Future Development of Multislice CT

The trend towards a larger number of slices will not be driven by need to increase scan speed in spiral acquisition modes, But rather by new clinical applications that potentially become possible with these detector and system designs. Dynamic volume imaging becomes feasible , opening up a whole spectrum of new applications, such as functional or volume perfusion studies. Recently the manufactures introduced systems targeting application, again pursuing different technological paths to reach the same goal. Toshiba introduced a 320 slice scanner that allow covering whole oragan during one rotation. It based on prototype a scanner scanner with 256 x 0.5 mm detector element(Mori, 2004-2006). Siemens introduced a 128 slice scanner with dynamic spiral shuttle mode that allow data of 4D was acquiring large volume.The figure shows an example of a perfusion scan of complete brain acquired with that technology.

Whole brain perfusion study on Siemen SOMTOM definition AS +, Using dtector configuration of 128 x 0,6 mm, whole brain perfusion studies can be carried out by using a spesial spiral shuttle mode that uses a sinosoidal motion of the patient table to cover the wohle brainb for a period 30s

Dual-source CT

Dual-source CT (DSCT) was designed to overcome the limitation in gantry rotation time by doubling the number of X-ray sources and detectors. By mounting two X ray sources and detector arrays at angle of 90^oon the rotating time. This has the effect of doubling temporal resolution compared to standard MSCT. For example, a dual source scanner with gantry rotation time of 330ms will have a temporal resolution of 83 ms. Several studies have shown the value of DSCT in producing diagnostic image quality in patients with heart rates up to 100bpm,potentially still improved if the heart rate controlled

Dual source data are acquired simultaneously in the same relative phase of a patients cardiac cycle and at the same anatomical level. The two rows detectors in DSCT tend to vary in the entire filed view (FOV; about 50cm), whilst the other is restricted to a smaller central FOV. The radiation burden from DSCT is roughly equivalent to a similar single source MSCT acquisition.

 

Comparison of (a) single –source and (b) dual-source CT. Doubling the number of X ray tube/detector array pairs reduces the arc required to obtain an image to 90⁰. thus improving temporal resolution two-fold

Another important feature of DSCT is that the voltage and current of each x ray can varied individually, allowing dual energy CT (DECT) scans to be performed. This technique uses the phenomenon that tissues attenuate x ray beams of different energies to differing energies to differing degrees, depending on their density. Although not extensively validated, cardiac applications of DECT potentially include digital subtraction of calcium from coronary arteries (although spatial resolution is currently a limiting factor) and characterization of myocardial enhancement patterns.

Collimators Component on CT System

Collimation on CT Serves two purpose :

1. to reduce unnecessary radiation doses to patient
2. To ensure good image quality

There are two types of collimator :

1. Prepatient : the position of collimator between the xray source and the paient. Since x ray photons emitted from the x ray cover a very wide range in z, the prepatient collimator restrict the x ray flux applied to narrow region as it sis applied to the patient. For a single slice CT, it not only reduces dose to the patient. It also defines the slice thickness of t he imaging plane. For multislice CT, however, the slice thickness is defined by detector aperture instead of the collimator. Because nearly 99% of the x ray photons emitted from the xray tube blocked by pre patient collimator , x ray efficiency for CT is poor.
2. Postpatient : Typically, two kinds of collimators are used : in plane and cross plane.  The in plane collimation (grid)n is used by third generation CT scanner to reject scattered x ray photons. This technique cannot be employed effectively by foruth generation scanners due to a wide acceptance angle. This type sre place in front of the detector, focusing on the x ray source. Since the path of scateered raediation generally deviates from the original x ray photon (primary photon) path, plat block the photon from watering the detector.Cross plane collimation is employed by both third and fourth generation scanner. For 3^rd generation scanner, it servers mainly as additional z collimator to improve the slice sensitivity profile of scanner. The slice thickness is determined by combination of umbra and penumbra region. Because of the geomethric limitation, it is often difficult to design a prepatient collimator that provides very thin slices profiles for the scanner . The achieve this objective, additional collimation is employed near the surface of the detector to futher restrict the x ray beam to a narrow slice thickness. The disadvantage to approach is the penalty to the dose efficiently of the scanner because a portion of the x ray photons that pass through the patient is not used

