MDCT

In the last years all new high frequency resolution (transform) based coding systems use MDCT techniques (see below) instead of DFT orDCT. 

Time domain aliasing cancellation based filter banks. The Modified Discrete Cosine Transform (MDCT) was first proposed in [Princen et al., 1987] as a sub-band/transform coding scheme using Time Domain Aliasing Cancellation (TDAC). It can be viewed as a dual to the QMF-approach doing frequency domain aliasing cancellation. The window is constructed in a way that satisfies the perfect reconstruction condition ... 

MDCT or similar schemes are used in several audio coding systems [Brandenburg, 1988, Mahieux et al., 1990, Brandenburg et al., 1991] [Davidson et al., 1990, Iwadare et al., 1992] because they combine critical sampling with the good frequency resolution provided by a sine window and the computational efficiency of a fast FFT-like algorithm. Typically, 128 to 2048 equally spaced bands are used. 

As a further advantage of MDCT-like filter banks it should be noted that the time domain aliasing property needs to be valid for each half of the window independently from the other. Thus hybrid window forms (with different types of window functions for the first or second half) can be used. This leads to the realization of adaptive window switching systems ([Edler, 1989], see below). 

The MDCT is known under the name Modulated Lapped Transform ([Malvar, 1990]) as well. Extensions using an overlap of more than a factor of two have been proposed [Vaupelt, 1991, Malvar, 1991] and used for coding of high quality audio [Vaupelt, 1991]. This type of filter banks can be described within the framework of cosine-modulated filter banks ([Koilpillai and Vaidyanathan, 1991][Ramstadt and T anem, 1991, Malvar, 1992]). 

In ISO/MPEG Layer 3, a different approach to hybrid coding has been used (see Figure 2.9. To ensure compatibility to Layers 1 and 2, the same polyphase filter bank is used as the first filter in the hybrid filter bank. Each of the 32 polyphase subbands is normally further subdivided into 18 frequency lines using an MDCT. By using the window switching technique described below the subdivision can be switched to 6... 

The technique is based on the fact that alias terms which are caused by subsampling in the frequency domain of the MDCT are constrained to either half of the window. Adaptive window switching as used in Layer 3 is based on [Edler, 1989], Figure 2.10 shows the different windows used in Layer 3, Figure 2.11 shows a typical sequence of window types if adaptive window switching is used. The function of the different... 

The short window has basically the same form as the long window, but with 1/3 of the window length. It is followed by an MDCT of 1/3 length. The time... 

Filterbank. A modified discrete cosine transform (MDCT/IMDCT) is used for the filter bank tool. The MDCT output consists of 1024 or 128 frequency lines. The window shape is selected between two alternative window shapes. 

Iwadare et al., 1992] Iwadare, M., Sugiyama, A., Flazu, F., Flirano, A., and T.Nishitani (1992). A 128 kb/s Hi-Fi Audio CODEC Based on Adaptive Transform Coding with Adaptive Block Size MDCT. IEEE Journal on Selected Areas in Communications, 10(0) 138 - 144. 

Speed. With a 64-slice MDCT scanner, the entire length of the neurovascular tree from arch to vertex can be scanned in under 15 s (the extracranial ICA alone in less than 10 s), minimizing misrecording from motion and breathing artifacts, as well as reducing contrast requirements. 

With regard to MDCT radiation exposure, several factors must be weighed. There is up to a 4.5% loss of efficiency vs. single-detector row scanners due to absorption of radiation in the z-axis by septa between the detector rows [31]. However, with MDCT, most of the X-ray beam is utilized per rotation due to the increased number of detector rows, with less of a penumbra [31]. Subsequently, these effects balance and the dose efficiency of MDCT has been shown to be comparable to that of single-slice helical CT [31]. 

Specific advances in MDCT have been directed toward lowering radiation dose. Among these, automatic tube current modulation is more important for body than for head imaging. With this technique, the tube current is adjusted according to body diameter on the scout image, so as to maintain the same total amount of noise in the image (proportional to photon flux) for any given slice (Table 4.2) [40]. 

Ideally, there should be no venous enhancement. Nevertheless, except for specific indications such as cavernous sinus aneurysm and arteriovenous malformation (AVM) assessment, this is seldom clinically limiting. Z-direction coverage, in-plane and longimdinal resolution, and signal-to-noise ratio should be maximized, while radiation dose, total amount of contrast administered, and acquisition slice thickness should be minimized. Our routine stroke CTA protocol for 16- or 64-slice MDCT scanners covers from the great vessel origins at the aortic arch to the cranial vertex. 

As MDCT scanner technology advances, both acquisition speed and extent of z-direction coverage continue to improve. These advances should make possible the use of lower total contrast volumes and more rapid scanning, without loss of image quality or significantly increased radiation dose. Indeed, with the introduction of 320-row MDCT scanners, which feature a detector width of 16 cm, time-resolved CTA and whole-brain CTP are on the horizon [105-107]. 

Increased coverage will not only be of benefit for whole-brain CTP smdies, which are limited to the detector size of the given MDCT scanner (see Chap. 5),... 

Table 5.2 Sample acute stroke protocol for 64-section multidetector row CT (MDCT) scanner NCCT, vertex-to-arch CTA and biphasic acquisition (one cine image per second for 40 s followed by nine additional cine images obtained at 3-s intervals) CTP...

Coverage volume will continue to increase with enlarging detector arrays and improved technology. Cutting edge scanners with 320 detector rows offer the possibility of 16-cm coverage without the decreased temporal resolution required with shuttle-mode technique. However, some early clinical reports noted inferior image quality with 320-slice MDCT compared to 64-slice MDCT [83, 84]. 

