pulmonary artery anatomy and pulmonary embolism

Pulmonary Artery Anatomy

and Pulmonary Embolism

Gamal Rabie Agmy, MD, FCCP Professor of Chest Diseases, Assiut university

◙ The main pulmonary artery (MPA) is intrapericardial and

courses posteriorly and superiorly from the pulmonic

valve.

◙ It divides into the left pulmonary artery (LPA) and right

pulmonary artery (RPA) at the level of the fifth thoracic

vertebra.

◙ The RPA is longer than the LPA and crosses the

mediastinum, sloping slightly inferiorly to the right lung

hilus. The LPA represents the continuation of the MPA.

Anatomy of Pulmonary Artery

◙ Segmental and subsegmental pulmonary arteries

generally parallel segmental and subsegmental bronchi

and run alongside them. This is in contrast to the course

of most pulmonary veins, which run independently of

bronchi within interlobular septa.

◙ The segmental arteries are named according to the

bronchopulmonary segments that they feed, and we

follow the Jackson and Huber classification in this

description.


◙ However, the proximal portions of the arteries to the

posterior subsegment of the left upper lobe and the

lingular arteries can run independently of their respective

bronchi for short segments.

◙ Also, there are frequently accessory arteries from

neighboring segments, particularly in the right upper

lobe. Segmental and subsegmental pulmonary arteries

vary considerably in the location of their origins, in

whether they arise as common trunks with other arteries

or as separate arteries, and in their number.


PA Anatomy: Right Upper Lobe

◙ At the right lung hilus, the RPA divides into a superior

trunk (the ascending branch or truncus anterior) and an

inferior trunk (the descending branch or interlobar

artery).

◙ The truncus anterior supplies the right upper lobe; and

the interlobar artery, which runs in the interlobar fissure

and parallels the bronchus intermedius, supplies the

right middle and right lower lobes.

◙ The truncus anterior (TA) has a course which is anterior

to the right upper lobe bronchus, and it usually

terminates in the apical (A1) and anterior (A3) segmental

arteries, both of which usually divide into paired

subsegmental arteries (A1a and A1b, and A3a and A3b,

respectively).


◙ The posterior segment has a quite variable pulmonary

arterial supply (A2). In approximately one-half of

individuals, the pulmonary arterial supply to the posterior

segment is split between an ascending artery which

arises from the interlobar artery, and a recurrent artery

which originates from either the truncus anterior or from

A1a .

◙ In this situation, the anterior subsegment is typically

fed by the recurrent artery (which becomes named A2a),

and the posterior subsegment is typically fed by the

ascending artery (which becomes named A2b), and these

two arteries have a reciprocal relationship in terms of

size and area of distribution .


◙ In approximately one-third of individuals, the posterior

segment’s sole supply is from the ascending artery. In

approximately 15% of individuals, the posterior segment

is fed entirely from the truncus anterior, in which case

there may be no ascending artery. However, the

ascending artery is present in approximately 90% of

individuals .

◙ It may also on occasion become part or all of A2a and

A3, especially if there are two ascending arteries. Note

that different series report varying proportions of these

branching patterns. For example, one study reports that

sole supply of the posterior segment by the ascending

artery is most common


◙ There is also an inconstant bifurcation of the

ascending branch of the RPA into superior and inferior

trunks (truncus anterior superior and inferior) and this

inferior trunk of the upper lobe artery may supply some

or all of the anterior segment (becoming A3, and

particularly A3a), in approximately 15-20% of individuals .

This inferior trunk may also arise independently from the

superior trunk.

