Comparison of contrast-enhanced fundamental imaging, second-harmonic imaging, and pulse-inversion harmonic imaging

RATIONALE AND OBJECTIVES: To investigate the feasibility of recent contrast-specific ultrasound techniques in depicting vascular flow and the effects of changing the output power of the transducer and insonation mode on contrast enhancement, the authors performed an experimental study with a flow phantom. METHODS: While changing the mechanical index and the sound insonation mode (continuous and intermittent), images were obtained with three contrast-enhanced ultrasound techniques: fundamental, second-harmonic, and pulse-inversion harmonic imaging (PIHI) after a bolus injection of microbubble contrast agent. The images were compared on a time-intensity curve. RESULTS: In assessing fixed flow (10 cm/s), PIHI showed the best depiction of flow signal. In intermittent scanning, increases in the mechanical index caused stronger flow signals and longer enhancement duration in all techniques. However, continuous scanning revealed poor depiction of flow signal regardless of the technique or changes in the mechanical index because of significant bubble destruction. CONCLUSIONS: Microbubble-enhanced PIHI with intermittent scanning at a high mechanical index can depict vascular flow highly effectively without shortening the duration of enhancement.     

Low mechanical index contrast-enhanced ultrasound better reflects high arterial perfusion of liver metastases than arterial phase computed tomography

RATIONALE AND OBJECTIVES: We investigated whether observing the arterial vascularization of liver metastases by contrast-enhanced ultrasound with low mechanical index (low-MI) imaging offers additional diagnostic information for the characterization of the liver lesions. METHODS: Twenty nine patients with untreated liver metastases of different primaries were examined. Measurements were performed using a low frame rate, low-MI pulse inversion technique after injection of 2.4 mL SonoVue. The relative maximum signal intensity of the liver lesions related to the normal liver tissue was quantified. Ultrasound findings were compared with contrast-enhanced, dual-phase computed tomography (CT) using a pattern-based classification scheme. RESULTS: Compared with contrast-enhanced CT, this modality better detects arterial perfusion. Metastases, even those usually considered hypovascularized, often showed homogeneous enhancement (66%) and higher arterial vascularization than normal liver tissue. CT did not show a comparable vascularization pattern (P < 0.001) or any similarly early signal intensity (P < 0.001). CONCLUSIONS: Contrast-enhanced CT may not be able to visualize short-lasting but large differences of the arterial perfusion of liver metastases, as does contrast-enhanced low-MI ultrasound. This offers new methods for their characterization and for monitoring of therapeutic effects.     

超声造影在移植肾排斥反应中的诊断价值

目的观测移植肾排斥反应的超声造影特点,分析造影图像,寻求超声造影诊断移植肾排斥反应的定量指标。方法选取患者30例,将患者分为A、B两组,A组20例肾功能异常和B组10例移植肾功能正常患者分别进行常规超声检查和超声造影检查,观察造影时微循环灌注情况并应用造影分析软件对感兴趣区域分析定量指标曲线下面积AUC(Area Under The Curve),然后进行统计分析。结果 A组移植肾微循环的灌注明显比B组差;A组与B组的分析指标AUC差异有统计学意义(P<0.05)。结论超声造影可以动态检测移植肾发生排斥反应时微循环灌注的改变;定量指标AUC为诊断移植肾排斥反应提供了较为可靠、客观的影像学依据。     

Ultrasound imaging of liver metastases in the delayed parenchymal phase following administration of Sonazoid using a destructive mode technique (Agent Detection Imaging)

