CARDIAC STRESS PERFUSION MRI
PREPARATION
- ECG, Film/Images with Reports
- Blood for Serum Creatinine (Contrast)
- Old Documents
- Operation Note (Post Oparation)
Indications for MRI cardiac perfusion scan
- For the assessment of blood flow after heart bypass surgery, angioplasty or artery stenting
- For the detection of microvascular dysfunction in various congenital heart diseases
- For the detection of ischemia before medical therapy or revascularization
- For the diagnosis and assessment of coronary artery disease (CAD)
- To identify the location and damage caused by a heart attack
- For the assessment of myocardial infarction complications
- For the assessment of cardiac masses
PLANNING
Picture: Cardiac MRI LVOT
STRESS PERFUSION
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Picture: Cardiac MRI Look Locker (TI Scout) Survey.
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Picture: Cardiac MRI Look Locker (TI Scout) Survey.
PHASE SENSETIVE INVERSION RECOVERY (PSIR)
Picture: PSIR Two Chember View Planning.
Picture: PSIR Four Chamber View Planning.
Picture: PSIR Short Axis Chamber View Planning.
Anatomy The great vessels of the heart
The great vessels of the heart function to carry blood to and from the heart as it pumps, located largely within the middle mediastinum.
In this article we will consider the structure and anatomical relationships of the aorta, pulmonary arteries and veins, and the superior and inferior vena cavae.
Aorta
The aorta is the largest artery in the body. It carries oxygenated blood (pumped by the left side of the heart) to the rest of the body.
The aorta arises from the aortic orifice at the base of the left ventricle, with inflow via the aortic valve. Its first segment is known as the ascending aorta, which lies within the pericardium (covered by the visceral layer). From it branch the coronary arteries. The second continuous segment is the arch of the aorta, from which branch the major arteries to the head, neck and upper limbs. These are:
Brachiocephalic trunk
Left common carotid artery
Left subclavian artery
After the arch of the aorta, the aorta then becomes the descending aorta which continues down through the diaphragm into the abdomen.
Fig 1 – The arch of aorta.
Clinical Relevance - Disorders of the Aorta
Aortic Dissection
Fig 1.1 – Aortic dissection, where blood enters the wall of the aorta.
Aortic dissection refers to a tear in the inner wall of the aorta. The tear creates two channels for blood flow; one is the normal lumen of the aorta, another is into the wall, where the blood becomes stationary.
Blood entering the wall can constrict the aortic lumen, reducing blood flow to the rest of the body. It can also cause further weakness and dilation of the wall, potentially leading to an aortic aneurysm.
Aortic Aneurysm
An aneurysm is a dilation (expansion) of an artery, which is greater than 50% of the normal diameter. An aortic aneurysm is due to an underlying weakness of the walls (such as Marfan’s syndrome), or a pathological process (such as aortic dissection).
The main concern with an aortic aneurysm is rupture of the aorta, which if not treated, will lead to death.
Pulmonary Arteries
The pulmonary arteries receive deoxygenated blood from the right ventricle and deliver it to the lungs for gas exchange to take place.
The arteries begin as the pulmonary trunk, a thick and short vessel, which is separated from the right ventricle by the pulmonary valve. The trunk is located anteriorly and medially to the right atrium, sharing a common layer of pericardium with the ascending aorta. It continues upwards, overlapping the root of the aorta and passing posteriorly.
At around the level of T5-T6, the pulmonary trunk splits into the right and left pulmonary arteries. The left pulmonary artery supplies blood to the left lung, bifurcating into two branches to supply each lobe of the lung. The right pulmonary artery is the thicker and longer artery of the two, supplying blood to the right lung. It also further divides into two branches.
Fig 1.2 – Anterior view of the heart, and its great vessels.
Pulmonary Veins
The pulmonary veins receive oxygenated blood from the lungs, delivering it to the left side of the heart to be pumped back around the body.
There are four pulmonary veins, with one superior and one inferior for each of the lungs. They enter the pericardium to drain into the superior left atrium, on the posterior surface. The oblique pericardial sinus can be found within the pericardium, between the left and right veins.
The superior pulmonary veins return blood from the upper lobes of the lung, with the inferior veins returning blood from the lower lobes. The inferior left pulmonary vein is found at the hilum of the lung, while the right inferior pulmonary vein runs posteriorly to the superior vena cava and the right atrium.