◙ The truncus anterior inferior is expected to pass

anterior to the anterior segmental bronchus (B3) (8). In

any event, the ascending artery is expected to arise from

the interlobar artery, caudal to both the superior and

inferior trunks of the right upper lobe arterial supply


◙ As mentioned, the anterior segment arterial supply

(A3) arises solely from the truncus anterior in the

majority of cases, but additional or replaced supply can

also be seen from the ascending artery (in approximately

25% of individuals) or from the inferior trunk of the upper

lobe arterial supply (in approximately 15%)

PA Anatomy: Middle Lobe

◙ The right middle lobe has lateral and medial segments,

which by definition have pulmonary arterial supply from

A4 and A5, respectively. In approximately one-half of

individuals, A4 and A5 arise as separate branches from

the anteromedial aspect of the interlobar artery, and in

the other one-half of individuals they arise as a common

trunk at this location before dividing

◙ Uncommonly, three separate trunks are seen . Either A4

or A5 may have a common origin with a lower lobe

segmental artery, however . A branch of the medial

segmental artery may cross segmental boundaries and

also supply the anterior aspect of the lateral segment

PA Anatomy: Right Lower Lobe

◙ Caudal to the origin of the right middle lobe arteries, the

interlobar artery becomes the right lower lobe artery. At

about the same level as the origin of the right middle lobe

arteries and opposite to them, the right lower lobe artery

gives off the posteriorly oriented artery to the superior

(apical) segment of the right lower lobe (A6).

◙ However, in approximately 20% of individuals, A6 arises

as two or very rarely three separate arteries from the right

lower lobe artery . There are often three identifiable

subsegmental arteries within the superior segment of the

right lower lobe (A6a, A6b, and A6c), any two of which

may have a common Trunk


◙ Caudal to the origin of the artery to the superior

segment of the right lower lobe, the right lower lobe artery

is renamed as the pars basalis, or basal trunk ).

◙ In approximately one-half of individuals, the order of

branching from the pars basalis is first the medial basal

segmental artery (A7), then the anterior basal segmental

artery (A8), and finally the lateral and posterior basal

segmental arteries (A9 and A10) . A7 and A8 may arise as

a common trunk (A7+8), as may A9 and A10 (A9+10), the

former coursing anteromedially and the latter coursing

posterolaterally.


◙ The order of branching from the pars basalis is

frequently random, however (9). Other variations can be

seen, such as the absence of A7; or the branching pattern

of A7 and A8+9+10. Although one basal segmental artery

around a segmental bronchus is the prevailing pattern,

two or even three subsegmental arteries around a

segmental bronchus may be seen .

◙ An inconstant subsuperior pulmonary segment is seen

in approximately 30% of individuals, located between the

superior and basal segments of the lower lobes

bilaterally . If present, its pulmonary arterial supply is

denoted as A8, and this artery generally takes origin from

the lateral aspect of the right pars basalis. It has a single

stem in the majority of cases, although two and three

stems are sometimes seen

PA Anatomy: Left Upper Lobe

◙ The LPA arches over the superior aspect of the left

upper lobe bronchus to first supply the left upper lobe.

There is no left truncus anterior, and there tend to be

more separate branches scattered along the course of

the left pulmonary artery than there are along the course

of the right pulmonary artery

◙ The left upper lobe pulmonary arterial branches may

arise in fairly random order, and they frequently form

small trunks of two or more branches . Two to seven

branches may originate from the proximal LPA to supply

the left upper lobe and lingula (Figure 1b, 1c).The anterior

segmental pulmonary arterial supply (A3) arises from the

anterior aspect of the mediastinal portion of the LPA

(pars anterior), and A3 typically subdivides into three

portions (A3a, A3b, and A3c)


◙ The apicoposterior subsegmental arteries (apical,

A1+2a; posterior, A1+2b; and lateral, A1+2c) usually arise

from the most superior or slightly posterior portion of the

mediastinal portion of the LPA, superior to the interlobar

segment.10 In approximately one-half of individuals,

A1+2a and A1+2b arise as a common trunk, with A1+2c

arising separately.

◙ In approximately 15% of individuals, all three

subsegmental arteries arise as a common stem,

paralleling the apicoposterior segmental bronchus. Less

frequently but not uncommonly, one may see separate

origins of each subsegmental artery, or a common trunk

of A1+2b and A1+2c with separate origin of A1+2a. Again,

A1+2 is associated with A3 in some way in approximately

40% of individuals


◙ The lingular arteries (superior lingular, A4; inferior

lingular, A5) arise from the anterior aspect of the left

interlobar artery in approximately 90% of individuals; in

the remainder, they arise with other upper lobe vessels

from the pars anterior of the mediastinal LPA. They may

arise as a single trunk or as two separate arteries.

