F.Jannath also makes it obvious to differentiate between

F.Jannath and A.Riaz prepared and
wrote the report, Comparison of CT and MRI in vascular pathologies. F.Jannath
carried out research of abdominal aortic aneurysms and the relevant cases.
A.Riaz carried out research of renal artery stenosis and the corresponding
cases.

Attributions

Taking
into account recent healthcare systems around the world, CT and MRI have proved
to be the most reliable and efficient methods for diagnosis in various hidden
pathologies. The research conducted for this report was aimed for general
features of CT/MRI and the corresponding vascular pathologies.

 

For
RAS, considering the cases analysed, MRI gives a better definition of the
pathology. CT, on the other hand, clearly identifies the stenosis and position
and is also a reliable method to diagnose RAS. However, in my opinion, MRI
locates the precise position of the stenosis and also makes it obvious to
differentiate between the blood vessels.

 

To conclude, for AAA’s,
CT is the golf standard, it provides a more detailed scan as it has higher
resolution. CT angiography determines the exact size of the aneurysm with close
to 100% sensitivity and specificity and also shows the involvement of any
surrounding anatomy. A vital difference between MRI and CT is that CT allows for
the detection of calcification. Which is an important feature in AAA’s as it
affects the peal wall stress of the aneurysm.

                                                                                                                                                        
V.        
Conclusion

Table
i Comparison of CT and MRI 16

“Table
1” summarises the main differences of CT and MRI. Regarding image specifics, CT
has higher resolution and less motion artefact due to the nature of the speed
of the scans. CT is excellent for visualising bones but also good for
visualising soft tissues, especially with the use intravenous contrast dye. MRI
on the other hand is excellent for detecting slight differences between tissues
and it better than CT in providing higher detail in soft tissues. The risks
associated with CT in terms of radiation are higher than MRI, as CT uses x-rays
which are ionising, unlike MRI which uses magnetic fields and radio waves.
However people may experience allergic reactions to the contrast dye used in
MRI although it is rare, and also those suffering from kidney or liver problems
are not recommended to undergo an MRI scan as the contrast can be damaging.
Allergic reaction due to contrast used in CT is also possible and is more
common than in MRI because CT uses iodine in the contrast. CT scans are taken
significantly quicker than MRI scans, and therefore exposure to x-rays is not
elongated. Whereas MRI scans can take up to 90 minutes and can also be noisy
and uncomfortable. People who have metal implants inside their bodies such as
pacemakers and some prosthetics are not suited for MRI, whereas CT is not
suitable for pregnant women and children because of the damaging radiation. In
terms of cost, MRI is much more expensive as MRI scans are at a much larger
scale and therefore more materials and equipment is required.

                                                                                                     
IV.       
Comparison Of Medical Imaging Modalities

To
examine the two images (“Fig. 5” and “Fig. 6”) in contrast, both imaging modalities
are reliable to detect and diagnose a renal artery stenosis. However, MRI gives
a precise location and the size of stenosis whereas CT detects the presence
only. MRI provides more accurate information of quantity of blood vessels
present, which is important information for the diagnosis and treatment of
stenosis. Also, MRI efficiently distinguishes between normal and abnormal
tissues.

 

This
MRI image effectively identifies normal/abnormal tissue and blood vessels, with
high definition. The size and location of the stenosis can be detected
efficiently.

 

Furthermore,
the image shows a dark/light left kidney, which implies a normal functioning
kidney. However, the imaging produced for the left kidney is of light grey,
which suggests that this kidney is malfunctioning possibly due to the lack of
oxygenated blood.

 

         Figure 6
MRI scan image of a 45 year old male 15

 

 

 

 

Analysing
the image further, the imaging produced shows a highly visible, white dense
aorta. The surrounding blood vessels to and from the kidney are also highly visible.
A stenosis can be accurately detected in the artery leading to the left kidney.
The exact location of the stenosis can also be determined for treatment.

