A Physics in the Diagnosis of Pulmonary

A Comparison
of Modalities and their Underpinning Physics in the Diagnosis of Pulmonary
Embolism: Computed Tomography Pulmonary Angiography and Ventilation Perfusion
(V/Q) Scan.

Overview of
Pulmonary Embolism

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Pathology and
Aetiology

Pulmonary embolism (PE) refers to the pulmonary arterial
system being occluded by an embolus. In most cases it is of thrombotic nature
hence the term pulmonary thrombo-embolism is often used.¹

Effective haemostasis in the body leads to a healthy
balance of coagulation and fibrinolysis. When a disease or other process
affects haemostasis, too many or too large a thrombus may be formed. PE results
after the thrombus detaches from the formation site and travels through the
cardiovascular system into the pulmonary arteries.² Recent surgery, injury,
medical illness and long distance travel (especially flying) are all
predisposing factors for developing  PE.
³

Signs and
Symptoms

Common symptoms include dyspnoea, pleuritic chest pain
and coughing including haemoptysis. Less common symptoms include wheezing,
non-pleuritic chest pain and palpitations. The signs and symptoms vary for each
patient however dyspnoea, chest pain and haemoptysis are described as the
classic triad in suspecting PE. ¹

Clinical
assessment

Patients can be given a plain chest radiograph to rule
out other causes. A negative D-dimer, a test that makes evident whether there
has been significant clot formation and breakdown in the body, can rule out PE
easily. Whereas a positive D-dimer suggests PE is a possibility.  Clinicians then use a system called the Wells
score to determine probability of PE as seen in figure 1. ?

Diagnosis and
Treatment

NICE guidelines advise to offer patients, who score
“likely” in the Wells score, a CTPA or V/Q SPECT scan. Pharmacological
interventions, mechanical interventions and thrombolytic therapy can then be
offered after the results of either scan.¹?

Computed
Tomography Physics and Technology

What is CT?

CT is a diagnostic imaging modality that uses
continuous X-ray output, that’s moves helically along the z-axis of the
patient, the data produced is digitally reconstructed in trans axial slices of
the anatomy. This is known as transmission type tomography.¹²

Mechanical

The X-ray source constructs a matrix of data cells
which are stored to the computer as attenuation values, which are relative to
the intensity of the x-ray beam after attenuation. Unlike conventional
radiography the matrix is made up of voxels, which have a volume depth, rather
than pixels. ?  ?

X-ray tube and
Generator

See figure 2 for the production of X-rays. The linear
attenuation coefficients are dependent on the kV, therefore any variations in
the kV would affect the image produced. To maintain the kV the generator must
be one that can produce very high energies, CT usually operates at 125 kV or
higher. To ensure even levels of energy that reaches the patient the from the
X-ray beam, it is highly filtered using copper and aluminium. Lower energy
X-ray photons are attenuated by the filter materials, leaving only higher
energy photons to reach the patient, this is known as beam hardening. ?

Image
Acquisition

Most CT systems use solid state detectors which use
Caesium Iodide (CsI). CsI crystals can be grown in a very fine columnar
structure, this allows for high spatial resolution. CsI works as part of the
detector due to it emitting visible light (scintillating) when absorbing electromagnetic
energy, (photo electric effect) in this case the X-rays. The analogous light
signal, is converted into a digital signal by a photodiode, which allows it to
then be processed by the computer software.?

Image
Reconstruction

Back projection is a principle used to reconstruct a
CT image from the digital information produced by the detectors, it takes the
total absorption for each matrix row and ‘smears’ the data along the path it
originally came from. This results in a blurry image with star streaked
artefacts. Consequently, filters are now applied to the raw data before back
projection to obtain a sharper image, this is known as filtered back
projection.?