Superior Vena Cava
The superior vena cava receives deoxygenated blood from the upper body (superior to the diaphragm, excluding the lungs and heart), delivering it to the right atrium.
It is formed by merging of the brachiocephalic veins, travelling inferiorly through the thoracic region until draining into the superior portion of the right atrium at the level of the 3rd rib.
As the superior vena cava makes its descent it is located in the right side of the superior mediastinum, before entering the middle mediastinum to lie beside the ascending aorta.
Inferior Vena Cava
The inferior vena cava receives deoxygenated blood from the lower body (all structures inferior to the diaphragm), delivering it back to the heart.
It is initially formed in the pelvis by the common iliac veins joining together. It travels through the abdomen, collecting blood from the hepatic, lumbar, gonadal, renal and phrenic veins. The inferior vena cava then passes through the diaphragm, entering the pericardium at the level of T8. It drains into the inferior portion of the right atrium.
Anatomy The Chambers of the Heart
The heart consists of four chambers: the two atria and the two ventricles.
Blood returning to the heart enters the atria, and is then pumped into the ventricles. From the left ventricle, blood passes into the aorta and enters the systemic circulation. From the right, it enters the pulmonary circulation via the pulmonary arteries.
In this article we shall look at the anatomy of the atria and the ventricles, and we will consider their clinical correlations.
Atria
Right Atrium
The right atrium receives deoxygenated blood from the superior and inferior vena cavae, and from the coronary veins. It pumps this blood through the right atrioventricular orifice (guarded by the tricuspid valve) into the right ventricle.
In the anatomical position, the right atrium forms the right border of the heart. Extending from the antero-medial portion of the chamber is the right auricle (right atrial appendage) – a muscular pouch that acts to increase the capacity of the atrium.
The interior surface of the right atrium can be divided into two parts, each with a distinct embryological origin. These two parts are separated by a muscular ridge called the crista terminalis:
Sinus venarum – located posterior to the crista terminalis. This part receives blood from the superior and inferior vena cavae. It has smooth walls and is derived from the embryonic sinus venosus.
Atrium proper – located anterior to the crista terminalis, and includes the right auricle. It is derived from the primitive atrium, and has rough, muscular walls formed by pectinate muscles.
The coronary sinus receives blood from the coronary veins. It opens into the right atrium between the inferior vena cava orifice and the right atrioventricular orifice.
Interatrial Septum
The interatrial septum is a solid muscular wall that separates the right and left atria.
The septal wall in the right atrium is marked by a small oval-shaped depression called the fossa ovalis. This is the remnant of the foramen ovale in the fetal heart, which allows right to left shunting of blood to bypass the lungs. It closes once the newborn takes its first breath.
Fig 1 – The right atrium and interatrial septum. The atrium proper is only partially visible on this illustration.
Clinical Relevance: Atrial Septal Defect
An atrial septal defect is an abnormal opening in the interatrial septum, persistent after birth. The most common site is the foramen ovale, and this is known as a patent foramen ovale.
In the adult, left atrial pressure is usually greater than that of the right atrium, so blood is shunted through the opening from left to right. In large septal defects, this can cause right ventricular overload, leading to pulmonary hypertension, right ventricular hypertrophy and ultimately right heart failure.
Definitive treatment is closure of the defect by surgical or transcatheter closure.
Left Atrium
The left atrium receives oxygenated blood from the four pulmonary veins, and pumps it through the left atrioventricular orifice (guarded by the mitral valve) into the left ventricle.
In the anatomical position, the left atrium forms the posterior border (base) of the heart. The left auricle extends from the superior aspect of the chamber, overlapping the root of the pulmonary trunk.
The interior surface of the left atrium can be divided into two parts, each with a distinct embryological origin:
Inflow portion – receives blood from the pulmonary veins. Its internal surface is smooth and it is derived from the pulmonary veins themselves.
Outflow portion – located anteriorly, and includes the left auricle. It is lined by pectinate muscles, and is derived from the embryonic atrium.
Ventricles
The left and right ventricles of the heart receive blood from the atria and pump it into the outflow vessels; the aorta and the pulmonary artery respectively.
Right Ventricle
The right ventricle receives deoxygenated blood from the right atrium, and pumps it through the pulmonary orifice (guarded by the pulmonary valve), into the pulmonary artery.