AIM: To determine whether delayed-phase liver imaging using a destructive imaging mode is able to provide similar information to phase-inversion imaging regarding detection and conspicuity of liver metastases. MATERIAL AND METHODS: Patients with a known primary malignancy with suspected liver metastases were recruited. Ultrasound was performed at baseline, and up to 5min after the administration of Sonazoidtrade mark, using phase-inversion imaging at both low and high mechanical indices (MI) and at 10-15min using destructive imaging. One of four doses of Sonazoidtrade mark was used: 0.008, 0.08, 0.12, and 0.36mul/kg of body weight. Two observers documented lesion number and conspicuity subjectively, and divided the patients into group A (no lesions), group B (one to seven lesions), and group C(I-III) (more than eight lesions, subdivided with increasing lesion number) depending on the number of lesions and categories I-IV based on lesion conspicuity. These parameters were compared with contrast-enhanced computed tomography (CECT) as the reference standard. RESULTS: Sixteen patients were examined (six women, 10 men), mean age 67.3 years (range 48-83 years). Based on CECT imaging, the division was as follows: group A n=1, group B n=8, group C(I)n=1, group C(II)n=4, group C(III)n=2. The accuracy of baseline ultrasound versus CECT was 75% (in 12 of the 16 patients the group concurred) and the accuracy for contrast-enhanced ultrasound (CEUS) versus CECT was 93.8% (15/16). There was a significant improvement in lesion conspicuity for both low (p=0.0029) and high MI phase-inversion (p=0.0004) and destructive (p=0.0015) CEUS imaging in comparison with baseline ultrasound. Artefact was noted at higher doses of Sonazoidtrade mark; and no side effects were recorded. CONCLUSION: Following a single, intravenous injection of Sonazoidtrade mark, the properties of this microbubble allow for a and robust examination of the liver using two different techniques with comparable results.     

Renal elasticity and perfusion changes associated with fibrosis on ultrasonography in a rabbit model of obstructive uropathy

To evaluate elasticity and perfusion change associated with fibrosis in a rabbit model of unilateral ureter obstruction using shear wave elastography (SWE) and contrast-enhanced ultrasonography (CEUS). Complete unilateral ureter obstruction by ligation was performed in the left kidney of 15 rabbits. Renal elasticity on SWE and perfusion change on CEUS at the renal cortex were measured before and after the operation. Histopathological renal fibrosis was quantified by the stained area ratio with Masson trichrome and Picrosirius red using ImageJ analysis. Renal elasticity and perfusion values were compared by the Mann-Whitney U test and Proc Mixed as a function of time. Spearman’s correlation was used to analyze differences between imaging values and fibrosis. The duration of imaging follow-up was up to 49 days, with interval imaging performed 1–3 times. Renal elasticity values were higher in obstructed kidneys compared to contralateral kidneys (31.0 kPa vs 16.4 kPa, p < 0.001) and increased according to postoperative time (0.46 kPa/day). With respect to renal fibrosis, SWE values were positively correlated with Masson trichrome (ρ = 0.651, p < 0.001) and Picrosirius red (ρ = 0.514, p = 0.007). Among CEUS parameters, mean transit time was negatively correlated with renal fibrosis by Masson trichrome (ρ = − 0.639, p = 0.001) and Picrosirius red (ρ = − 0.625, p = 0.001). Rise time and time to peak were positively correlated with renal fibrosis. Obstructive uropathy resulted in changes to both renal elasticity and perfusion. Renal fibrosis was moderately associated with increased renal cortical stiffness and both delayed and decreased cortical perfusion. • Obstructive uropathy causes changes in elasticity and perfusion in a rabbit model. • Renal fibrosis from obstructive uropathy increases renal cortical stiffness, and both delay and decrease cortical perfusion.     

Diffusion and perfusion of the kidney

MRI of the kidney currently makes the transition from depiction of morphology to assessment of function. Functional renal imaging methods provide information on diffusion and perfusion on a microstructural level. This review article presents the current status of functional renal imaging with focus on DWI (diffusion-weighted imaging) and DCE-MRI (dynamic contrast-enhanced MRI), as well as BOLD (blood-oxygenation level dependent) MRI, DTI (diffusion tensor imaging) and arterial spin labeling (ASL). Technical background of these techniques is explained and clinical assessment of renal function, parenchymal disease, transplant function and solid masses is discussed.     

Patient identification data on medical images

MRI of the kidney currently makes the transition from depiction of morphology to assessment of function. Functional renal imaging methods provide information on diffusion and perfusion on a microstructural level. This review article presents the current status of functional renal imaging with focus on DWI (diffusion-weighted imaging) and DCE-MRI (dynamic contrast-enhanced MRI), as well as BOLD (blood-oxygenation level dependent) MRI, DTI (diffusion tensor imaging) and arterial spin labeling (ASL). Technical background of these techniques is explained and clinical assessment of renal function, parenchymal disease, transplant function and solid masses is discussed.     

