in attendance and Nesdonal or Ketalar is used for sedation. No medications are used to reduce the heart rate.
5- For studying fast-moving structures, ie coronaries, aortic root and ventricles EKG synchronization is necesssary. Thus for EKG-gated CT angiography, in adolescents and adults metoprolol is given 50-100 mg orally prior to the study. When the heart rate drops to the level of 60-65 beats/min data acquisition could be started.
6-For coronary artery disease, sublingual administration of nitroglycerin is recommended to improve visualization of the coronary arteries.
7-Arrhythmia artifacts, due to atrial fibrillation, premature ventricular or atrial contractions could be eliminated by synchronization with peak of R-waves, instead of RR-interval.
8- Some general remarks regarding radiation dose.
a-The radiation energy absorbed may be of various types: X rays, electron ie beta rays, neutrons, gamma rays, or other particles.
b-The radiation energy absorbed by the patient's tissue or organ is important, not the radiation generated by the equipment.
c-Therefore the energy absorbed must be divided by the mass of the matter. The energy is expressed in joules and mass in kilogram. In the international System of quantities and units, one joule /1kg is the special unit called "gray" (Gy) for absorbed X-ray.
Older units for absorbed radiation energy are:
1 Gy = 100 rad; 1mGy = 0.1 rad.
d-Radiation weighting factor or constant. This unit is a dimensionless unit used for reporting the magnitude of the biological effects of different types of radiation. The type of radiation used, affects the value of the radiation weighting factor or constant. Radiation weighting factor or constant shows the absorbed dose in Gy averaged over an equivalent dose (given in sievert ie Sv).
Thus this relation is expressed as:
equivalent dose (Sv) = absorbed dose in tissue (Gy)x radiation weighting factor.
The older equivalent dose (Sv) was given in rem, thus : 1 Sv = 100 rem; 1 mSv=0.1 rem. As X-ray is used, in CT angio, the radiation weighting factor is equal to 1.0. Therefore in CT angiography, the absorbed dose in a tissue, in Gy, is equal to the equivalent dose Sv.
For Computed Tomography, other special dose quantities are used. These are:
1-Computed Tomography Dose Index (CTDI)
2-weighted CTDI (CTDIW)
3-volume CTDI (CTDIVOL)
4-multiple scan average dose (MSAD)
The radiation dose received by the patient during cardiac, eg coronary artery study varies greatly depending on the type of equipment, EKG-gated and nongated studies. The radiation dose in one double-blind, multicenter report ranged between 568-1259 mGy x cm. This median dose-length product (DLP) was 885 mGy x cm. The dose received increases depending on the patient's weight, rhythm abnormalities and whether the study is EKG-gated or nongated.
See references under "Radiation dose".CHAPTER 2
APROACH TO CONGENITAL HEART DEFECTS BY CT ANGIOGRAPHY
Using CT angio for proper decision-making and to guide the surgeon for corrective or palliative procedures while dealing with congenital heart defects, is quite safe, easy and comprehensive. However to achieve this goal certain preconditions must be osbserved.
The cardiologist in charge must know the patient in depth and must have history, and physical examination findings in detail. EKG, chest-X-ray and echocardiographic data must be studied and recorded in detail. These informations must be used while discussing the case with the radiologist, and answers to major questions as delineated below should be sought with utmost attention.
When reading CT-angios the cardiologist and the radiologist must sit together, and while knowing the pathology, the radiologist finds the pathology and both physicians study the case in depth. Once through with the joint conference a comprehensive report is made which although including the CT-angio findings, but also giving an overall picture of the patient and the pathology concerned. In this approach we report about the patient and the entire pathology concerned, not just a radiographic description of the images.
In almost all cases, the data are quite adequate for surgical management. Of course when catherization is needed for essential problems, such as pressure gradients, ie direct pressure measurement, cardiac catheterization will be performed.
In this monograph, the approach to diagnosis of congenital heart disease is discussed in three sections:
1-Segmental approach to the definition of congenital heart defect.
2-Important negative findings.
3-Special pathology under examination, and final diagnosis.
Following these 3 steps, case studies of a wide variety of complex congenital heart defects regardless of age or parhology are presented.
STEP I: SEGMENTAL APROACH TO DIAGNOSIS OF CONGENITAL HEART DEFECTS.
To diagnose congenital heart defects, the authors use the segmental approach proposed by van Praagh(1).
In this approach one has to define situs first. Following that the position of three segments of the heart (atria, ventricles, and the great arteries) must be defined accurately.
Thus instead of using many confusing terms, the segmental approach allows a specific diagnosis of the cardiac segments, thus allowing a dynamic approach to diagnosis and surgical procedure.