It is triangular in shape, and forms the majority of the anterior border of the heart. The right ventricle can be divided into an inflow and outflow portion, which are separated by a muscular ridge known as the supraventricular crest.
Inflow Portion
The interior of the inflow part of the right ventricle is covered by a series of irregular muscular elevations, called trabeculae carnae. They give the ventricle a ‘sponge-like’ appearance, and can be grouped into three main types:
Ridges – attached along their entire length on one side to form ridges along the interior surface of the ventricle.
Bridges – attached to the ventricle at both ends, but free in the middle. The most important example of this type is the moderator band, which spans between the interventricular septum and the anterior wall of the right ventricle. It has an important conductive function, containing the right bundle branches.
Pillars (papillary muscles) – anchored by their base to the ventricles. Their apices are attached to fibrous cords (chordae tendineae), which are in turn attached to the three tricuspid valve cusps. By contracting, the papillary muscles ‘pull’ on the chordae tendineae to prevent prolapse of the valve leaflets during ventricular systole.
Outflow Portion (Conus arteriosus)
The outflow portion (leading to the pulmonary artery) is located in the superior aspect of the ventricle. It is derived from the embryonic bulbus cordis. It is visibly different from the rest of the right ventricle, with smooth walls and no trabeculae carneae.
Fig 2 – Frontal section of the heart, showing the attachment of the papillary muscles to the tricuspid and mitral valves.
Interventricular Septum
The interventricular septum separates the two ventricles, and is composed of a superior membranous part and an inferior muscular part.
The muscular part forms the majority of the septum and is the same thickness as the left ventricular wall. The membranous part is thinner, and part of the fibrous skeleton of the heart.
Left Ventricle
The left ventricle receives oxygenated blood from the left atrium, and pumps it through the aortic orifice (guarded by the aortic valve) into the aorta.
In the anatomical position, the left ventricle forms the apex of the heart, as well as the left and diaphragmatic borders. Much like the right ventricle, it can be divided into an inflow portion and an outflow portion.
Inflow Portion
The walls of the inflow portion of the left ventricle are lined by trabeculae carneae, as described with the right ventricle. There are two papillary muscles present which attach to the cusps of the mitral valve.
Outflow Portion
The outflow part of the left ventricle is known as the aortic vestibule. It is smooth-walled with no trabeculae carneae, and is a derivative of the embryonic bulbus cordis.
Fig 3 – The papillary muscles and inflow portion of the left ventricle.
Clinical Relevance: Tetralogy of Fallot
Tetralogy of Fallot is a cyanotic congenital heart disease, comprising four abnormalities as a result of a single development defect. The four abnormalities are:
Ventricular septal defect
Overriding aorta (this is where the aorta is positioned directly over the VSD)
Pulmonary valve stenosis
Right ventricular hypertrophy
Stenosis of the pulmonary valve increase the force needed to pump blood through it, resulting in right ventricular hypertrophy. Eventually, the pressure in the right ventricle becomes higher than that of the left – and blood then shunts from right to left through the ventricular septal defect. The overriding aorta lies over the ventricular septal defect, resulting in deoxygenated blood passing into the aorta.
It is usually treated surgically in the first few months of life or in severe cases, soon after birth.
Fig 4 – The four structural defects in Tetralogy of Fallot.
Anatomy The valves of the heart
The valves of the heart are structures which ensure blood flows in only one direction. They are composed of connective tissue and endocardium (the inner layer of the heart).
There are four valves of the heart, which are divided into two categories:
Atrioventricular valves: The tricuspid valve and mitral (bicuspid) valve. They are located between the atria and corresponding ventricle.
Semilunar valves: The pulmonary valve and aortic valve. They are located between the ventricles and their corresponding artery, and regulate the flow of blood leaving the heart.
In this article, we will look at the anatomy of these valves – their structure, function, and their clinical correlations
Fig 1 – The four valves of the heart, visible with the atria and great vessels removed.
Atrioventricular Valves
The atrioventricular valves are located between the atria and the ventricles. They close during the start of ventricular contraction (systole), producing the first heart sound. There are two AV valves:
Tricuspid valve – located between the right atrium and the right ventricle (right atrioventricular orifice). It consists of three cusps (anterior, septal and posterior), with the base of each cusp anchored to a fibrous ring that surrounds the orifice.