 

                Considering another RAS case,
“Fig. 6” shows an image of MRI scan that is taken from a 45 year old male with
symptoms relating to renal artery stenosis. The image precisely shows the
location of the kidneys and surrounding tissue. Imaging of numerous blood
vessels is also produced by the scanner. The size of the kidneys can be
accurately measured and compared using this MRI image.

 

This
CT image visualizes clearly large organs, tissue and blood vessels. The
presence of stenosis can be easily detected. The sizes of the kidneys can also
be measured (not accurately).

 

The
CT image produced by the scanner reveals complete occlusion of the left renal
artery. Due to the narrowing of the arteries, the image shows a ‘blank black
part’, which leads us to believe that stenosis of the artery has occurred.
Furthermore, the left kidney does not appear to be fully grey as the right
kidney. This suggests damage to the kidney tissue due to lack of oxygenated
blood in the left kidney. On the other hand, the image of the right kidney
(light/dark grey) suggests a normal functioning kidney. The blood vessels
linked to the right kidney are also visible.

 

             Discussing vascular pathologies
and their diagnosis in further detail, “Fig. 5” shows a CT scan image taken
from a 65 year old male, with symptoms of renal artery stenosis. The two kidneys
are easily visible in the image. Surrounding organs and tissue or fluid are
also visible, mostly in light/dark grey colour. The dense white part leading to
the kidneys is the aorta.

               

                    Figure 5 CT scan image of a 65
year old male 14

 

      

 

B.    Renal
Artery Stenosis

 

The
two modalities are both sufficient in identifying the AAA and determining its
size. However, CT demonstrates higher contrast and therefore important details
such as calcification can be identified.

 

Here
in “Fig. 4” the lumen can be seen and hence the diameter of the AAA can be
measured.

 

Figure 4 MRI scan 13

 

 

“Fig.
3” shows an MRI scan of an AAA, of a 65 year old male. The MRI visualises the
soft tissues and the surrounding anatomy. The kidneys can be seen as well as
the renal arteries, so any involvement of surrounding anatomy in the
development of the AAA can be determined. The AAA itself can be identified
because of the contrast between the organs which are darker and the arteries
which are brighter and the dilation in the aorta is very clear. However to be
able to determine if there is thrombosis or calcification and also the diameter
of the aneurysm a cross sectional MRI is required. Which is shown in Figure…

 

                    
Figure 3 MRI scan image of 65 year old male 13

 

 

“Fig. 2” also shows a CT scan of a
male with an AAA. Similarly, to figure …, the aorta is clearly visible and
therefore diameter of AAA can be measured. Surrounding the aorta is
circumferential mural thrombosis, this occurs in 70%-80% of AAA patients and can cause hypoxia in places where
thrombosis entirely covers the aorta. This can also contribute to stress in the
arterial wall. The contrast between the aorta and thrombosis is very high
allowing them to be distinguished from each other. The small white dots surrounding
the thrombosis called calcification are calcium deposits which increase peak
wall stress. Thrombosis and calcification are both considered in the evaluation
of wall stress for the risk assessment of AAA rupture which is why it is highly
important that these features of AAA can be identified in screening.

Figure 2 CT scan image of a male 12

 

“Fig. 1” shows a CT scan of a 70 year old male with AAA. The
aorta is clearly visible hence the diameter of AAA can be measured. The grey area surrounding the
aorta is a fat strand,
which suggests a contained
rupture as no extensive retroperitoneal haemorrhage Can be seen, the haemorrhage would have been characterised  An
intra-luminary and extra-luminal air crescent is also visible. Moreover, there are extra-luminary air bubbles, which also suggest a
possible leak of the aneurysm. These findings highly suggest an infected AAA,
with gas producing bacteria. Surrounding anatomy can also be observed from the
scan, and the spine is clearly visible as it is bright white.

 

Figure 1 CT scan image of a 70
year old male 11

 

 

A.    Abdominal
Aortic Aneurysms

                                                                                                                 
III.       
Diagnosis Of Vascular Pathologies

For the diagnosis of AAA and RAS, MRI is considered to be
one of the most useful imaging modality. It accurately identifies abnormal blood
vessels and tissue.

