DESCRIPTION ULTRASOUND DIAGNOSTIC APPARATUS AND ULTRASOUND DIAGNOSTIC APPARATUS CONTROL METHOD TECHNICAL FIELD [0001] The present invention relates to an ultrasonic diagnostic apparatus and a method for controlling the ultrasonic diagnostic apparatus. BACKGROUND ART [0002] An ultrasonic diagnostic apparatus can capture a tomographic image representing an internal body tissue of a subject, and is currently used extensively to monitor the shape of an internal body tissue. Also, in diagnosing arterial sclerosis, an ultrasonic diagnostic apparatus is used to measure the intima-media thickness (which will be abbreviated herein as "IMT") of the carotid artery, which is an important index to detect initial signs of atherosclerosis. That is to say, IMT is the thickness of an intima-media complex of the vascular wall of the carotid artery, and the thickness of a layer seen between the vascular lumen 201 and the adventitia 202 (which will be referred to herein as " in tima -media 203") as shown in FIGS. 11(a) and 11(b). The ultrasonic diagnostic apparatus detects the boundary between the vascular lumen 201 and the intima (which will be referred to herein as a "lumen-intima boundary 204") and the boundary between the media and the adventitia 202 (which will be referred to herein as a "media-adventitia boundary 205") and calculates the distance between those two boundaries as the IMT. [0003] In measuring the IMT, with an IMT measuring range 206 is set in the blood vessel running direction in which the carotid artery runs (which will be referred to herein as a "longer axis direction"), either the maximum thickness max IMT or the mean thickness mean IMT in that range is ordinarily measured as the IMT value as shown in FIG. 11(a). For example, Non-Patent Document No. 1 recommends setting the IMT measuring range 206 1 cm away from one far end (i.e., the end that is closer to the head) of the common carotid artery (CCA) 207 of the carotid artery. In the example illustrated in FIGS. 11(a) and 11(b), the portion on the right-hand side of the dotted line L is the common carotid artery, and the range shown in FIG. 11(b) is the recommended IMT measuring range 208. [0004] Patent Document No. 1 discloses an ultrasonic diagnostic apparatus that measures the IMT automatically. The ultrasonic diagnostic apparatus of Patent Document No. 1 includes region setting means for setting a region to evaluate the reliability of the blood vessel's IMT measured, reliability calculating means for calculating the reliability of the IMT measured in the region that has been set by the region setting means and presenting the calculated reliability of the IMT measured on a display section, and IMT measuring means for measuring the IMT automatically in the region that has been set by . the region setting means. Hereinafter, it will be described as an example how the ultrasonic diagnostic apparatus disclosed in Patent Document No. 2 may measure the IMT. [0005] The IMT cannot be measured accurately unless the lumen-intima boundary 204 and media-adventitia boundary 205 of the blood vessel are clearly rendered by making the probe send and receive ultrasonic waves from/at around the center of cross section of the blood vessel that is defined to cross the longer axis direction of the blood vessel at right angles. Such a cross section will be referred to herein as a "shorter-axis cross section") and the longer axis direction means the direction in which the blood vessel runs. That is why first of all, the operator puts the probe on the subject's neck surface and adjusts the position and orientation of the probe surface, from/at which ultrasonic waves are sent or received, so that the ultrasonic wave can be sent and received from/at around the center of the shorter-axis cross section of the blood vessel of the carotid artery. [0006] Once the position and orientation of the probe have been adjusted, the recommended IMT measuring range 208 shown in FIG. 11(b) (i.e., the range set by the region setting means according to Patent Document No. 1) is set. Suppose in the initial state in which the probe is put at an appropriate position, the IMT measuring range has shifted from the recommended position to the right just like the IMT measuring range 206 shown in FIG. 11(a). In that case, using a trackball, a touchscreen panel, or any other interface {i.e., region setting means) for setting the position of the IMT measuring range, the operator moves the IMT measuring range to the recommended IMT measuring range 208 shown in FIG. 11(b). If a predetermined condition is satisfied after the IMT measuring range has been moved to the recommended one 208 with the probe held at an appropriate position, the IMT is measured automatically and an IMT value is calculated. CITATION LIST PATENT LITERATURE [0007] Patent Document No.1: Japanese Laid-Open Patent Publication No. 2010-022565 NON-PATENT LITERATURE [0008] Non-Patent Document No.1: Journal -of the American Society of Echocardiography February 2008, pp. 93 to 111 SOMMRRY OF INVENTION TECHNICAL PROBLEM [0009] In the apparatus disclosed in Patent Document No. 1, the IMT calculated is presented on the display section. However, if the ultrasonic diagnostic apparatus needs to continue the measurement, the tomographic image presented on the display section keeps on being updated even after the IMT has been measured so that the latest tomographic image is always displayed. That is why if the tomographic image currently displayed is different from the one from which the IMT was obtained, the IMT is not associated with the tomographic image. In that case, it could be difficult for the operator to determine, by reference to the information displayed on the display section, whether or not the IMT has been measured properly. Also, if the IMT has once been measured successfully but currently cannot be measured properly because the probe has lost proper contact with the subject after that, then the latest measuring condition is not appropriate for making measurement, and therefore, neither an appropriate tomographic image nor a proper result of measurement can be presented. Nevertheless, as the IMT is still presented on the display section even in such a disturbed condition, the operator may sometimes find it unnatural. Furthermore, in a situation where the measurement needs to be continued, even if the IMT is presented, it could be difficult for him or her to decide when to finish the measurement. [0010] Such a problem could arise not just when the IMT is measured but also when the length of a predetermined internal body tissue needs to be