A post patient cross plane grid is sometimes used by the fourth generation scanner to reject scattered radiation is a manner similar t the in plane collimation (grid) for the 3^rd generation scanner. Although the role of 4th generation scanners is fading, the use of the grid is likely to be employed by 3^rd generation scanner. As the volume coverage of the detector increases, the scatter to primary ratio (the amount of scattered radiation compared to primary radiation) will increase . In the not too distance future , it may become necessary for the multislice or volume metric scanners to employ suck collimation for scatter rejection

Gantry and Slip Ring

The gantry servers as the backbone of a CT System. With increasingly demanding scan speeds, requirement on the gantry performance have increased significantly (the centrifugal force increases with the square of the rotation speed). The CT Component mounted on the gantry weigh several hundred pounds. With such a large load , the gantry still must maintain angular accuracy as well as position accuracy. Angular accuracy requires the gantry to be free of significant vibration in all directions (both in-plane and cross –plane)/ Lets us consider, for example , clinical applications in which submilimeter slice thickness is required. Since the width of the x ray beam is less than a millimeter, the position of the x ray beam should not vary more than a small fraction of bean, width during gantry rotation to ensure true sub millimeter imaging.  The gantry needs to be stable within a fraction of millimeter for all projection angles.

Another key component of the CT System is the slip ring. Although the slip ring was introduced to CT in nearly 1980s to facilitate continues  gantry and sthe stat rotation , it became the de facto standard only after x ray tube flow between continuously rotating gantry and stationary CT component through electrical, optical or RF connection on the slip rings, The amount of data that must be transmitted is large. For a typical multi slice scanner there are roughly 1000 channels per detector row. Each rotation contains nearly 1000 projections or views. To avoid large storage side of the scanner, the data transfer rate must be sync with the data generation rate. For illustration the data transfer rate, R. for a 0,5 sec per revolution eight slice scanner is :

R=number of samples per rotation/time per revolution= 1.6 x 10 ⁷ sample/sec

If each sample is digitized in a floating point format (32 Bits), the bandwidth of the slip ring must be at least 500 Mbaud. The bandwidth required of slip rings will certainly increase with the introduction of new CT Scanners.

The X-ray Tube a Major component of CT Scanner.

The x ray tube is one of most component of CT System. Indeed, x ray tubes supply the necessary x ray photons to performs a scan. In the early days of CT, pulsed x ray tube was generally used. In the pulsed mode, x ray tube produced x ray photons in pulse of short duration. This pulsed time is varied between 1 ms and 4 ms. This was mainly to accommodate the fact that x ray detector could not Take measurement while signals were sampled.

Four CT Scanner Generations

From historical perspective, four scanner generations are known in CT. Head scanners, which the patient by translation and rotation of the measurement system with pencil beam(a) or a small fan beam(b) and fan beam system. The “3^rd Generation” featuring a rotating detector(c)has clearly outdistance the “4^th generation” ,which utilizes stationary detector rings(d)

 Most commercial translation scanner offered a field of measurement for head examinations only. The first attempt to scan typical body cross sections by CT was also based on translation rotation principle. The ACTA scanner (automatic Computerized Axial Scanner) sampled a field of measurement with 48 cm diameter in a six minutes scans taken during a single brathhold. The instruction of fan beam scanning was the solution to this problem.

Instead of sampling a transmission profile, i.e a projection , by pencil beam with translator motion, a fan beam and a larger detector are were used to measure a complete projection simultaneously. In this approach, the available x ray power is utilized much more efficiently. The translator motion becomes obsolete and the system only executes a rotatory motion. The first whole body scanner with fan beam system came to market

 In 1976 and with this envisaged goal of 20 second per image scan time was achieved. In the first scanners of this type both the x ray tube and the detector rotated around the patient demanding  higher technical efforts but offering advantages with respect a component cost and image quality and image quality (3^rd generation). Only a little later scanner followed with a ring like stationary detector fully encircling the patient, so that only the x ray tube rotated (fourth generation). Only a little later scanner followed with a ring like stationary detector fully encircling the patient, so that quickly accepted, and translation rotation systems meanwhile disappeared almost type is over, both with respect to numbers installed and more importantly, with respect to development of multi row detectors, which will be discussed below, the third generation has prevailed