Mitral valve – located between the left atrium and the left ventricle (left atrioventricular orifice). It is also known as the bicuspid valve because it has two cusps (anterior and posterior). Like the tricuspid valve, the base of each cusp is secured to fibrous ring that surrounds the orifice.
The mitral and tricuspid valves are supported by the attachment of fibrous cords (chordae tendineae) to the free edges of the valve cusps. The chordae tendineae are, in turn, attached to papillary muscles, located on the interior surface of the ventricles – these muscles contract during ventricular systole to prevent prolapse of the valve leaflets into the atria.
There are five papillary muscles in total. Three are located in the right ventricle, and support the tricuspid valve. The remaining two are located within the left ventricle, and act on the mitral valve.
Fig 2 – The papillary muscles and inflow portion of the left ventricle.
Semilunar Valves
The semilunar valves are located between the ventricles and outflow vessels. They close at the beginning of ventricular relaxation (diastole), producing the second heart sounds. There are two semilunar valves:
Pulmonary valve – located between the right ventricle and the pulmonary trunk (pulmonary orifice). The valve consists of three cusps – left, right and anterior (named by their position in the foetus before the heart undergoes rotation).
Aortic valve – located between the left ventricle and the ascending aorta (aortic orifice). The aortic valve consists of three cusps – right, left and posterior.
The left and right aortic sinuses mark the origin of the left and right coronary arteries. As blood recoils during ventricular diastole, it fills the aortic sinuses and enters the coronary arteries to supply the myocardium.
The pulmonary and aortic valves have a similar structure. The sides of each valve leaflet are attached to the walls of the outflow vessel, which is slightly dilated to form a sinus. The free superior edge of each leaflet is thickened (the lunule), and is widest in the midline (the nodule).
At the beginning of ventricular diastole, blood flows back towards the heart, filling the sinuses and pushing the valve cusps together. This closes the valve.
Fig 3 – The aortic valve cusps, aortic sinuses, and the origin of the coronary arteries.
Clinical Relevance: Aortic Stenosis
Aortic stenosis refers to narrowing of the aortic valve, restricting the flow of blood leaving the heart. The main three causes are:
Age-related calcification
Congenital defects
Most commonly a bicuspid aortic valve, which predisposes the valve to calcification later in life.
Rheumatic fever
The classical triad seen in severe aortic stenosis is shortness of breath, syncope and angina. The increasing workload for the left ventricle can also result in left ventricular hypertrophy.
Definitive treatment is surgical, and can be achieved via valve replacement or balloon valvuloplasty.
Fig 4 – Aortic stenosis, secondary to rheumatic heart disease. The aorta has been removed to show thickened, fused aortic valve leaflets and opened coronary arteries from above.
Anatomy Vasculature of the Heart
The entire body must be supplied with nutrients and
oxygen via the circulatory system and the heart is no
exception. The coronary circulation refers to the vessels that supply and
drain the heart. Coronary arteries are named as such due to the way they
encircle the heart, much like a crown.
This article will outline the naming, distribution, and
clinical relevance of vessels in the coronary circulation.
Naming
Coronary Arteries
There are two main coronary arteries which branch to
supply the entire heart. They are named the left and right coronary
arteries, and arise from the left and right aortic sinuses within
the aorta.
The aortic sinuses are small openings found within the
aorta behind the left and right flaps of the aortic valve. When the heart
is relaxed, the back-flow of blood fills these valve pockets, therefore
allowing blood to enter the coronary arteries.
The left coronary artery (LCA) initially
branches to yield the left anterior descending (LAD), also called the anterior
interventricular artery. The LCA also gives off the left marginal artery
(LMA) and the left circumflex artery (Cx). In ~20-25% of individuals, the
left circumflex artery contributes to the posterior interventricular artery
(PIv).
The right coronary artery (RCA) branches
to form the right marginal artery (RMA) anteriorly. In 80-85% of
individuals, it also branches into the posterior
interventricular artery (PIv) posteriorly.
Fig 1 – Anterior view of the arterial supply to the
heart.
Fig 2 – Overview of the branching structure of the
coronary arteries.
Cardiac Veins
The venous drainage of the heart is mostly through
the coronary sinus – a large venous structure located on the
posterior aspect of the heart. The cardiac veins drain into the coronary sinus,
which in turn, empties into the right atrium. There are also smaller cardiac
veins which pass directly into the right atrium.