MRI is highly recommended and used in the diagnosis of
vascular pathologies. It is suitable to examine the brain and many internal
organs due to its ability to define anatomy extensively. Also suitable to
examine blood vessels for blood flow. Although MRI is costly, it is greatly
used worldwide seeing that it uses non ionizing radiation throughout the
process (which is harmful for human health).

Two thirds of the body is made of water (oxygen and
hydrogen). The protons located in the centre of the water molecules are highly
attracted towards a magnetic field. During an MRI scan, the scanner produces a
strong magnetic field. This causes the protons in the body to line up. Short
bursts of radio waves are then sent to certain parts of the body which are to
be examined causing the protons to be knocked off alignment. The radio waves
are then turned off resulting in the re-alignment of the protons. Consequently,
radio signals are produced which determine the exact location of the protons
and are detected by the receivers 10.

Magnetic resonance imaging (MRI) is a type of diagnostic
san that uses strong magnetic fields and radio waves to produce detailed images
of inside the body. It measures properties of high hydrogen tissues. MRI can be
used to examine almost anybody part such as bones, tissue, muscle, blood
vessels, brain, heart, lungs and breasts. The person to be examined is laid
flat on a bed. The bed moves inside the scanner. The scanner is controlled by a
computer.

B.    Magnetic
Resonance Imaging

The cross-sectional images generated
during a CT scan can be reformatted in multiple planes, and can generate
three-dimensional images which can be viewed on a computer monitor, printed on
film or transferred to electronic media. CT has proved to be highly effective
in the diagnosis of abdominal aortic aneurysms and renal artery stenosis due to
its imaging properties. Differences between normal/abnormal blood vessels and
tissues is efficiently distinguishable through CT with the aid of processing
techniques.

CT
in the recent years has allowed detailed evaluation of vascular diseases.
Multiphase contrast enhancement plays a particularly important role in
revealing abnormalities associated with these diseases 9.
Usually a dye is injected to the patient to aid multiphase contrast enhancement
and show blood flow. Recent implementations in CT include post-processing
techniques, such as multi planar reformatting, shaded surface display, maximum
intensity projections, and 3D perspectives of surface and volume rendering,
which simulate virtual intravascular endoscopy.

 

      
Computed tomography (CT) is a diagnostic imaging test used to create
detailed images of internal organs, bones, soft tissue and blood vessels. It
also one of the best methods to detect the presence of a tumor and determine
the precise size and location 8. The CT scanner looks like a big doughnut. The scanner includes an x-ray
tube on one side and a detector mounted on the other side. The patient lies
down inside the patient aperture which is normally 60cm to 70cm in diameter. A
narrow beam of x-rays is produced by the x-ray tube as the scanner rotates around
the tube and detector. This beam rapidly rotates around the body. Each
rotation is of 360° and takes about one second. The detector records the x-rays
exiting the patient’s body and creates a snapshot at one position. Many
different snapshots are created during one rotation. The data are sent to a
computer which reconstructs all the snapshots and creates a ‘slice’ image of
the particular body part. Computed tomography works on the x-ray principle.
Depending on the amount of absorption, different amounts of x-rays will pass
and leave the respective parts of the body A1.  Dense bones absorb most of the radiation,
while soft tissue and fat allow most of the x-rays to pass through them.
Therefore, bones appear as white on the x-ray mage and tissue appear in shades
of grey.

A.    Computed
Tomography

                                                                                                                             
II.        
Medical Imaging Modalities

 

The
imaging techniques used to diagnose renal artery stenosis also vary, the most
currently used method is intra-arterial digital subtraction angiography 7. The gold standard for the diagnosis of RAS is
considered to be invasive angiography. Similarly to abdominal aortic aneurysms,
ultrasonography is also used to screen for RAS, and the lack of ionising
radiation makes this method much safer. The type of ultrasonography used is
called duplex ultrasonography, where peak systolic and end diastolic velocities
of the renal artery and the ratio of velocities in the renal artery to the
aorta are obtained. This method boasts high sensitivities of 92.5 to 98% and specificities
of 96% to 98%. Several factors however can reduce the image quality such as
obesity and recent food intake.