PA Anatomy: Left Lower Lobe

◙ The left lower lobe bronchus is shorter than the right

lower lobe bronchus, so the left lower lobe bronchi and

their accompanying arteries originate at a slightly higher

level than those on the right . The superior (apical)

segmental arterial supply (A6) of the left lower lobe

originates from the interlobar artery at or above the level

of the lingular arteries, usually as a single trunk (in

approximately 70% of individuals)

◙ However, the superior segmental pulmonary arterial

supply frequently arises as two or more separate trunks .

Occasionally, A6 arises from a lingular artery. There are

often three identifiable subsegmental arteries within the

superior segment of the left lower lobe (A6a, A6b, and

A6c), any two of which may have a common trunk


◙ Caudal to the origin of the superior segmental artery,

the left interlobar artery becomes the pars basalis or

basal trunk. There is usually a single anteromedial basal

segmental bronchus; and so there is also, in

approximately 60% of individuals, a single common

pulmonary arterial trunk for segments 7 and 8, denoted

as A7+8.8,9,12. However, the artery to the lateral basal

segment (A9) frequently arises in this common trunk as

well, which would give us the notation of A7+8+9, with

A10 arising separately from the pars basalis .

◙ A9 and A10 also frequently arise as a common trunk, or

in other configurations. Additionally, the medial basal

segmental artery is inconstant, and a true A7 may not be

identified. Again, the basal subsegmental arteries may be

duplicated or even triplicated around a segmental

bronchus


◙ The inconstant subsuperior pulmonary segment is

seen in approximately 30% of individuals . If present, A8

generally takes origin from the posterolateral aspect of

the left pars basalis. It has a single stem in the majority of

cases, although two and three stems are sometimes seen

Systematic Approach to the Evaluation

of the Pulmonary Arteries

◙ This approach can be used to help evaluate the

pulmonary arteries in a systematic fashion. Once the

approach becomes routine, common patterns of

anatomical variants (described above) should be

recognized and the system modified to accommodate.

◙ Evaluation of the pulmonary arteries is best performed

at a workstation, where blood vessels can be more easily

followed along their courses. As well, a workstation

allows rapid switching between vascular (W 550 / L 150)

and lung (W 1400 / L -400) windows, which is necessary

to evaluate the spatial relationships of vessels to bronchi.

Systematic Approach to the Evaluation

of the Pulmonary Arteries

In the central portions of the lungs, the upper lobe

arteries are generally central to their associated bronchi;

and the middle and lower lobe and lingular arteries are

generally peripheral to their associated bronchi . The

pulmonary veins are generally anterior to the arteries

except in the right upper lobe.

Main and Right Pulmonary Arteries

Follow the main pulmonary artery superiorly from the heart and go right at

its bifurcation into the main RPA as it passes behind the ascending aorta.

Within the mediastinum, the RPA gives rise to the truncus anterior (TA),

anterior to the right upper lobe bronchus.

Right Upper Lobe

1. Apical (A1) (Fig 2a,d,e) -- Go superiorly within the TA to

a superiorly oriented branch -> A1a, A1b; also look for a

posteriorly oriented recurrent artery from the truncus

anterior -> A2a

Right Upper Lobe

2. Anterior (A3) (Fig 2b,c,d,e) -- Go back to the TA and find an

anteriorly oriented branch -> A3a, A3b

3. Posterior (A2 ) (Fig 2c,d,e) -- Go back to the right pulmonary artery

(RPA) and proceed just caudal to the TA, and look for the superiorly

oriented ascending artery -> A2b and possibly A2a

Middle Lobe

4. Lateral (A4) (Fig 3b,c,d,e) -- From the anteromedial

aspect of the interlobar artery, find a laterally coursing

branch -> A4a, A4b

Middle Lobe

5. Medial (A5) (Fig 3c,d,e) -- From the anteromedial aspect

of the interlobar artery, find a medially coursing branch

either arising alone or with A4 -> A5a, A5b

Right Lower Lobe

6. Superior (apical) (A6) (Fig 3a,d,e in previous page) -- From the

interlobar artery at about the same level as the middle lobe arteries and

opposite their origin, find a posteriorly oriented branch(es) -> A6a, A6b,

A6c 7. Medial basal (paracardiac) (A7) (Fig 4a,b,c,d) -- From the basal

trunk, go caudally and find a medially oriented branch -> A7a, A7b *Note

that an inconstant subsuperior pulmonary segment is sometimes seen,

with arterial supply A* taking origin from the lateral aspect of the right

basal trunk.