Filter Component of CT System

The x ray photons emitted from the x ray tube exhibit wide spectrum;many soft (low energy) x rays are present. The low energy x rays mostly absorbed by the patient and contribute little to the detected signal. There fore, it sis necessary to remove these soft x ray to reduce the dose to the patient. To achieve this objective, most CT manufacture employ additional x ray filtration to improve the quality of the beam. The most commonly used filters are the flat filter and the bowtie filter. The flat filter is typically made of copper or aluminum, and is placed between the x ray source and the patient. The flat filter modifies the x ray spectrum uniformly across the entire field of view. Observing the fact the the cross-section of a patient is mostly oval shaped, some manufactures employ a bowie filter to modify the intensity of the x ray beam inside the field of view to further reduce dose to patient. 

To achieve this objective, most CT manufacturers employ additional x ray filtration to improve the quality of the beam. The most commonly used filter is the flat filter and the bowtie filter. The flat filter is typically made of copper or aluminum and is placed between the x ray source and the patient. The flat filter modifies the x ray spectrum uniformly across he entire field of view. Observing the fact that cross section of the patient is mostly oval shaped, some manufactures employ filter to modify the intensity of the x ray beam inside the field of view to further reduce dose the patient.

 

Stress Testing on CT & Changes in Reading Images Comparing 64-Slice to 256-, 320-Slice CT

Stress Testing upon CT

Min said the actual higher-slice systems may present an advantage in perfusion image resolution compared with 64-slice, because of their capability to image the entire coronary heart at once. Up until now, he explained only 64-slice systems happen to be evaluated for CT perfusion image resolution. The higher-slice systems will probably show CT can be used for high-quality heart perfusion imaging, but studies are needed. These systems also provide additional perfusion applications with regard to liver and mind imaging. This may assist justify the higher price, if shared in between several specialities.

The actual Aquilion One 320-slice scanner may complete an entire physique scan in 3 seconds and a coronary heart scan without desk movement in less than fifty percent a second. Poon said this particular speed and protection will allow CT adenosine stress image resolution, which is primarily restricted to magnetic resonance imaging (MRI) as well as nuclear imaging.

“Nuclear continues to be the standard of treatment (for perfusion imaging), however the temporal spatial resolution isn't good at all,” Poon stated. He added the reduced resolution can skip small perfusion defects which are easily seen around the 320-slice images.

Changes in Studying Images

The minor misalignments in 64-slice cardiac pictures lead to many items, which radiologists and cardiologists consider when reading. Nevertheless, a 320-slice scanner picture eliminates such items. Poon said what you see may be the actual, full coronary heart frozen at a particular time.

“I had to alter my reading routines,” Poon said. “I was so accustomed to reading 64-slice that I simply assumed many things We saw were items. But now what I 'm seeing is a sore; it’s not an artifact anymore. You have to look at the 330 under a different mild. You can’t assume you will find artifacts anymore.”

The actual University of Chi town Medical Center uses 16-, 64- as well as 256-slice systems. Some individuals had exams carried out at different occasions by two or even all three of the methods. Vannier said the radiologists in comparison these images as well as were able to pick out the actual 256-slice system immediately. Just about all motion was freezing and the images didn't have the “blocky” misaligned sewing artifacts, he said. Our prime motion area close to the diaphragm also contributed to movement blur that made an appearance as a thickening of cells, which would have resulted in an improper prognosis. “We started asking ourself if we were phoning things abnormal once they were actually Okay. It was really sobering,” Vannier stated.

