Introduction within the brain weakens and the



For this essay the patient has been given the name Lilly.
The purpose of this essay is to understand how two different modalities, such
as computed tomography angiography and magnetic resonance angiography, can
alter 36-year-old Lilly ‘s treatment for suspected subarachnoid haemorrhage
(SAH). In addition to finding out, out of the two modalities, which will be the
better suited modality for the patient in questioning.

The subarachnoid lies between the brain and the tissues
which enclose the brain; the pia mater and arachnoid mater. The surface of the
brain contains cerebrospinal fluid which acts as a cushioning. When a
haemorrhage occurs within this layer, prompt and accurate imaging is required,
as delay can lead to the patient facing a coma, paralysis or in the worst case,
death. Lilly was complaining of nausea and a ‘thunderclap’ headache. Some
symptoms that indicate SAH are confusion, loss of alertness and vomiting. If
any of these symptoms are combined with an intense headache, it leads to the
belief of a SAH (AARP,2017). The A/E consultant has requested an urgent CT scan
which is the first line of diagnosis for
a suspected SAH, however it should not be relied upon on its own. MRI is
believed to be more sensitive to blood in the subarachnoid, than CT(Gaillard,2017). 

When Lilly was being rushed to A/E, after collapsing, her
husband confirmed that intracerebral aneurysms have previously occurred within
the family and that she was facing a lot of stress at work. A cerebral aneurysm
occurs when a blood vessel within the brain weakens and the blood flow causes
the vessel’s wall to protrude out. This then can lead to a SAH when the vessel
ruptures and causes bleeding onto the brain. Facing stress at work lead Lilly
to smoke more; both these factors combined could have increased Lilly’s blood
pressure, which increases the likelihood of an aneurysm forming. As her GCS was
11; which indicates a moderate injury/ level of responsiveness (BrainLine,2017)
and not at the normal rate of 15, the assumption of her having a haemorrhage
increased, hence immediate CT scanning and interpretation of the image was
suggested (NICE 2017). When Lilly regained consciousness she had several episodes
of vomiting and started feeling numbness all over her body. All these factors
and symptoms combined further increases the suspicion of a SAH. The sooner the
images are taken and interpreted, the sooner surgery can take place for either endovascular
coiling or clipping surgery, both of which are treatments to prevent further
bleeding (NHS,2017).

Computed Tomography (CT)

A CT scan is provided for Lilly as it is the first step
taken for a suspected brain haemorrhage and is highly sensitive to SAH. It
combines a series of x-rays which form a thorough 3D image, using coronal and
sagittal planes. In emergency cases It is useful to detecting blood in and around
the brain and locating the haemorrhage. In majority of cases, the location of
the blood correlates with the site of the aneurysm. It is painless, fast and
easy to set up, allowing quick diagnosis which can save lives. The sensitivity
and specificity are said to be 100% within the first 6 hours of the ‘thunderclap’
headache and reduces to a 50% after a week. The sooner the image is taken, the
more accurate the diagnosis will be, as Lilly was rushed into the hospital, her
CT scan was taken within the 6-hour time frame. Severe anaemia or a
small-volume SAH can result in a false negative scan which effect around 10-15%
of patients, hence this should be taken into consideration. Another scan should
be taken alongside the initial CT scan to provide further confirmation for SAH
and help plan the treatment. If the CT scan has shown no SAH to be present, a
lumbar puncture (LP) can be performed (figure 1). LP takes a sample of the
cerebrospinal fluid to identify the presence of red blood cells which indicates
a SAH has taken place and is more accurate 12 hours after the symptoms are
presented, in comparison to doing the procedure 2 hours after, which can
indicate a false negative. The procedure is painful and time consuming. Drawbacks
with timing for the procedure and the different scanning modalities should be
taken into consideration, along with swiftly obtaining the diagnosis of the
patient for treatment, as delay can lead to consequences for the patient (Becske ,2017). For Lilly her CT scan showed a positive SAH
presence in the posterior region, hence a LP procedure was not needed.  