See References under "Cardiac Malpositions".
1-DEFINITION OF THE SITUS AND VENO-ATRIAL LOCATION:
Situs of the heart could be determined by EKG, plain chest X-Ray, echocardiography and CT angio.
1-In situs solitus the liver is on the right side, the stomach bubble is on the left, and the heart is in the left hemithorax. Inferior vena cava is right-sided and enters right atrium on the right side.
2-In situs inversus the liver is on the left side, the stomach bubble is on the right, and the heart is in the right hemithorax. Inferior vena cava is left-sided and enters right atrium on the left side.
3-In situs ambiguus the position of these structures ie liver, stomach bubble and heart is indeterminate. Thus the heart or liver may be on the midline. Inferior vena cava is on the midline and enters right atrium which is on the midline.
Situs is mentioned first, ie:S= solitus; I=Inversus or A=Ambiguus.
What is most important, especially for surgical procedure, is the determination of the venoatrial situs. By this we have to determine where is inferior vena cava located. Once inferior vena cava situs is determined the atrium which is connected to it, is the right atrium. This is important for the surgeon. CT angio is a superb tool to show the inferior vena cava (which may be interrupted or absent) or the suprahepatic vein and the atrium into which it drains. Thus by looking for venoatrial situs (inferior vena cava, right atrium) one can locate the situs as solitus, ie on the right, or inversus, ie inferior vena cava and right atrium on the left side of the spine or ambiguus, in which the inferior vena cava and right atrium are on the midline.
CT angio frames showing venoatrial situs:
2-DEFINING THE POSITION OF THE VENTRICLES AND ATRIOVENTRICULAR CONNECTION:
The physician who cooperates with the radiologist must use all the means, ie EKG, chest X-Ray and echocardiogram to define which ventricle is located on which side. Having this preliminary information one can use CT angio to refine the diagnosis. The important problem to solve is to define which ventricle (ie right or left ventricle) connects with which atrium (ie right or left atrium).Thus in normal state right atrium is connected to the right ventricle and left atrium is connected to the left ventricle.
Here it is important to use every possible method to identify the ventricles as right or left.
Specific criteria to diagnose the right ventricle:
1-Right ventricle is triangular in shape, in frontal view, with its tail accommodating the left ventricular body.
2-Right ventricle has a tricuspid valve.
3- Right ventricle has a conus which makes the outflow tract.
4- Right ventricle has a papillary muscle of the conus.
5-Inside the right ventricular cavity is heavily trabeculated.
6-Right ventricle has three papillary muscles.
7-Insertion of the medial cusp of the tricuspid valve on the ventricular septum is more caudad as compared to the medial cusp of the mitral valve.
Specific criteria to diagnose the left ventricle:
1-Left ventricle is conical in shape and rounded in cross section.
2- Left ventricle has a bicuspid atrioventricular valve.
3- Left ventricle has no conus and therefore no separate outflow tract.
4-Inside left ventricle is smooth.
5-Left ventricle has two papillary muscles.
6- Insertion of the medial cusp of the mitral valve on the ventricular septum is more cephalad as compared to the medial cusp of the tricuspid valve valve.
Not all of the criteria mentioned above, can be documented by any single method, except at autopsy or to some extent at the time of open heart surgery. One should use all possible means, to determine the location of the right ventricle and left ventricle in relation to each other (ie right or left side), and the atria. The physician who sits in conference with the radiologist to read CT angios must have done his homework to the extreme detail. In this regard, in a virgin heart one can use EKG for laterality of the ventricles. The rSr' in right the right precordial leads means that right ventricle (RV) is on the right side. In angiography during the catheterization one uses the shape of the ventricles (triangular for right ventricle in anteroposterior position and rounded for the left ventricle in anteroposterior position), the 2 papillary muscles in the left ventricle, smooth inside surface of the left ventricle and trabeculated inside for the right ventricle, could be easily seen and diagnosed.
Insertion of the medial cusps of the mitral and tricuspid valves on the ventricular septum could easily be examined on four-chamber view by echocardiography. However, one could see this also on special sections in CT angio. The ventricle under the valve leaflet more cephalad, is the left ventricle.
However in CT angio studies, using only selected frames, one has to be extremely careful, using these criteria for determining the laterality of the ventricles.
Although isolated inversion of the conus is described [See the second case described by JGS in epilogue], one can use conus for determining which chamber is the right ventricle.
For all practical purposes using CT angio only for determining the laterality of the ventricles, conus is the most reliable and easiest to see. So using CT angio, if the conus is seen the ventricle under this structure is the right ventricle and the ventricle which has no conus, is the left ventricle.