Right Lower Lobe

8. Anterior basal (A8) (Fig 4a,b,c,d) -- From the basal trunk, find an

anteriorly oriented branch -> A8a, A8b

9. Lateral basal (A9) (Fig 4a,b,c,d) — From the basal trunk, find a laterally

oriented branch -> A9a, A9b 10. Posterior basal (A10) (Fig 4a,b,c,d,) —

From the basal trunk, find a posteriorly oriented branch -> A10a, A10b,

A10c

Left pulmonary arteries

The LPA leaves the main PA, passing superiorly over the left mainstem

bronchus, to descend posterior to the bronchus.

Left Upper Lobe

1,2. Apicoposterior (A1+2) (Fig 5a,b,c,d,e) -- From the superior apex of the

mediastinal left pulmonary artery (LPA), find superiorly oriented branch(es)

-> A1+2a, A1+2b; A1+2c may arise as a separate branch or in a common

origin.

Left Upper Lobe

3. Anterior (A3) (Fig 5a,b,d,e) -- From the anterior aspect of the

mediastinal LPA, find anteriorly oriented branch(es) -> A3a, A3b, A3c

Left Upper Lobe

4. Superior lingular (A4) (Fig 6b,d,e) -- From the anterior aspect of the left

interlobar artery, find an anteriorly oriented branch which courses to the

superior lingula -> A4a, A4b

Left Upper Lobe

5. Inferior lingular (A5) (Fig 6c,d,e) -- From the anterior aspect of the left

interlobar artery, find anteriorly oriented branch which

Left Lower Lobe

6. Superior (apical) (A6) (Fig 6a,d,e ) -- From the interlobar artery at or just

superior to the origin of the lingular arteries, find a posteriorly oriented

branch(es) -> A6a, A6b, A6c

Left Lower Lobe

7,8. Anteromedial (A7+8) (Fig 7a,b,c) -- From the basal trunk, go caudally

and find an anteromedially oriented branch(es); a common branch may

divide to anterior and medial branches -> A7a, A7b; A8a, A8b *Note that

an inconstant subsuperior pulmonary segment is sometimes seen, with

arterial supply A* taking origin from the posterolateral aspect of the left

basal trunk.

Left Lower Lobe

9. Lateral basal (A9) (Fig 7a,b,c) -- From the basal trunk, find a laterally

oriented branch -> A9a, A9b

10. Posterior basal (A10) (Fig 7a,b,c) -- From the basal trunk, find a

posteriorly oriented branch -> A10a, A10b, A10c

Significance of Pulmonary Embolism

◙ The incidence of pulmonary embolism (PE) is unknown, however it

is estimated that approximately 600,000 cases of PE occur annually

in the United States. Of these, approximately 250,000 patients are

hospitalized. The mortality rate for pulmonary embolism is known to

be high and in the “Prospective Investigation of Pulmonary

Embolism Diagnosis I (PIOPED I)” study the fatality during one year

follow up in patients diagnosed with pulmonary embolism was 10%.

A Medicare study including patients 65 years of age or older reported

30 day case fatality rates in patients with PE of 13.7%

◙ It has been shown that if the diagnosis of PE is made promptly and

appropriate therapy initiated immediately, the mortality rate can be

reduced. The key for the correct diagnosis of PE is a high index of

clinical suspicion. However, the clinical manifestations of pulmonary

embolism are many fold and clinical assessment has been shown to

be both nonspecific and not very sensitive. There are a multitude of

laboratory tests such as arterial blood gases, electrocardiography,

and chest radiography which can be helpful in excluding other

diseases mimicking PE, but they typically do not allow the reliable

diagnosis of PE.