Fully MR-guided hepatic artery catheterization for selective drug delivery: a feasibility study in pigs

PURPOSE: To demonstrate the feasibility of hepatic catheterization for selective delivery of therapeutic agents using a clinical MRI scanner for real-time image guidance. MATERIALS AND METHODS: Experiments were performed in three domestic pigs (70-80 kg) using a clinical 1.5-T MR scanner. After abdominal three-dimensional contrast-enhanced MR angiography (3D-CE-MRA) was performed, endovascular devices with susceptibility markers were tracked with passive tracking techniques. Catheters were maneuvered into the primary and secondary hepatic arteries. Selective catheterization was verified using selective time-resolved CE angiography. Paramagnetic microspheres were administered to a different region for each liver. The resulting biodistributions were investigated using MR images. RESULTS: Successful selective hepatic catheterization was repeatedly demonstrated using passive tracking techniques. 3D-CE-MRA significantly aided the interventional procedure by showing the vascular anatomy, and maximum-intensity projections (MIPs) were used as roadmaps during the interventions. In all cases, microspheres were successfully delivered to the selected regions. The catheters were visualized at a maximum frame rate of five frames per second, allowing a good depiction of the devices and a reliable catheterization of the hepatic arteries. CONCLUSION: Fully MR-guided real-time navigation of endovascular devices permits complex procedures such as selective intra-arterial delivery of therapeutic agents to parts of the liver.     

Phase-inversion sonography during the liver-specific late phase of contrast enhancement: improved detection of liver metastases

OBJECTIVE: The purpose of our study was to assess whether phase-inversion sonography during the late, liver-specific phase of contrast enhancement using Levovist improves the detection of hepatic metastases relative to unenhanced conventional B-mode sonography. SUBJECTS AND METHODS: Sixty-two patients were studied with unenhanced B-mode sonography and phase-inversion sonography 2.5 min after the injection of Levovist. All patients underwent one reference examination (CT, MR imaging, or intraoperative sonography). The conspicuity, number, size, and distribution of metastases before and after contrast administration as judged by a sonographer (who was unaware of other imaging findings) were compared with each other and with reference imaging. RESULTS: The conspicuity of metastases was improved by contrast-enhanced phase inversion in 94% of patients. Thirty-nine patients showed metastases on reference imaging; 36 of these were positive on baseline sonography and 38 on phase-inversion sonography. Phase-inversion sonography showed more reference imaging-confirmed metastases than baseline sonography in 28 patients (45%). The average number of confirmed metastases per patient was 3.06 for baseline sonography and 5.42 for contrast-enhanced phase-inversion sonography (p < 0.01). The average sensitivity for detecting individual metastases improved from 63% to 91%. Metastases of less than 1 cm were shown in 14 patients on baseline sonography, in 24 patients on phase-inversion sonography, and in 26 on reference imaging. Both sonographic techniques showed false-positive lesions in six patients. CONCLUSION: Contrast-enhanced phase-inversion sonography in the liver-specific phase of contrast enhancement using Levovist provides a marked improvement in the detection of hepatic metastases relative to unenhanced conventional sonography, without loss of specificity. Phase-inversion sonography was particularly advantageous in detecting small metastases and may be a competitive alternative to CT and MR imaging.     

Pseudoaneurysm of the renal artery following partial nephrectomy: imaging findings and coil embolization

AIM: To present the imaging findings of five patients with renal artery pseudoaneurysm (RAP) after partial nephrectomy. METHODS: Five patients (four men and one woman) with RAP as a complication of partial nephrectomy were studied. The diagnosis of RAP was established using contrast-enhanced computed tomography (CT) in three patients and renal angiography in two patients. In two cases, the diagnosis was evident on ultrasound with colour Doppler. RESULTS: The indication for partial nephrectomy (open approach in four patients and laparoscopic in one patient) was a space-occupying lesion, which proved to be a renal cell carcinoma. All patients presented with macroscopic haematuria, 1-21 days (mean 12.2 days) after surgery. In three of patients the definitive diagnostic imaging method was contrast-enhanced CT. The arterial phase of CT showed a well-circumscribed dense collection of contrast material located within the renal parenchyma. In two other patients the initial and conclusive diagnostic imaging method was renal angiography. All patients underwent selective renal angiography with therapeutic coil embolization. The procedure failed in one patient, which necessitated nephrectomy. CONCLUSIONS: Pseudoaneurysm of the renal artery should be considered in patients presenting with macrohaematuria after nephron-sparing surgery. The diagnosis can be established using contrast-enhanced CT, ultrasound with colour Doppler, or angiography. Renal angiography with selective embolization is a safe and efficacious technique for managing the condition.     