Cardiac CT With >64 Slices

Here is advantages and disadvantages of Cardiac CT With >64 Slices

256 Slice CT

1. Currenly the only commercially available 256 Slice CT Scanner(Philips) has a 128 detector configuration with double sample. Air bearings allow a rapid gantry location time of 0,027s, giving a temporal resolution of 135ms. The unit is larger that typical 64 slice scanner due to the wider detector array and the need with stand the high g-forces generated by rapid gantry rotation.
2. Z axis coverage is double(8 cm) that of a typical 64-slice scanner, but still short enough to require at least two gantry rotations to cover the heart completely, images may still be susceptible to step and misregistration artifacts. However , as fewer overlapping slices are required to cover the heart, scan acquisition time reduced.

320 Slice CT

1. The only commercially available 320 detector scanner (Toshiba) has ab ultrawide detector width of 16cm(320 x 0.5mm), which is wide enough to cover the entire heart in a single rotation without the need to move the patient through the gantry. The 350ms gantry rotation time gives a temporal resolution of 175ms using half scan reconstruction . The scanner has several technology advantages
2. Low radiation dose :single rotation cardiac acquisition allow complete data set to be obtained in a single prospective pulse. As there is no need for overlap between successive rotations, radiation dose is reduced(typically 1-3 mSv)
3. Arrthyhmia management :single rotation cardiac acquisition enables the scanner to wait for an appropriately long R-R interval before the acquisition ‘pulse’. This allows a static, isophasic datasheet in all patients providing ventricular rate is well controlled(heart rate <70 bpm)
4. Prospective single-beat functional : ultrawide detector array allows acquisition of functional data from all phases of cardiac cycle using only a single heartbeat

However , there also several disadvantages

1. The wide cone-beam angle (>14^o) requires substantial computer processing power to produce artifact -free  images, particularity to corrector scatter
2. Very wide detectors are substantially heavier than conventional detectors, resulting in higher centrifugal forces during rotation
3. As a result , temporal resolution is limited to 175 ms restricting single beat  prospective scanning to patient with heath rate<65bpm.

How a CT Scan?

Doing a CT scan is the reason CT (computed tomography) scan to scan a clear picture of the various internal organs, including bone, a variety of tissues, and blood vessels. The scans are very useful in the diagnosis of various diseases and conditions, including cancer, heart disease, vascular disease, stroke, reproductive problems, and digestive problems.

Unlike conventional scanning X-ray, CT scan is able to scan the organs in much more detail to allow doctors to diagnose correctly. CT scans are not always conducted in a serious situation and does not necessarily mean the patient is suffering from a serious condition. CT scan is a diagnostic tool regardless of the severity of the patient.

CT scan machine looks like a large tube which is equipped with some sort of table where the patient is lying down. An X-ray scanners at the tube and then scan the patient's body. Every single round of scanning about 1,000 pictures taken of the body. This image is then compiled by a computer to form a two-dimensional cross-sectional images of the body which are then analyzed.

Preparation of CT Scan

CT scans do not require elaborate preparation. Will be very helpful if the patient wear loose clothing, remove the metal accessories such as jewelry or glasses, and drink only clear liquids (water) before the CT scan. Depending on the reason for a CT scan, the patient may be asked to swallow a contrast agent prior to scanning. Make the scan contrast agents have a better resolution to facilitate the analysis. Contrast agents are often given to patients with CT scans that have digestive problems.

Consideration

CT scans usually takes 10 to 45 minutes, or longer in some cases. For this reason, CT scans were performed only when doctors feel the procedure is really necessary. Due to radiation exposure of the body, a CT scan is generally performed at most twice a year. Pregnant women in the first trimester is recommended not to undergo a CT scan or other X-ray procedures. While helping to get better results scanning, contrast agents can cause allergic reactions such as nausea and vomiting. In the event of allergy in patients who underwent more than one CT scan should consult a doctor because of an allergic reaction to contrast agents for future use is generally more severe than the first reaction

How The difference between PET Scan and CT Scan?

PET and CT scans include the use of high-tech equipment to picture the internal structure of the physique. PET scan can expose changes at the cellular degree, while the CT scan showed a general change in the overall structure of the body organ or tissue. Because of this capability of PET scans may detect the disease at an initial phase that can not be done by normal CT scan.

PET Scan

Dog stands for "positron emission tomography". Scanning (check out) uses a small amount of radioactive material tend to be inserted in the body by shot, inhaled, or swallowed. Dog scan will show where the radioactive materials accumulates to detect issues in blood flow, oxygen usage, and metabolism.