Significance of Pulmonary Embolism

◙ The advent of the D-dimer blood test has shown promising results

and the test has good sensitivity and specificity in patients who

present without other systemic illnesses. However, imaging remains

the mainstay in the diagnosis of PE. Several imaging tests are

currently available in order to assess the pulmonary vasculature for

presence of PE. These include ultrasonography, nuclear medicine

(ventilation/perfusion scanning), pulmonary angiography, computed

tomography and magnetic resonance angiography. In this section of

the tutorial we review the current knowledge on the imaging

diagnosis of acute PE with special emphasis on the noninvasive

techniques.

Appearance of Pulmonary Embolus on CTPA

◙ The key diagnostic findings of PE on CTPA are central or wall-

adherent filling defects in the pulmonary arteries leading to either

complete or partial obstruction. There are a number of mimickers of

PE, which can be divided into anatomic and technical mimickers.

Familiarity with these helps to avoid diagnostic errors. These pitfalls

are discussed later in this tutorial. In the CTPA sequence of images

below scroll to see the filling defects created by the PE.

◙ Below you can find the CTPA of an acute PE in first image, followed by a

1-month follow-up and then a 1-year follow-up.

A

B C

Ultrasonography

◙ If there is an ultrasound confirmation of deep venous thrombosis

(DVT) associated with pulmonary symptoms, this may indirectly

confirm the diagnosis of PE. Since therapy is often the same for both

conditions, further investigation to exclude PE may not be necessary.

Prevalence of DVT is 82% in patients with PE

◙ . Compression Duplex ultrasound is the primary diagnostic test for

DVT in patients having proximal leg symptoms, with a reported

sensitivity and specificity of 97 and 94%, respectively. Although the

diagnosis of a DVT below the knee is more controversial, reported

prevalence of lower limb (infra-popliteal level) DVT is as low as 13%

in the patients with proven acute PE. Major restrictions of US

imaging limiting its sensitivity in the diagnosis of DVT are the

challenging evaluation of calf veins, iliac veins and the inferior vena

cava as well as its operator/equipment dependency.

Ultrasonography

Ultrasonography

Schematic representation of the parenchymal, pleural and vascular features

associated with pulmonary embolism.(Angelika Reissig, Claus Kroegel.

Respiration 2003;70:441-452 )

V/Q Scan

◙ Pulmonary perfusion scanning was the most widely utilized

screening test to rule out clinically important pulmonary embolism

prior to the advent of computed tomography pulmonary angiography

(CTPA). It has been shown that a normal test result almost certainly

excludes the presence of pulmonary embolism.

◙ A high probability ventilation-perfusion (VQ) scan usually indicates

the presence of clinically significant PE. 88% of patients with a high

probability VQ scan had angiographic evidence of PE in the PIOPED I

study.

◙ However, in patients with a prior history of PE and high probability

VQ scans the presence of acute PE was proven in only 74%

angiographically. In addition, of all patients diagnosed with acute PE

in the PIOPED I study, only 41% had a scan pattern thought to

represent a high probability VQ scan.

V/Q Scan

◙ Patients presenting in either the intermediate or indeterminate group of

VQ scans are difficult to interpret and the technique is also not helpful in

patients with a low probability scan in the setting of a high clinical

suspicion. Unfortunately, the majority of patients undergoing perfusion

scanning have nondiagnostic results (more than 60% of all patients in the

PIOPED I study). Also, in the PIOPED II study, VQ scanning has been

used for the diagnosis or exclusion of PE evaluated by CTA. Only 30% of

all patients had a high probability and normal VQ findings. It is for this

reason that other screening techniques for PE have been developed.

◙ Compression Duplex ultrasound is the primary diagnostic test for DVT

in patients having proximal leg symptoms, with a reported sensitivity and

specificity of 97 and 94%, respectively. Although the diagnosis of a DVT

below the knee is more controversial, reported prevalence of lower limb

(infra-popliteal level) DVT is as low as 13% in the patients with proven

acute PE. Major restrictions of US imaging limiting its sensitivity in the

diagnosis of DVT are the challenging evaluation of calf veins, iliac veins

and the inferior vena cava as well as its operator/equipment dependency.

The addition of a ventilation scan improves the overall test performance

only marginally.