 

Due
to the high mortality rate associated with AAA’s, it is essential that they are
diagnosed accurately and early to prevent further complications and to improve
the outcome of the patient’s health. The main roles of imaging in AAA diagnosis
are; the detection of AAA, monitoring the growth rate of the aneurysm,
preoperative planning and postoperative follow-up. History of AAA’s show that
as the aneurysm expands in size, the rate of expansion becomes greater and
therefore the likelihood of a rupture also, which is why regular monitoring
using imaging is required 5. In England, screening
for AAA is offered to men once they turn 65. The screening is highly encouraged
especially for those who have a history of smoking. The screening involves a
quick, painless ultrasound of the tummy and is very important in identifying an
AAA before it becomes bigger or bursts. Ultrasonography is the standard method
used in the diagnosis of AAA and in the monitoring of any known AAA’s and has a
high sensitivity of nearly 95% and specificity of nearly 100%.  Aortography has also been used in the
diagnosis of AAA’s, in the past to evaluate the stage of AAA’s before operating
and more recently to address issues not resolved by less invasive methods,
issues such as; other nearby vessel stenosis’s 6.
Another imaging modality used is CT, which is considered the imaging gold
standard, further advances in CT including helical CT and CT angiography
provide significant advantages over traditional, such as the development of 3D
images of the aneurysm and an ability to produce more rapid scans.
Additionally, CT angiography and aortography can determine the size and
surrounding anatomy of the aneurysm, which aids in the process of selecting the
right candidates for the use of endovascular stent grafts.

 

Renal
artery stenosis is the narrowing of one or both the renal arteries. It is a
major cause of secondary hypertension and it is often caused by atherosclerosis
in cases of patients older than 55, which is a hardening of arteries caused by
a build-up of plaque, and less often by fibromuscular dysplasia, which is the
abnormal growth of tissue within the artery wall, usually in younger patients 4. Secondary hypertension is unlike primary
hypertension, the most common form of high blood pressure for which the cause
is unknown 5. The endothelial damage responsible
for atherosclerosis is not clear, however some contributors may be; smoking,
diabetes, viral infections and immune injuries. 

 

Abdominal
aortic aneurysms are quite common and can be life threatening. Aneurysms can be
simply defined as a focal dilation in an artery, hence AAAs are a result of the
dilation of 3cm or more of the abdominal aorta, which is located in the
distance between the diaphragm and the aortic bifurcation 1.
The cause of AAA is the failure of the main structural proteins in the aorta;
elastin and collagen, however the events that lead to this failure of the
proteins are not yet fully known or understood. Although, some biological
processes have been identified that contribute to AAA, these include;
inflammation, vascular smooth muscle cell (VSMC) apoptosis, extracellular
matrix degradation and oxidative stress. If left untreated and the expansion of
the aneurysm progresses, the aortic wall continues to weaken and ultimately
becomes unable to withstand the blood pressure which will likely lead to a
rupture. AAA is asymptomatic and therefore many of them are identified via
diagnostic imaging, ordered for different reasons 2.
Patients who are more prone to developing AAA are men older than 65 years who
have peripheral atherosclerotic vascular disease. A recent review found that
the strongest risk factor for AAA that can be controlled, is smoking. Others
include, age, male gender, family history of AAA, coronary artery disease,
hypertension, peripheral artery disease and previous myocardial infarction 3.

 

                                                                                                                                                      
I.         
Introduction

Abstract — The purpose of this report is to compare the features
of CT (computed tomography) and MRI (magnetic imaging resonance) in the
diagnosis of aortic aneurysms and renal artery stenosis and to determine which
modality is more effective in diagnosing these diseases. Both CT and MRI are
widely used in the medical setting as a tool to help diagnose vascular
pathologies, including the ones discussed in this report. The features discussed
include image contrast, spatial resolution, scan duration, risks associated etc.