Focal liver lesions: role of contrast-enhanced ultrasound

The introduction of microbubble contrast agents and the development of contrast-specific techniques have opened new possibilities in liver imaging. Initially, only intermittent imaging with Doppler detection was available. Second-generation contrast agents and low mechanical index real-time scanning techniques are decisive advances that enable convenient liver examinations with high sensitivity and specificity. Hepatic lesions usually show typical perfusion and enhancement patterns through the various contrast phases, which help their characterization. Several published studies and the daily clinical routine show that, as opposed to conventional ultrasound (US), contrast-enhanced US can substantially improve detection and differentiation of focal liver lesions. Today, contrast-enhanced US is the dynamic imaging modality of choice for differentiation of focal liver lesions. Contrast uptake patterns of the most relevant liver lesions, as well as important clinical indications are presented and discussed.     

Current and potential renal applications of contrast-enhanced ultrasound

The combination of microbubble technology and complementary ultrasound techniques has resulted in the development of contrast-enhanced ultrasound (CEUS) and, although initial clinical applications largely focussed on the liver, these are now becoming more diverse. With?regard to the kidney, it is a safe, well-tolerated, and reproducible technique, and in selected cases, can obviate the need for computed tomography or magnetic resonance imaging. A clear advantage is the absence of nephrotoxicity. With respect to the current and potential renal applications, it is a useful technique in the evaluation of pseudotumours, acute pyelonephritis, renal tumours, cystic lesions, vascular insults, and renal transplantation. It may also be of value for monitoring the kidney following anti-angiogenic treatment or nephron-sparing interventional techniques for renal tumours. Assessment of microvascular perfusion using time-intensity?curves is also likely to have further far-reaching applications in the kidney as well as other organs.     

超声造影对肾脏小占位的诊断分析

目的:探讨不同病理类型肾脏肿瘤的造影特点,评价超声造影诊断肾脏良恶性肿瘤的应用价值,为临床诊断提供依据。方法:应用声学造影剂声诺维对27例超声诊断为肾脏占位病变或可疑肾脏占位病变的患者进行超声造影检查,分析其造影增强图像特点。结果:27例患者均满意获得清晰的肿瘤动态造影灌注图像,大部分与其正常肾组织回声等强,10例肾恶性肿瘤的造影显像比其正常肾组织快,3例肾良性肿瘤造影显像为"同进慢出";4例肾柱肥大造影后与正常肾组织造影剂显像一致;2例肾囊肿内无造影剂显示。结论:超声造影成像技术能清晰显示肾肿瘤的血流灌注特点,对肾脏肿瘤的诊断具有一定实用价值。     

肾乳头状细胞癌的超声造影特征分析并与增强CT对照研究

目的 探讨超声造影与增强CT在肾乳头状细胞癌诊断中的应用价值.方法 分析12例肾乳头状细胞癌的超声造影及增强CT的增强特征,并比较两者敏感性、假包膜显示率、坏死区显示率.结果 肾乳头状细胞癌超声造影模式主要为“慢进”、增强程度为低增强;增强CT主要表现为轻度强化或不强化;两者的敏感性及假包膜显示率无统计学差异,超声造影坏死区显示率高于增强CT.结论 超声造影结合增强CT有助于肾乳头状细胞癌的诊断与鉴别诊断.     

Evaluating the perfusion of occupying lesions of kidney and bladder with contrast-enhanced ultrasound

PurposeTo evaluate the clinical diagnostic value of contrast-enhanced ultrasound (CEUS) for occupying lesions of kidney and bladder.Materials and MethodsCEUS was performed for a total of 50 kidney and bladder occupying lesions in 47 cases, and CEUS manifestations of these lesions were observed and analyzed. Patterns of dynamic changes in perfusion phases of CEUS were summarized and compared with results of postoperative pathology, enhanced computed tomography (CT), magnetic resonance imaging (MRI), and follow-ups.ResultsCEUS results showed that there were 22 cases of malignant renal tumors, 11 cases of benign lesions, two cases of renal column hypertrophy, and 12 cases of malignant bladder tumors (15 lesions). Renal cell carcinoma exhibited various CEUS manifestation, with the majority showing fast filling and hyper-enhancement. CEUS manifestation of renal hamartoma was characterized by slow filling and slow outflow. Renal cystic lesions always exhibited no enhancement within the cysts. Renal column hypertrophy exhibited the same enhancement pattern as the renal cortex. CEUS manifestation of bladder carcinoma was mainly characterized by quick filling, quick outflow, and hyperenhancement.ConclusionsCEUS offers real-time observation of perfusion in occupying lesions of kidney and bladder, but the enhancement pattern of kidney occupying lesions was complex; therefore, combination of enhanced CT, MRI, and CEUS may be necessary.     