Magnetic Resonance Pulmonary Angiography (MRPA)

◙ Magnetic resonance pulmonary angiography (MRPA), although still

considered a second-line imagine technique behind CTPA, has been

gaining acceptance in the evaluation of VTE. In contrast to CT, MR

imaging does not expose the patient to ionizing radiation, and its

main contrast agent, gadolinium chelate, shows a much lower

potential for allergic reactions and nephrotoxicity than does

iodinated contrast. There is however the risk of developing

nephrogenic systemic fibrosis (NSF) in patients on dialysis or with

renal insufficiency (reference). However, longer acquisition times and

poorer spatial resolution have historically limited its applicability in

the imaging of the pulmonary arterial system.

Magnetic Resonance Pulmonary Angiography (MRPA)

◙ As MR techniques improve these historic limitations are slowly

being overcome. The advent of more rapid imaging sequences in

conjunction with parallel imaging has diminished motion artifact,

allowing for improved resolution of the pulmonary vascular system.

Additionally, dynamic imaging during contrast injection now offers

the functional information of perfusion imaging. The systemic

venous structures can be simultaneously evaluated, using MR

venography (MRV), to assess for pelvic and lower extremity VTE.

Recent published literature suggests that MR imaging of acute VTE

(MRI, MRPA, MR perfusion, MRV) indeed may rival the accuracy of CT

in the detection of VTE and PE (at least centrally). These data have

yet to be validated in larger prospective studies. As it continues to

evolve, MR imaging is expected to play an increased role in the

future evaluation of PE.

MR Direct Thrombus Imaging

◙ MR direct thrombus imaging (MRDTI) makes use of direct

detection of methemoglobin in a thrombus, which appears bright on

T1-weighted sequences owing to its T1 shortening effect. Utilization

of this endogenous contrast permits visualization of a thrombus

without the use of any contrast agent (hence the term “direct

thrombus imaging”), and it is non-invasive. MRDTI can be used to

detect subacute thrombosis. Deep vein thrombosis and pulmonary

embolism can be both confirmed by MRDTI. Also, it does allow the

detection of venous thromboembolic disease with a single imaging

modality. The most significant advantage of MRDTI is the direct

visualization of the thrombus, while other sequences rely on the

detection of thrombus through a filling defect. Another advantage is

that MRDTI allows to differentiate between old and new clots (high

signal intensity indicates subacute thrombosis).

MR Direct Thrombus Imaging

◙ Several studies have been performed to determine the efficacy of

MRDTI in detection of thrombus. In a study comparing MRDTI with

contrast venography the sensitivity of MRDTI was 96% in 338

patients. In a smaller PE study involving 13 patients the sensitivity

and specificity of MRDTI was 100% for both. MRDTI allowed the

detection of three additional emboli not seen on conventional

pulmonary angiography. A randomized trial of MRDTI for diagnosis of

pulmonary embolism in 157 patients demonstrated similar patient

outcomes compared with more extensive diagnostic strategies,

including CT, VQ and D-dimer testing. These results make it possible

that MRDTI may have a significant role in the future for the diagnosis

and management of venous thromboembolism.

MR Perfusion Imaging ◙ Contrast-based perfusion MRI is generally used in combination with

ultra-fast three dimensional MR angiography (3D MRA), demonstrating

contrast agent entering into the pulmonary circulation as well as imaging

of the vessel morphology. Techniques without or with parallel imaging

technology are effective. These permit both the identification of perfusion

defects and imaging of the vessel morphology, hence facilitating the

diagnosis of pulmonary vascular disease. Highly promising results have

been reported from the initial work in this field.

◙ A recent study of 33 patients evaluated the feasibility of MR perfusion for

short-term follow-up of patients with acute PE. The study also purposed to

evaluate temporal changes of pulmonary perfusion and thrombus

characteristics of a thrombus that might be helpful in determining the age

of the thrombus. All patients were examined by CT and, MRA, real-time

MRI and MR pulmonary perfusion imaging initially and 1 week after

treatment. A follow-up diagnostic work-up was feasible for all patients after

treatment. MRA and CT were concordant for a diagnosis of PE in all

patients. They have also indicated that MRI has potential role for

determining the age of embolic material. The technique currently has to be

considered experimental and its value in clinical practice remains to be

demonstrated.