CT Scan

CT means "computerized tomography" (also referred to as CAT scan). CT check out uses X-rays to obtain cross-sectional images from the body. These images can be seen individually (two dimensional) or even combined to produce three-dimensional view.

Perform

PET scans can identify the spread of cancer, heart disease identified, and examined mind disorders, and nervous system.As the CT scan can quickly identify accidents due to accidents and other stress, evaluate the condition of individuals after surgery and radiation treatment, as well as the detection of most cancers and vascular disease.

Impact

PET and CT scan is really a procedure that has been used for decades as well as proven to have minimal negative effects. But still there are some things to consider. Each types of scanning involves the utilization of radiation. CT scans may somewhat increase the risk of cancer improvement. However, this risk is extremely small compared with the benefits of the actual CT scan itself. CT scan ought to be used only when necessary. Expecting or lactating women ought to inform their doctor prior to undergoing PET or CT check out.

Scanning Combination

PET checking can scan combined with CT check out. This combination makes the diagnosis can be created more accurately.

X-ray Detector on CT Sanner Component

The x ray detector is equally important to the performance of a CT Scanner. Like x ray tubes, detector technology has experienced tremendous growth over pas 30 years. The earlier vintage detectors had smaller fields of view mainly to perform head scans. The later vintage detector must have larger fields of view to handle large bodies.

Detector of thirds generation scanner use either high pressure inert gas (usually xenon) or solid state scintillates coupled with photo bodies. The operating principle of xenon detector is illustrated on the picture. The detector is constructed with May thin tungsten plates inside a high pressure xenon chamber. Every other plate is connected to a high voltage dc supply. For example on GD 8800 detector, the plate connected to 500 v. The remaining plates are electricity floating ans are at approximately 0 V. A pair of low –high voltage plates forms a single detector cell. When x ray photon strikes a cell, it causes ionization of xenon gas in a photoelectric interaction. The interaction release energetic photo electrons to ionize more ion gas ions. The ionized gas xenon nuclei are collected by the o V plates and free electrons are collected by positive biased plated (500 V) to produce current signal. The bias voltage across the plates it set high enough such that are ion collected rapidly by plates with little recombination of positive and negative ions in the gas. At the some time, the bias voltage is below the point of the avalanche effect, where a nonlinear magnification of signal occurs. Improperly biased region, the amount of ionization is linearly proportional to the total energy of the absorbed x ray photons.

Solid detectors overcome many of the shortcomings of the xenon detector. The detector is made of small blocks of scintillating materially, such as CdWO4, Gd2O2S or HiLight, coated with reflective material and coupled to set of photodiodes.

The light photon produced by the scintillation process travel in all directions. The scintillatator is coated with highly reflective material to direct the light toward the photodiodes at the bottom of the detector. Because of the reflection and absorption in the scintillator , only small percentage of the light photon reaches the photodiodes to produce electrical signals.

CT Scanner Component: High Voltage Generator

To Produce and maintain the desired x ray flux output, the voltage and current to the x ray tube must be kept at a constant or desired level. Unfortunately, the voltage of the power fluctuates sinusoidally from negative to positive relative to the front. This is undesirable for two reason :

1. If cathode voltage becomes positive relative to anode, electron emitted from the hot anode will accelerate toward the cathode, causing the cathode filament to age prematurely and generate undesired x ray photons(located significant off the x ray focal spot)
2. High fluctuation of the voltage causes significant difficulties in the calibration and data conditioning for the CT systems. Note that the energy spectrum of the x ray flux is linked closely to the voltage potential across the cathode and anode. When significant variation exist in the power supply voltage, the x ray spectrum changer constantly from view to view. This makes some of correction process, such as beam hardening, nearly impossible.  To ensure adequate image quality and prevent premature x ray tube failure, the power supply to the x ray is typically rectified and additional efforts are made to ensure the voltages is as close to a constant as possible ( to simulate a dc power supply). The voltage generator from single phase rectified typically has ripple of 100 %. Ripple is defined as t he peaks to valley voltage over peak voltage