Noninfarcted myocardium: correlation between dynamic first-pass contrast-enhanced myocardial MR imaging and quantitative coronary angiography

PURPOSE: To determine the accuracy of first-pass contrast material-enhanced stress myocardial magnetic resonance (MR) imaging for depiction of myocardial ischemia in patients without myocardial infarction. MATERIALS AND METHODS: First-pass contrast-enhanced MR images of the entire left ventricle were acquired in 104 patients at rest and during dipyridamole-induced stress by using an interleaved notched saturation technique. Coronary angiography was performed in all patients, and stress perfusion single photon emission computed tomography (SPECT) was performed in 69 patients. Receiver operating characteristic curve analysis was performed to compare the diagnostic accuracies of first-pass contrast-enhanced stress MR imaging and stress SPECT, with coronary angiography as the reference standard. RESULTS: The overall sensitivity of MR imaging for depicting at least one coronary artery with significant stenosis was 90% (69 of 77 patients). The sensitivities of MR imaging for depiction of single-, double-, and triple-vessel stenoses were 85% (33 of 39 patients), 96% (22 of 23 patients), and 100% (15 of 15 patients), respectively. The specificity of MR imaging for identification of patients with significant coronary artery stenoses was 85% (23 of 27 patients). The areas under the receiver operating characteristic curve for detection of significant stenosis in individual coronary arteries were 0.888 (observer 1) and 0.911 (observer 2) for MR imaging and 0.707 (observer 1, P <.001) and 0.750 (observer 2, P <.001) for SPECT. CONCLUSION: In patients without myocardial infarction, stress enhancement at dynamic MR imaging correlates more closely with quantitative coronary angiography results than does stress enhancement at SPECT.     

Contrast-enhanced computed tomography and ultrasound for the evaluation of tumor blood flow

OBJECTIVE: We evaluated implanted rat mammary adenocarcinoma tumors during a 5-week period using ultrasound, computed tomography (CT), and histology. MATERIALS AND METHODS: Contrast-enhanced ultrasound with a destruction-replenishment imaging scheme was used to derive estimates of blood volume and flow. These ultrasound-derived measures of microvascular physiology were compared with contrast-enhanced CT-derived measures of perfusion and vascular volume made by the Mullani-Gould formula and Patlak analysis, respectively. RESULTS: The tumor cross-sectional area and necrotic core cross-sectional area determined by the 3 methods were correlated (r>0.8, P<0.001, n=15). The spatial integral of perfusion estimated by CT correlated with the spatial integral of flow from ultrasound (P<0.05). The contrast-enhanced tumor area calculated from the ultrasound analysis was highly correlated with the contrast-enhanced area estimated by CT images (r=0.89, P<0.001, n=15). However, the fraction of the tumor area enhanced by the CT contrast agent was significantly larger than either the fraction enhanced by ultrasound contrast agent or than the viable area as estimated from histology slides. CONCLUSION: Destruction-replenishment ultrasound provides valuable information about the spatial distribution of blood flow and vascular volume in tumors and ultrasound analysis compares favorably with a validated contrast-enhanced CT method.     