The main tributaries of the coronary sinus are:
Great cardiac vein (anterior interventricular vein)
– the largest tributary of the coronary sinus. It originates at the apex of the
heart and ascends in the anterior interventricular groove. It then curves to
the left and continues onto the posterior surface of the heart. Here, it
gradually enlarges to form the coronary sinus.
Small cardiac vein – located on the anterior surface
of the heart, in a groove between the right atrium and right ventricle. It
travels within this groove onto the posterior surface of the heart, where it
empties into the coronary sinus.
Middle cardiac vein (posterior interventricular
vein) – begins at the apex of the heart and ascends in the posterior
interventricular groove to empty into the coronary sinus.
Posterior cardiac vein – located on the posterior
surface of the left ventricle. It lies to the left of the middle cardiac vein
and empties into the coronary sinus.
Fig 3 – Anterior view of the venous drainage of the
heart. Supplied by the great and small cardiac veins
Fig 4 – Posterior view of the heart, showing the venous
drainage.
Distribution of the Coronary Arteries
In general, the area of the heart which an artery passes
over will be the area that it perfuses. The following describes the
anatomical course of the coronary arteries. See Appendix A for a tabular
overview of the arterial distribution.
The RCA passes to the right of the pulmonary trunk and
runs along the coronary sulcus before branching. The right marginal artery
arises from the RCA and moves along the right and inferior border of the
heart towards the apex. The RCA continues to the posterior surface of the
heart, still running along the coronary sulcus. The posterior
interventricular artery then arises from the RCA and follows the posterior
interventricular groove towards the apex of the heart.
The LCA passes between the left side of the pulmonary
trunk and the left auricle. The LCA divides into the anterior
interventricular branch and the circumflex branch. The anterior
interventricular branch (LAD) follows the anterior
interventricular groove towards the apex of the heart where it
continues on the posterior surface to anastomose with the posterior
interventricular branch. The circumflex branch follows the
coronary sulcus to the left border and onto the posterior surface of the heart.
This gives rise to the left marginal branch which follows the left border of
the heart.
Fig 4 – Anterior view of territorial arterial supply to
the heart.
Fig 5 – Posterior view of territorial arterial supply to
the heart.
Clinical Relevance: Coronary Artery Disease
Coronary artery disease or coronary heart disease (CHD)
is a leading cause of death, both in the UK and worldwide. It describes a
reduction in blood flow to the myocardium and has several causes and
consequences.
CHD can result in reduced blood flow to the heart as a
result of narrowing or blockage of the coronary arteries. This may be due
to atherosclerosis, thrombosis, high blood pressure, diabetes
or smoking. All these factors lead to a reduced flow of blood to
the heart through physical obstruction or changes in the vessel wall.
Angina pectoris is one consequence of CHD. Angina
pectoris describes the transient pain a person may feel on
exercise as a result of lack of oxygen supplied to the heart.
This pain is felt across the chest but is quickly resolved upon
rest. Exercise is a trigger for angina as the coronary arteries fill
during the diastolic period of the cardiac cycle. On exercising, the
diastolic period is shortened meaning that there is less time for blood flow to
overcome a blockage in one of the coronary vessels in order to supply the
heart.
If left untreated, angina can soon progress to more
severe consequences, such as a myocardial infarction. The sudden occlusion
of an artery results in infarction and necrosis of the myocardium. This
means a section of the heart is unable to beat (which part of the heart depends
on which artery has become occluded). The ECG leads on which an MI change
appears can be used to locate the artery that had been occluded as shown in the
table.
Diagnosis and Treatment of Coronary Artery Disease
Fig 1.6 – A coronary angiogram. Two critical narrowings
have been labelled.
A blockage in a coronary artery can be rapidly identified
by performing a coronary angiogram. The imaging modality involves the
insertion of a catheter into the aorta via the femoral artery. A contrast
dye is injected into the coronary arteries and x-ray based imaging is then used
to visualise the coronary arteries and any blockage that may be present.
Immediate treatment of a blockage can be performed by way
of a coronary angioplasty, which involves the inflation of a
balloon within the affected artery. The balloon pushes aside the atherosclerotic
plaque and restores the blood flow to the myocardium. The artery may then
be supported by the addition of an intravascular stent to maintain
its volume.
Appendix A – Tabular Overview of the Vasculature of the Heart