Lilly’s CT image had
similar characteristics to the one in figure 2 (with the presence of SAH).
Blood from a haemorrhage appears white, which is high attenuation. Radiodensity
is measured in Hounsfield Units (HU). The brain has 40 HU and blood is said to
be within the range 55-75 HU which will appear brighter on the image
(University of Wisconsin, 2011), hence confirming a SAH on Lilly as she had
brighter areas in her CT scan.

Computed Tomography Angiography (CTA)

 A CTA scan can be done additionally, straight after a CT
scan, to find the aneurysmal source of bleeding. It uses an iodinated
intravenous contrast agent that is injected into the bloodstream of the arm/hand
to highlight blood vessels and tissues, which help identify the location shape
and size of the aneurysm/s. It is accessible and is more available than MRI (Tampieri et al, 2017).

 Its advantages include being; cost effective,
non-invasive (in comparison to angiography as no catheter is being inserted),
low risk of complications, reliably identifies aneurysms and is said to be
suitable for unstable patients (who may not be able to undergo angiography or
in emergency settings). CT is said to have better spatial resolution
than MRI, which will establish a greater sensitivity in finding smaller
aneurysms and smaller branching vessels. Another
positive is that CTA can be carried out on the same bed straight after the CT
scan, allowing there to be less time spent on moving the patient for each scan
(Meurer and Walsh,2014).

However, a CTA after a CT scan,
provides additional radiation which MRA does not do. Other negatives include
more time spent on scanning, Intravenous contrast being administered, and it can
identify aneurysms that may not be the source of the bleed (which could lead to
further testing that may not be needed) (Meurer and Walsh,2014). The risks that
need to be considered when administrating the contrast is whether the patient
has allergies/heart/renal problems which can cause further problems to the
patient, however Lilly’s husband was able to confirm that she has no known
problems and that a possible CTA scan can go forward. The iodinated contrast
material at an iodine concentration of 320-400 mg/mL is injected at a flow rate
of 3-5mL/second (Kapsalaki, 2012), which can also leak in the surrounding IV site and can irritate
skin and blood vessels; this can cause tissue damage as a result
(Hopkinsmedicine, 2017). Another factor that needs to be considered, is when a
negative CT head occurs within 6 hours, there is a high probability of a negative
CTA also occurring. Hence it should not be carried out until a SAH is confirmed
(Long and Koyfman,2015), however, this is not a problem in Lilly’s case as her
CT was positive.