Kontrastverst?rkter Ultraschall der Skelettmuskulatur

Functional imaging can increase the role of imaging in muscular diseases, as alterations of muscle morphology alone are non-specific for a particular disease. A good example for these functional imaging techniques is to use contrast-enhanced ultrasound (CEUS) to visualize and quantify in vivo (patho-) physiological information about the skeletal muscle microcirculation. Perfusion, i.e. the blood flow per tissue unit including capillary flow, is an important functional parameter. Pathological changes of skeletal muscle perfusion can be found in various clinical conditions, such as degenerative or inflammatory myopathy or peripheral arterial disease (PAD). This article reviews the theoretical basics of functional radiological techniques for assessing skeletal muscle perfusion and focuses on applications of microvascular imaging by CEUS which has improved the diagnosis of these muscular disorders. For evaluation of myositis, CEUS is more efficient in the diagnostic work-up than routine b-mode ultrasound because CEUS can detect inflammation-induced muscular hyperperfusion in acute myositis. This has already been demonstrated by high-mechanical index techniques using a first generation ultrasound contrast agent. Low-mechanical index CEUS techniques that require the use of a second generation contrast agent allow real-time quantification of muscular microcirculation at rest and during exercise. Using this CEUS method, the influence of different exercise intensities on the microcirculation of the exercising muscle becomes detectable. Moreover, the arterial perfusion reserve in PAD can be adequately examined using low-mechanical index CEUS. Initial findings have shown that the arterial perfusion reserve in patients suffering from PAD is reduced in comparison to healthy volunteers. In conclusion, modern CEUS techniques can offer deeper insights in muscular (patho-) physiology than just illustrating unspecific myopathic manifestations using conventional diagnostic imaging, such as edematous or lipomatous changes, hypertrophy or atrophy.     

肾细胞癌MSCT灌注成像研究

目的：测定正常肾脏皮质的各灌注参数,探讨肾细胞癌的多层螺旋CT灌注表现,分析不同病理亚型肾细胞癌的灌注是否有差异。方法：73例肾细胞癌,其中透明细胞癌65例、乳头状癌3例、嫌色细胞癌5例。术前行多层螺旋CT灌注扫描,分别测量肾癌病灶、患侧肾脏及对侧肾脏正常皮质的血容量（BV）、血流量（BF）、平均通过时间（MTT）和表面通透性（PS）,并进行统计学分析。结果：正常肾脏皮质BV、BF、MTT及PS分别为（23．53±5．71）ml／100g、（454．32±110．90）ml／（min·100g）、（3．62±1．38）S、（63．95±18．85）ml／（min·100g）。肾癌病灶BV、BF、MTT及PS分别为（17．17±8．34）ml／100g,（261．96±175．86）ml／（min·100g）、（7．08±3．42）s、（25．07±13．20）ml／（min·100g）。其中透明细胞癌BV、BF、MTT及PS分别为（17．97±8．30）ml／100g,（279．61±177．02）ml／（min·100g）、（6．85±3．39）S、（25．78±12．94）ml／（min·100g）;乳头状癌BV、BF、MTT及PS分别为（4．82±2．93）ml／100g,（52．00±51．77）ml／（min·100g）、（11．74±3．63）s、（11．90±5．12）ml／（min·100g）;嫌色细胞癌BV、BF、MTT及PS分别为（14．22±3．21）ml／100g,（158．49±49．79）ml／（min·100g）、（7．26±1．77）s、（23．69±17．41）ml／（min·100g）,肾细胞癌BV、BF、PS较正常肾脏皮质减低（P〈0．01）,MTT较正常肾皮质增加（P〈0．01）。透明细胞癌BV、BF均较乳头状癌高（P〈0．05）,透明细胞癌MTT较乳头状癌低（P〈0．05）,透明细胞癌BV、BF均较嫌色细胞癌高（P〈0．05）,透明细胞癌BV、BF均较非透明细胞癌高（P〈0．05）,余各病理亚型间各参数无差异。结论：多层螺旋CT灌注扫描可以直观地反映肾细胞癌与正常肾脏组织的灌注差异,且各病理亚型的血流灌注有明显的差异。     

MRT der Lungenperfusion

With technical improvements in gradient hardware and the implementation of innovative k-space sampling techniques, such as parallel imaging, the feasibility of pulmonary perfusion MRI could be demonstrated in several studies. Dynamic contrast-enhanced 3D gradient echo sequences as used for time-resolved MR angiography have been established as the preferred pulse sequences for lung perfusion MRI. With these techniques perfusion of the entire lung can be visualized with a sufficiently high temporal and spatial resolution. In several trials in patients with acute pulmonary embolism, pulmonary hypertension and airway diseases, the clinical benefit and good correlation with perfusion scintigraphy have been demonstrated. The following review article describes the technical prerequisites, current post-processing techniques and the clinical indications for MR pulmonary perfusion imaging using MRI.