obtained in making various physical checkups or when a high definition tomographic image needs to be obtained as well. [0011] An object of the present invention is to overcome at least one of these problems with the related art by providing a highly handy ultrasonic diagnostic apparatus and a method for controlling such an ultrasonic diagnostic apparatus. SOLUTION TO PROBLEM [0012] An ultrasonic diagnostic apparatus according to the present invention is an apparatus to which a probe with transducers is connectible. The apparatus includes a controller which performs transmission processing for sending out an ultrasonic wave toward a subject by driving the probe and reception processing for generating a received signal based on the ultrasonic wave that has been reflected from the subject and received at the probe and which performs, if a predetermined condition based on the received signal is satisfied, automatic freeze processing for stopping at least one of the transmission processing and the reception processing. The controller restricts the automatic freeze processing for a predetermined period of time after having accepted a predetermined operating event. [0013] An ultrasonic diagnostic apparatus controlling method according to the present invention is a method for controlling an ultrasonic diagnostic apparatus to which a probe with transducers is connectible. The method includes the steps of: (i) performing transmission processing for sending out an ultrasonic wave by driving the probe and reception processing for generating a received signal based on the ultrasonic wave that has been reflected from a subject and received at the probe; (iii) restricting, if a predetermined condition based on the received signal is satisfied, automatic freeze processing for stopping at least one of the transmission processing and the reception processing for a predetermined period of time after having accepted a predetermined operating event; and {iv) performing the automatic freeze processing after the predetermined period of time has passed. [0014] Another ultrasonic diagnostic apparatus according to the present invention is an apparatus to which a probe with transducers is connectible. The apparatus includes an ultrasonic wave transmission and reception processing section which performs transmission processing for sequentially sending out ultrasonic waves one after another toward a subject by driving the probe and also performs reception processing for sequentially generating received signals one after another based on the ultrasonic waves that have been reflected from the subject and received at the probe; a tomographic image processing section which sequentially generates tomographic images based on the received signals and which determines whether or not the probe put on the subject is propagating an ultrasonic wave through the vicinity of the center of a short-axis cross section of a blood vessel included in the subject; a vascular wall thickness calculating section which calculates, based on the received signals, a value representing the vascular wall thickness of the blood vessel; a pulsation detecting section which determines whether the pulsating status of the blood vessel is detected properly or not and which outputs a signal at a predetermined timing during one cardiac cycle of the subject; a reliability decision section which outputs, depending on results of the decisions made by the tomographic image processing section and the pulsation detecting section, a signal indicating the degree of reliability of the vascular wall thickness value and the vascular wall thickness value; an image synthesizing section which synthesizes together the tomographic image and the signal representing the thickness of the vascular wall and which generates image data to be presented on a display section; and a control section which controls the ultrasonic wave transmission and reception processing section and the image synthesizing section so that in accordance with the signal indicating the degree of reliability, the ultrasonic wave transmission or reception processing is stopped and/or the tomographic image generated is put into a pause. ADVANTAGEOUS EFFECTS OF INVENTION [0015] According to the present invention, if a predetermined condition about a received signal is satisfied, automatic freeze processing which stops at least one of transmission processing and reception processing is carried out. That is why if the length of a predetermined tissue is measured automatically or if a high-definition ultrasonic image is obtained when an IMT is measured or when any of various internal body tissues is inspected, the measurement is finished. Consequently, when any intended measurement is made automatically, the operator can find this apparatus very easy to handle. [0016] In addition, by restricting the automatic freeze processing for a predetermined period of time after having accepted a predetermined operating event, the apparatus never enters the frozen state at an inappropriate timing even if measurement needs to be made once again by adjusting the measurement range after the intended measurement has gotten done once. Consequently, the handiness can be increased. BRIEF DESCRIPTION OF DRAWINGS [0017] [FIG. 1] A block diagram illustrating an embodiment of an ultrasonic diagnostic apparatus according to the present invention. [FIG. 2] A detailed block diagram of the ultrasonic diagnostic apparatus shown in FIG. 1. [FIG. 3] (a) and (b) illustrate the relative position of the probe put on the subject with respect to a short-axis cross section of the blood vessel. [FIG. 4] (a) is a schematic cross-sectional view illustrating a long-axis cross section of the carotid artery and (b) illustrates a waveform showing how the inside diameter of the carotid artery changes with time as blood pumps out of the heart. [FIG. 5] A flowchart showing how the ultrasonic diagnostic apparatus shown in FIG. 1 operates. [FIG. 6] Shows in detail a variation in the inside diameter of the carotid artery. [FIG. 7] (a) shows a correlation between a carotid artery model waveform and an inside diameter variation waveform, while (b) shows how to extend the carotid artery model waveform on the time axis. [FIG. 8] Shows the blood vessel inside diameter variation waveform, the IMT value variation waveform, and the ECG waveform. [FIG. 9] Illustrates a carotid artery inside diameter variation waveform representing both a situation where the blood vessel is being inspected properly and a situation where the blood vessel is not being inspected properly. [FIG. 10] (a) and