Pulmonary Angriography

◙ Pulmonary angiography is considered the gold standard for the

diagnosis of PE, although recent evidence does not necessarily

always support that. Pulmonary angiography is an invasive

procedure and due to its costs and potential risks is usually reserved

for patients in whom more information or certainty of the diagnosis

of PE are necessary. Indications for angiographic evaluation of

patients suspected of having PE are the need to confirm the

diagnosis of PE in the presence of contraindications to

anticoagulation or if IVC filter placement or surgical embolectomy are

contemplated.

◙ In addition, patients with a high index of clinical suspicion but

nondiagnostic noninvasive studies and patients with pulmonary

hypertension of unknown cause commonly undergo the exam . The

unequivocal establishment of the diagnosis of PE in younger

patients facing life-long anticoagulation therapy or IVC filter

placement is another indication. With modern techniques and

nonionic contrast media, the risks of the procedure are exceedingly

low with major nonfatal complication rates less than 2% and a

mortality of the procedure of 0.1%.

Pulmonary Angriography

◙ Life threatening complications are typically secondary to acute cor

pulmonale in patients with pre-existing severe pulmonary

hypertension and failing right ventricle. Therefore, the measurement

of the right ventricular enddiastolic pressure (RVEDP) is mandatory

before performing the angiographic runs and if this is 20 mmHg or

higher it should be acknowledged that the patient has a significantly

higher risk of a serious complication and therefore either should not

have the study performed or have a superselective study using more

runs with smaller amounts of contrast.

◙ It has been shown that digital angiography helps to reduce both

time and amount of contrast necessary to perform pulmonary

angiography if compared to conventional cut-film angiography. It

also has been shown to have similar performance without sacrifices

in sensitivity or specificity

Pulmonary Embolism During Pregnancy

◙ The diagnosis of PE during pregnancy imposes special

challenges. Both CTPA and VQ scanning expose the mother and the

fetus to radiation, with the fetus being particularly susceptible. The

International Commission on Radiological Protection (ICRP) has

made recommendations regarding the minimization of radiation

exposure to both patient and fetus and many countries have

introduced legislation to this end. In general terms, the fetal dose is

much higher with VQ scanning than with CTPA (depending on the

protocol up to 200 times higher) (700 – 800 µGy, vs 3 – 131 µGy) while

the maternal dose is typically lower with VQ scanning (1.4 mSv

versus 2.2 – 6.0 mSv).


◙ This fetal dose advantage for CTPA even applies to protocols

specifically modified for pregnant patients like half-dose perfusion scans

(140 – 250 µGy). Other considerations are the somewhat higher risk for

the mother to develop breast cancer with CTPA and the lack of human

safety data for the effects of iodinated contrast media on the fetus,

although they approved to be safe in animal experiments. Therefore, any

search for PE in a pregnant patient should involve a thorough discussion

of the risks and benefits with the patient. If imaging is necessary with

ionizing radiation, the currently available data support the preferential use

of CTPA, if possible with a modified low-dose technique and limited

anatomical coverage.

Automated tube modulation should be used for all scans, regardless

if the patient is pregnant or not. However, as a first step, an

ultrasound of the legs is recommended as a surrogate test for

pulmonary embolism.


◙ The D-dimer concentration is usually increased in the second and

third trimester of the pregnancy and normalizes 4 to 6 weeks after

delivery. Thus, the test is clearly less useful during pregnancy. As

significant controversy exists on this topic and because no study

has ever validated a negative D-dimer test for t he exclusion of PE in

pregnancy, the use D-dimer testing as a stand-alone test to exclude a

PE can currently not be recommended in this setting.

Flowchart for Diagnosing Pulmonary Embolus


◙ This fetal dose advantage for CTPA even applies to protocols

specifically modified for pregnant patients like half-dose perfusion scans

(140 – 250 µGy). Other considerations are the somewhat higher risk for

the mother to develop breast cancer with CTPA and the lack of human

safety data for the effects of iodinated contrast media on the fetus,

although they approved to be safe in animal experiments. Therefore, any

search for PE in a pregnant patient should involve a thorough discussion

of the risks and benefits with the patient. If imaging is necessary with

ionizing radiation, the currently available data support the preferential use

of CTPA, if possible with a modified low-dose technique and limited

anatomical coverage.