Imanishi, Y, et al., Radiation-induced temporary hair loss as a radiation damage only occurring in patients who had the combination of MDCT and DSA. Eur Radiol, 2005. 15(1) p. 41-6. 

Computed tomography (CT) has developed dramatically with the introduction of multi-row detector technology. Especially the abdomen, where motion artifacts due to respiratory motion and bowel peristalsis are disturbing, benefits greatly from this technique. While scanners with 64 or more detector rows are still most common in large community or university hospitals, scanners with between 2 and 16 slices are widely available even in private practice or in small hospitals. With the introduction of multi-detector CT (MDCT) bi- or even tri-phasic examinations of the liver can be combined into a thoraco-abdominal CT examination without compromise with regard to spatial or temporal resolution. The acquisition of the liver with a 64-slice scanner, for example, only requires a few seconds... 

According to the literature liver metastases are detected with spiral CT with a sensitivity ranging from 58% to 85% [31, 53, 56]. Data from single-row spiral CT and MDCT show that the optimal reconstructed slice thickness for reading CT examinations of the liver on transversal sections is in the range of 2.5 mm-5 mm [17, 57]. Since a reconstructed slice thickness of 2.5 mm is difficult to obtain without motion artifacts on single-row scanners, the use of... 

MDCT can be regarded as helpful and advantageous per se. As mentioned above, modern scanners allow for submillimeter collimations, so that slices with <1 mm can be obtained without problems. Our own experience and the data from the literature, however, showed that such thin slices are not superior in the detection of liver metastases. On the contrary, the large number of slices to be reviewed and the strong increase in image noise are rather disadvantageous [17,25]. 

Fig. 3.4a,b. MDCT in the late arterial (a) and portal venous phase (b) in a male patient suffering from a neuro-endocrine carcinoma with liver metastases (arrows). Note the strong wash-out of the metastases to nearly isointensity, so that even the larger lesions can retrospectively not be properly detected in the portal venous phase in contrast to the excellent conspicuity of the lesions in the arterial phase. This example strikingly demonstrates the importance of a correctly timed late arterial phase... 

Fig. 3.6a-C. MDCT in portal venous phase (a) and MRI with a Tl-w 3D gradient-echo sequence after gadolinium-injection (b) and a T2 -w gradient-echo sequence after injection of the SPIO contrast agent ferucarbotran (c) in a male patient suffering from a colorectal carcinoma in whom atypical resection of a liver metastasis in the right lobe of the liver had been performed previously. Note the postoperative bilioma (marked by the asterisk). In the MDCT examination no further liver lesions can be detected, whereas the gadolinium-enhanced MRI faintly shows a hypervascular lesion arrow). The SPlO-enhanced MRI scan clearly depicts a newly developed metastasis with a high contrast between lesion and adjacent liver parenchyma...

Fig.3.7a-d. MDCT in the arterial phase (a) and MRI with the hepato-biliary contrast agent Gd-EOB-DTPA in the arterial phase (b), portal venous phase (c) and liver-specific phase 20 min after injection (c) each examined with a Tl-w 3D GRE sequence with fat-saturation in a patient with a neuroendocrine tumor and liver metastases. The hypervascular metastases were not detected with MDCT. In the arterial phase after Gd-EOB-DTPA injection hyperintense lesions are demarcated. The hypervascular metastases show a wash-out to isointensity in the portal venous phase. With Gd-EOB-DTPA liver-specific phase imaging can be performed in addition to the early dynamic phase. Note the signal increase in the normal liver parenchyma in (d) caused by the physiological Gd-EOB-DTPA up-take whereas the suspected liver metastases are demarcated as areas spared from specific Gd-EOB-DTPA up-take... 

Fig. 3.8. Male patient suffering from a multifocal hepatocellular carcinoma (HCC), treated with repeated sessions of transarterial chemoembolization (TACE). Dyna-CT (Siemens Medical Solutions, Erlangen, Germany) image started together with injection of 10 ml iodinated contrast agent via a super-selective catheter system placed in the right hepatic artery compared with a corresponding MDCT section in the arterial phase after i.v. injection of 120 ml iodinated contrast agent. Note the excellent, direct depiction of arterial blood supply of the HCC nodule in liver segment 5/8 (arrow). The artifacts in the left and right liver lobe are caused by spots of Lipiodol in already treated HCC nodules after earlier transarterial chemoembolization...

Even if the difference between the diagnostic value of CT and MRI has become smaller due to the introduction of MDCT, MRI with liver-specific contrast agents has to be considered the modality of choice for dedicated liver examinations. MRI has an excellent contrast resolution which allows for sensitive detection of intrahepatic lesions. 3D dynamic gadolinium chelate-enhanced scans enable the as-... 

For dedicated examinations of unclear, potentially benign liver lesions and for hepatic staging prior to liver surgery, MRl can be recommended as the method of choice. In case a CT scan has already been performed (e.g. a staging CT scan of the thorax and abdomen), MRI should not be omitted if the intra-hepatic findings have direct influence on the further treatment. MDCT is suitable for all emergency situations, since it allows for easy patient access and is associated with very short acquisition times, so that uncooperative or clinically unstable patients can be easily examined. 

Romano S, Tortora G, Scaglione M et al (2005) MDCT imaging of post interventional liver a pictorial essay. Eur J Radiol 53 425-432... 

Schima W, Hammerstingl R, Catalano C et al (2006) Quadruple-phase MDCT of the liver in patients with suspected hepatocellular carcinoma effect of contrast material flow rate. AJR Am J Roentgenol 186 1571-1579... 

Vogt FM, Herborn CU, Hunold P, Lauenstein TC, Schroder T, Debatin JF, Barkhausen J (2004) HASTE MRI versus chest radiography in the detection of pulmonary nodules comparison with MDCT. AJR Am J Roentgenol 183 71-78... 

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