Using the papillary muscles, smooth or trabeculated endocardial surface, the general shape of the ventricles could all be misleading, unless supported by findings on EKG, echocardiography, cardiac catheterization, etc.
Once right and left ventricles are known one can describe the atrioventricular connection. Thus right atrium may be connected to the right ventricle or left ventricle and vice versa for the left atrium and the right and left ventricle.
By this stage one can say one is dealing with
SD; SL; ID, IL: or AD, AL;
SD means situs solitus, d-loop ventricles ie right ventricle is on the right and left ventricle is on the left.
SL means situs solitus right ventricle is on the left, left ventricle is on the right side.
AD means situs ambiguus, right ventricle is on the right and left ventricle is on the left, etc.
CT angio frames showing right and left ventricles:
As noted before left ventricle has no subarterial conus, whereas right ventricle is anatomically distinguished from left ventricle by having a subpulmonary conus.
Right ventricle is crescent shaped in axial section, triangular on longitudinal section or 3-dimensional view; and inside it is heavily trabeculated.
On cross section left ventricle is circular, whereas right ventricle is crescent shaped, covering left ventricle.
This frame shows aorta originating from a heavily trabeculated ventricle, ie right ventricle.
For ventricles, if in situs solitus, right ventricle is on the right and left ventricle on the left, we have D-loop of the ventricles. So designation is given as:
SD; situs solitus D-loop ventricles
SL: situs solitus; L-loop ventrticles
ID: situs inversus; D-loop ventricles
IL: situs inversus; L-loop ventricles.
AD: situs ambiguus; D-loop ventricles
AL: situs ambiguus; L-loop ventricles
3-DEFINING THE POSITION OF THE GREAT ARTERIES AND VENTRICULO-ARTERIAL CONNECTION:
At this step, the position of the great arteries must be defined. Thus one must give exact description of the aortic root and pulmonary valve location.
For each pulmonic valve and aortic valve one must define the position as right or left, anterior or posterior, superior or inferior. Thus in normal individuals:
The pulmonic valve is anterior superior to the left.
The aortic valve is posterior inferior to the right.
This is normal d-loop position.
If the aortic valve is anterior, superior to the left and
pulmonic valve is posterior, inferior to the right
we are dealing with l-loop great vessels ie corrected transposition.
CT angio frames showing the position of the great arteries:
Aortic and pulmonic valve position: d-loop, situs solitus. Normal.
In the following case of a 3- month- old baby boy with d-TGV (d-transposition of the great vessels), aorta is anterior, superior to the right and pulmonic valve is posterior, inferior to the left, thus we are dealing with d-TGV. This frame showing the position of the aortic and pulmonic valves in full lateral position.
Note d-TGV in a 2 month old baby boy with situs inversus and TGV. This patient has situs inversus totalis, with d-TGV. Pulmonary artery to the right and aorta to the left ie L-loop great vessels, so d-TGV for situs inversus.
This frame belongs to a 17-year-old -female, with situs solitus, dextrocardia. This frame shows double outlet right ventricle.
The following CT-angio belongs to a 41-year-old man who presented with heart failure, obesity (101 Kg), who claimed to be unaware of his heart disease until 2 months prior to the outpatient department visit.
In normal individuals the great vessels are in d-loop position.
So to define the situs, ventricles and the great arteries, 3 letters are used:
eg: SDD, means situs solitus, D-loop ventricles, and 2nd D = d-loop great arteries ie aorta to the right (posterior and inferior), or if the 3rd letter is L, then it means, aorta to the left of the MPA (main pulmonary artery).
To practice, define these three cases:
SDD; ILL; ADD;
Excerpted from Computed-Tomography a Powerful Tool for Diagnosis of Pediatric and Adult Congenital Heart Disease by Jami G. Shakibi, Mahmood Tehrai. Copyright © 2015 Jami G. Shakibi, M.D., FACC. Excerpted by permission of AuthorHouse.
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Table of Contents
CHAPTER 1 BASIC TECHNICAL INFORMATION FOR CARDIOLOGIST USING CT ANGIOGRAPHY, 1,
CHAPTER 2 APROACH TO CONGENITAL HEART DEFECTS BY CT ANGIOGRAPHY, 3,
STEP I: SEGMENTAL APROACH TO DIAGNOSIS OF CONGENITAL HEART DEFECTS, 4,
CHAPTER 3 STEP II: IMPORTANT NEGATIVE FINDINGS, 21,
CHAPTER 4 STEP III: SPECIFIC ANOMALIES. CASE STUDIES, 37,
CHAPTER 5 REFERENCES, 168,
ABOUT THE AUTHORS, 191,