Automated tube modulation should be used for all scans, regardless

if the patient is pregnant or not. However, as a first step, an

ultrasound of the legs is recommended as a surrogate test for

pulmonary embolism.

Pitfalls of CTPA

◙ Anatomic Pitfalls: PA’s run with bronchi, PV’s run

independent, Unopacified veins, Mucoid impaction,

Lymphadenopathy and perivascular tissue, Other

pathologies (Shunts, Sarcomas)

◙ Imaging Artifacts: Streak artifacts, Motion artifacts,

Improper bolus timing, inconsistent (“fractured”) bolus

Edge-enhancing reconstruction algorithm, Inadequate

window settings, Patient size

Pitfalls of CTPA

Computed tomography

pulmonary arteriography:

pitfalls lymphadenopathy.

Lymphadenopathy in a patient

with silicosis and shortness of breath mimicking acute

pulmonary embolus. There are

multiple lymph nodes adjacent to

vessels that occasionally could

impose problems differentiating from partially organized thrombus

(arrows).

Pitfalls of CTPA

Computed tomography pulmonary arteriography: pitfalls - nonopacified

pulmonary vein. Image on the left has been occasionally misdiagnosed as

acute pulmonary embolus (arrow). However, following the brnach towards the

left atrium helps clarify this question in all cases (arrow).

Pitfalls of CTPA

Computed tomography pulmonary arteriography: pitfalls - mucoid

impaction. This shows the typical appearance of mucous-filled bronchi (arrows)

adjacent to the enhanced arteries. This finding should not be mistaken for

pulmonary embolism.

Pitfalls of CTPA

Computed tomography pulmonary arteriography: pitfalls - volume

rendering. (Left) Pulmonary arteries that run in-plane parallel with bronchi can

occasionally appear less dense than the arteries running perpendicular to the imaging

plane. This is caused by partial volume averaging of the air -filled bronchi into the

pulmonary artery, thus artificially reducing its density (arrow). Reconstruction of the dataset

with thinner slices and/or oblique perpendicular multiplanar reconstructed images usually

help resolve this issue. (Right) True pulmonary embolus in a small branch to the middle

lobe running in-plane (arrow). There are also other emboli (arrowheads).

Conclusion Clinical diagnosis of PE is difficult and it is frequently under- and over-

diagnosed. Imaging diagnosis required in most cases because treatment

has complications and costs. CTPA (or V/Q scan) first test if no DVT

symptoms and Pulmonary angiography as ultimate confirmatory test with

low morbidity and mortality. In patients suspected of having pulmonary

embolism further workup with imaging is usually necessary. Nowadays, CT

pulmonary angiography is the most widely used diagnostic modality with

higher sensitivity and specificity than VQ scanning. VQ scanning remains

a valid modality, although its result will be non-diagnostic in more than

60% of all screened patients.

◙ Since the ability of CTPA to depict smaller, isolated subsegmental

pulmonary emboli appears to be suboptimal, the key question is what is

the outcome of patients with negative spiral CT scans who do not undergo

anticoagulation. Several studies looked into this, both pro- and

retrospective in design. They uniformly found that the recurrence of PE in

these patients is low and comparable to that after a negative or low

probability VQ scan or negative pulmonary arteriogram. Thus, CTPA in

conjunction with D-Dimer testing is considered a reliable imaging tool for

the exclusion of clinically important PE.

Conclusion

◙ CTPA valuable diagnostic tool with overall high sensitivities and

specificities. CTPA is possibly more effective than V/Q scanning in

the diagnosis and screening of PE. CTPA reduces likelihood of

requiring further diagnostic tests as compared to scinthigraphy (Van

Rossum Eur Radiol 1999). CTPA is more likely to yield alternative

diagnosis if no PE exists (Shah Radiology 1999, Remy-Jardin

Radiology 1999).

Conclusion

pulmonary artery anatomy and pulmonary embolism

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