CT: Pulmonary
Angiogram Technique

Figure 3 shows an example of parameters used for a
CTPA, they result in an acquisition time of 6.4 seconds for z-axis coverage of
160mm. An automated bolus tracking system is used and an iodinated non-ionic
contrast is injected intravenously into the patient, preferably at 4ml/sec. A predefined
region of interest, the main pulmonary artery, is monitored on a scan of that
area only. When the area reaches a pre-set threshold of Hounsfield units (HU),
this demonstrates there is an adequate amount of contrast in the pulmonary
artery for optimal imaging, and the CT scan of the entire chest area is
acquired¹?. Evidence of a PE would show as a filling defect in the
pulmonary arteries, of which the shape and position would indicate where it is
and which type (see figure 3¹¹).

Nuclear
Medicine V/Q Scan Physics and Technology

What is
nuclear medicine?

Nuclear medicine is a diagnostic imaging modality
which involves administering a radiopharmaceutical to the patient, to enhance
pathological processes, which is then detected by specially designed
equipment.¹³ This is known as emission radiography.¹²

Equipment

For the diagnosis of PE, a gamma camera is used. It
consists of detectors and collimators mounted on a gantry that allows them to
be rotated and tilted into different positions. The detectors, which have a
large surface area, consist of crystals made of NaI(Tl) and photomultiplier
tubes. They are housed inside an aluminium light proof shielding. This all,
excluding the input surface, are also cased in lead. On top of the crystals is
a honeycomb shaped collimator made with finely cast lead. Pharmaceuticals of
radioactive nature are used due to the radioactive decay process they
undertake. ¹?

Radiopharmaceuticals
and Radioactive Decay

When something is described as radioactive it is
emitting energy to stabilise the nucleus. The emission of this radiation from
the radionuclide is called nuclear decay. For medical use gamma decay is used
as its high energy electromagnetic waves and can pass through the body. They
need a reasonable half-life, long enough to produce enough activity to obtain
adequate imaging, but short enough to minimise the patient’s exposure to
ionising radiation.¹?

Image
acquisition

See figure 4.

V/Q Scan
Technique

A ventilation/perfusion scan, also called V/Q, is a
scan of the lungs performed in two stages. Firstly during ventilation, which
assesses lung airflow, the patient inhales a radioactive gas/aerosol. The
particles of which are small enough to travel to the distal bronchi in the
lungs and reflects true airflow. Secondly during perfusion, which assess the
lungs blood flow, the patient is injected with Tc-99m MAA intravenously. The
MAA particles get lodged in the lungs capillaries, this reflects gaseous exchange
and shows which parts of the lungs are receiving correct blood flow.

CTPA vs V/Q
Scan: Clinical Effectiveness, Sensitivity and Specificity

Although V/Q scans have a much lower dose there is
growing demand being put on CT departments with CTPA requests. This may be due
to being able to visualise the PE itself

within the vessels on a CT image whereas you can only
see the effects of a possible PE on a V/Q scan. CT is available more so than a
V/Q, there is more likely to be a radiographer on call in CT in an emergency
than in nuclear medicine.  The results
are available faster, are more accurate and the scan can diagnose or rule out
other pathologies within the thorax such as acute pneumonia, pleural effusion
or malignancy. ²? PE is more common in women. Including pregnant, and
with the higher dose from CT there is a concern about radiation the breast tissue,
see figure 8. The urgency of the symptoms must be considered but it could be
beneficial to wait for a V/Q scan to reduce breast cancer possibility in the future.
With a CTPA scan, iodinated contrast has to be administered intravenously which
has some contraindications, see figure 9. Pregnancy must also be considered
when comparing the modalities, pregnant woman may have hyper dynamic circulation
meaning inadequate opacification of the vessels in CT. Also dose is an issue as
the foetus is more at risk to ionising radiation than the mother.

Many
things need to be considered when choosing a modality to diagnose or rule out
PE. PE can be fatal and can be difficult to diagnose, so diagnosing and
treating correctly it extremely beneficial to the patient. CTPA and V/Q are
both modalities that are proven in capably diagnosing PE. ALARP should be
practiced when imaging younger patients, especially women and pregnant women. If
non-urgent the wait for a V/Q scan seems beneficial in terms of dose.