For a SAH aneurysm, CTA is very
sensitive and specific as shown by these figures from a study carried out with
a 64-slice CTA; it was found that it was 98% sensitive and 100% specific for
aneurysms >3mm, if carried within 6 hours of the ‘thunderclap’ headache (Meurer
and Walsh,2014). Aneurysms that are <10mm tend to have low rates of rupture and are considered small. Nevertheless, from clinical practice it is more common for patients who have SAH to have small aneurysms. As indicated from the complete data gathered from Oxford university, aneurysms <10mm accounted for 84% of SAH within the last 5 years and the average size was 8mm from 1991-2016 (Bender et al., 2017). The sensitivity is strongly dependent on aneurysmal size and anatomic location of the aneurysm. Lilly's aneurysm was 7mm which has a high sensitivity and specificity rate in CTA and is considered to have a common aneurysm size. Magnetic Resonance Angiography (MRA) MRI uses magnetic fields along with radio waves to produce images, hence no harmful radiation is given to the patient, it is known for its ability to produce detailed images of the soft tissue. MRA is a MRI scan which shows specific blood vessels of the brain and visualises blood flow at the level of the circle of Willis. It identifies blood well; however, it is not usually used in emergency settings. MRA can also identify the different causes for a headache. It is a favourable method for unruptured intracranial aneurysms as there is little evaluation on ruptured aneurysms (Pierot et al., 2013), hence more studies may be needed to fully appreciate MRA. MRI is said to be better in contrast sensitivities than CT. Aneurysms and blood can be reliably identified without the use of radiation to the patient. MRA can also find the characteristics of the aneurysms such as; the size, location and the wall texture of the aneurysmal dome. MRA has a sensitivity of 87% and a specificity of 95%. The overall sensitivity increases for aneurysms larger than 3mm to 93-97% (Kapsalaki, 2012). Other advantages over CTA include no harmful iodinated contrast being used and no image degradation occurring from vascular calcifications or immediate bony structures (Zwam, 2012). On the other hand, MRA is similar to CTA in regard to recognising aneurysms that have no link to the headache. It isn't used commonly in an emergency department setting, hence less accessible in comparison to CTA. It has a lower sensitivity than cerebral angiography in discovering small aneurysms and it usually fails to detect posterior inferior communicating artery along with anterior communicating artery aneurysms (Becske ,2017). Other limitations include being liable to motion artefact, the lengthy time to carry out the scan and being unable to scan patients who carry metallic objects e.g. pacemakers. Patients who suffer claustrophobia or sensitive to loud noises may also find it difficult to go through the MRA scan procedure, as the MRI machine is noisy and places the patient in a tight space. Newer MRA techniques have been developed such as 3D Time-of-flight MRA (3D TOF-MRA) and contrast-enhanced MRA (CE-MRA) to improve diagnostic accuracy and sensitivity, along with the introduction of 3 Tesla magnetic field which allows the detection of aneurysms <3mm (Kapsalaki, 2012). Sensitivity of CE-MRA is 95% and 3D TOF-MRA 86% and both having alike specificity of 80%. The limitation of MRA occurs due to having a low viability during the acute phase of bleeding (Pierot et al., 2013). Development of scan techniques has allowed better detection of small aneurysms. CE-MRA uses a gadolinium-based agent; approximately 20mL that is injected over 8-12seconds to portray the neurovascular anatomy more accurately than 2D TOF-MRA e.g. Injecting gadolinium in CE-MRA to detect internal carotid artery stenosis was not a significant advantage to TOF-MRA, hence TOF-MRA may be a sympathetic alternative for patients who cannot be administrated contrast in CE-MRA and CTA (Babiarz et al., 2009). Treatment Treatment includes preventing rebleeding as it one of the early complications after a SAH. Rebleeding is prevented by bed rest, analgesia (painkiller) and sedation. Surgical procedures include: Coiling (figure 3): The coils are permanent and can undergo MRI. In rare cases coils can move and create an empty space within the aneurysm. Every 5 years follow-up angiograms and MRA will occur. Clipping surgery (figure 4): At the base of the aneurysm a clip is placed to close off the weakened area and like coiling it is permanent (Tampieri et al, 2017). The location of the lesion and the neck of the aneurysm usually determines the surgical procedure. Coiling is favoured for patients with comorbidities and posterior circulation aneurysms as surgical clipping has substantial illness and mortality rates associated with it. Surgery within 72 hours allows lower rate of rebleeding (Becske ,2017). Some aneurysms may require a combined approach. However, coiling tends to be first line of approach in Europe (Watson, 2017). Lilly was able to undergo coiling as it is more commonly practiced and is less invasive in comparison to surgical clipping.                                                                                            Conclusion The aim of this essay was to determine if CTA or MRA was better suited for Lilly. CT scan is provided as the initial method of choice for a SAH, a CTA occurring straight after CTA is accepted to be the initial method of evaluating a SAH patient by most centres, as MRA isn't readily available and time consuming in comparison. Lilly was known not to have any allergies or renal/heart problems; hence a CTA was undertaken; it also has a higher sensitivity and specificity than MRA as stated from the figures previously. It was found that she had a 7mm aneurysm in the posterior communicating artery in the CTA, although as previously mentioned, CTA usually fails to pick this up and MRA may have been a better option. Coiling is the best treatment for posterior circulating aneurysms and was provided for Lilly.


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